Stefania Viola

Marina Cavaiuolo

Dominique Drapier

Stephan Eberhard

Olivier Vallon

Francis-André Wollman

Yves Choquet
email: choquet@ibpc.fr

Stabilization and processing of the atpA mRNA 1 MDA1, a nucleus-encoded factor involved in the stabilization and processing of the atpA transcript in the chloroplast of Chlamydomonas

Keywords: Chloroplast gene expression, RNA stability, OctotricoPeptide Repeat Protein, Nucleo-Chloroplastic interactions, Chlamydomonas reinhardtii

In Chlamydomonas reinhardtii, chloroplast gene expression is tightly regulated posttranscriptionally by gene-specific trans-acting protein factors. Here we report the molecular identification of an OctotricoPeptide Repeat (OPR) protein, MDA1, which governs the maturation and accumulation of the atpA transcript, encoding subunit α of the chloroplast ATP synthase. As does TDA1, another OPR protein required for the translation of the atpA mRNA, MDA1 targets the atpA 5'UnTranslated Region (UTR). Unexpectedly, it binds within a region of ~100 nt in the middle of the atpA 5'UTR, at variance with the stabilisation factors characterised so far, which bind to the 5'end of their target mRNA to protect it from 5'  3' exonucleases. It binds the same region as TDA1, with which it forms a high molecular weight complex that also comprises the atpA mRNA. This complex dissociates upon translation, promoting degradation of the atpA mRNA. We suggest that atpA transcripts, once translated, enter the degradation pathway because they cannot reassemble with MDA1 and TDA1, which preferentially bind to de novo transcribed mRNAs.

Significance statement: Two OctotricoPeptide Repeat (OPR) proteins, MDA1 and TDA1, respectively required for the .accumulation and for the translation of the atpA mRNA interact with a region in the middle of the atpA 5'UTR, instead of the usual interaction with its 5' end, to form a ternary complex. This complex dissociates upon translation, which promotes the degradation of the atpA mRNA.

INTRODUCTION

Oxygenic photosynthesis evolved about 2.7 billion years ago [START_REF] Holland | The Oxygenation of the Atmosphere and Oceans[END_REF], when cyanobacteria started to convert sunlight energy into chemical bonds and reducing power, then used to fix carbon into organic molecules, while oxidising water to molecular dioxygen. In photosynthetic eukaryotes, photosynthesis takes place in specialized organelles, the chloroplasts, which derive from an ancestral cyanobacterium engulfed by a heterotrophic eukaryote [START_REF] Howe | Evolution of the chloroplast genome[END_REF][START_REF] Douglas | Genomes at the interface between bacteria and organelles[END_REF][START_REF] Ponce-Toledo | An early-branching freshwater cyanobacterium at the origin of plastids[END_REF]. Following this initial event, most genes of the endosymbiont were lost or transferred to the nuclear genome of the host cell [START_REF] Kleine | DNA transfer from organelles to the nucleus: the idiosyncratic genetics of endosymbiosis[END_REF]. As a result, chloroplast protein complexes are mosaics of plastid-and nucleus-encoded subunits, and their biogenesis and function require an efficient bidirectional coordination of gene expression between the two genetic compartments (reviewed in [START_REF] Pesaresi | Interorganellar communication[END_REF][START_REF] Woodson | Coordination of gene expression between organellar and nuclear genomes[END_REF].

The unicellular green alga Chlamydomonas reinhardtii has been extensively used as a model organism to study the regulation of chloroplast gene expression. Remarkably, the genetic analysis of nuclear mutants affected in chloroplast gene expression has uncovered an extensive post-transcriptional regulation by nucleus-encoded proteins termed Organelle Trans-Acting Factors (OTAFs). These factors control the stability and maturation (M factors) and/or translation activation (T factors) of specific chloroplast mRNA targets and typically act on their 5' untranslated [START_REF] Barneche | ATAB2 is a novel factor in the signalling pathway of light-controlled synthesis of photosystem proteins[END_REF]Dauvillee et al., 2003;[START_REF] Drager | In vivo evidence for 5'-3' exoribonuclease degradation of an unstable chloroplast mRNA[END_REF][START_REF] Eberhard | Dual functions of the nucleus-encoded factor TDA1 in trapping and translation activation of atpA transcripts in Chlamydomonas reinhardtii chloroplasts[END_REF][START_REF] Loiselay | Molecular identification and function of cis-and trans-acting determinants for petA transcript stability in Chlamydomonas reinhardtii chloroplasts[END_REF][START_REF] Nickelsen | Identification of cis-acting RNA leader elements required for chloroplast psbD gene expression in Chlamydomonas[END_REF][START_REF] Rahire | Identification of an OPR protein involved in the translation initiation of the PsaB subunit of photosystem I[END_REF][START_REF] Stampacchia | A nuclear-encoded function essential for translation of the chloroplast psaB mRNA in Chlamydomonas[END_REF][START_REF] Vaistij | Stability determinants in the chloroplast psbB/T/H mRNAs of Chlamydomonas reinhardtii[END_REF][START_REF] Wostrikoff | TCA1, a single nuclear-encoded translational activator specific for petA mRNA in Chlamydomonas reinhardtii chloroplast[END_REF][START_REF] Zerges | The 5' leader of a chloroplast mRNA mediates the translational requirements for two nucleus-encoded functions in Chlamydomonas reinhardtii[END_REF]reviewed in Stern et al, 2010;[START_REF] Choquet | Translational regulations as specific traits of chloroplast gene expression[END_REF]. In Chlamydomonas, the M factors characterised so far bind the very 5' end of their target mRNA to prevent its degradation by 5'  3'exonucleases [START_REF] Drager | In vivo evidence for 5'-3' exoribonuclease degradation of an unstable chloroplast mRNA[END_REF][START_REF] Loiselay | Molecular identification and function of cis-and trans-acting determinants for petA transcript stability in Chlamydomonas reinhardtii chloroplasts[END_REF][START_REF] Loizeau | Small RNAs reveal two target sites of the RNA-maturation factor Mbb1 in the chloroplast of Chlamydomonas[END_REF][START_REF] Wang | Two Chlamydomonas OPR proteins stabilize chloroplast mRNAs encoding small subunits of photosystem II and cytochrome b6 f[END_REF][START_REF] Pfalz | Site-specific binding of a PPR protein defines and stabilizes 5′ and 3′ mRNA termini in chloroplasts[END_REF][START_REF] Cavaiuolo | Small RNA profiling in Chlamydomonas: Insights into chloroplast RNA metabolism[END_REF]. . They often generate "footprints", i.e. a cluster with a sharp 5'end of small RNAs (sRNAs) protected from ribonucleolytic degradation by their interaction with their cognate M factors [START_REF] Ruwe | Short non-coding RNA fragments accumulating in chloroplasts: Footprints of RNA binding proteins?[END_REF][START_REF] Zhelyazkova | Protein-mediated protection as the predominant mechanism for defining processed mRNA termini in land plant chloroplasts[END_REF][START_REF] Cavaiuolo | Small RNA profiling in Chlamydomonas: Insights into chloroplast RNA metabolism[END_REF][START_REF] Loizeau | Small RNAs reveal two target sites of the RNA-maturation factor Mbb1 in the chloroplast of Chlamydomonas[END_REF]. Most of these footprints disappear in mutant strains defective for the corresponding M factor [START_REF] Loizeau | Small RNAs reveal two target sites of the RNA-maturation factor Mbb1 in the chloroplast of Chlamydomonas[END_REF][START_REF] Ruwe | Systematic analysis of plant mitochondrial and chloroplast small RNAs suggests organelle-specific mRNA stabilization mechanisms[END_REF][START_REF] Cavaiuolo | Small RNA profiling in Chlamydomonas: Insights into chloroplast RNA metabolism[END_REF]. By contrast, how T factors activate translation remains poorly understood. They may recruit ribosomes at the appropriate translation start sites, possibly compensating the lack of bona fide Shine-Dalgarno sequences in most chloroplast genes [START_REF] Schwarz | Synthesis of the D2 protein of photosystem II in Chlamydomonas is controlled by a high molecular mass complex containing the RNA stabilization factor Nac2 and the translational activator RBP40[END_REF][START_REF] Fargo | Shine-Dalgarno-like sequences are not required for translation of chloroplast mRNAs in Chlamydomonas reinhardtii chloroplasts or in Escherichia coli[END_REF][START_REF] Hirose | Functional Shine-Dalgarno-like sequences for translational initiation of chloroplast mRNAs[END_REF][START_REF] Lim | Alterations in rRNA-mRNA interaction during plastid evolution[END_REF]; but see [START_REF] Scharff | Shine-Dalgarno sequences play an essential role in the translation of plastid mRNAs in Tobacco[END_REF]. RBP40, the psbD translation activator, facilitates the recruitment of ribosomes on the psbD start codon by melting a secondary structure sequestering the initiation codon [START_REF] Schwarz | Synthesis of the D2 protein of photosystem II in Chlamydomonas is controlled by a high molecular mass complex containing the RNA stabilization factor Nac2 and the translational activator RBP40[END_REF], as does TAB1, the psaB translation activator in Chlamydomonas [START_REF] Stampacchia | A nuclear-encoded function essential for translation of the chloroplast psaB mRNA in Chlamydomonas[END_REF] or the PentatricoPeptide Repeat (PPR) PPR10 factor controlling atpH mRNA translation and stability in maize [START_REF] Pfalz | Site-specific binding of a PPR protein defines and stabilizes 5′ and 3′ mRNA termini in chloroplasts[END_REF][START_REF] Zhelyazkova | Protein-mediated protection as the predominant mechanism for defining processed mRNA termini in land plant chloroplasts[END_REF].

We previously showed that the expression of the atpA chloroplast gene encoding the α subunit of chloroplast ATP synthase (CF1α) requires the OctotricoPeptide Repeat (OPR) protein TDA1 in C. reinhardtii, which activates its translation but does not control its stability [START_REF] Drapier | Evidence for nuclear control of the expression of the atpA and atpB chloroplast genes in Chlamydomonas[END_REF][START_REF] Eberhard | Dual functions of the nucleus-encoded factor TDA1 in trapping and translation activation of atpA transcripts in Chlamydomonas reinhardtii chloroplasts[END_REF]. TDA1 belongs to the OPR protein family, defined by tandem repeats of a degenerate motif of 38 residues. OPR proteins, abundant in green algae, bind mRNAs in a sequence-specific manner [START_REF] Eberhard | Dual functions of the nucleus-encoded factor TDA1 in trapping and translation activation of atpA transcripts in Chlamydomonas reinhardtii chloroplasts[END_REF][START_REF] Rahire | Identification of an OPR protein involved in the translation initiation of the PsaB subunit of photosystem I[END_REF][START_REF] Boulouis | Spontaneous dominant mutations in Chlamydomonas highlight ongoing evolution by gene diversification[END_REF], as do PPR proteins, another helical repeat protein family prevalent in land plants.

Both families are almost exclusively dedicated to the post-transcriptional control of gene expression in chloroplasts and mitochondria [START_REF] Pfalz | Site-specific binding of a PPR protein defines and stabilizes 5′ and 3′ mRNA termini in chloroplasts[END_REF][START_REF] Hammani | A PPR protein in the PLS subfamily stabilizes the 5′-end of processed rpl16 mRNAs in maize chloroplasts[END_REF][START_REF] Zhou | PPR-SMR protein SOT1 has RNA endonuclease activity[END_REF][START_REF] Loiselay | Molecular identification and function of cis-and trans-acting determinants for petA transcript stability in Chlamydomonas reinhardtii chloroplasts[END_REF].

Here we characterise another OPR protein, MDA1, required for the stable accumulation and 5'end maturation of the atpA transcript in Chlamydomonas. Together, MDA1 and TDA1 constitute one of the few couples of M and T factors acting on the same mRNA identified so far and, therefore, represent an ideal tool to study the molecular basis of the regulation of chloroplast gene expression.

RESULTS

The soluble OPR protein MDA1 is required for the stable accumulation of the atpA mRNA.

The C. reinhardtii mutant mda1-1 was isolated as a non-phototrophic strain in an insertional mutant library [START_REF] Dent | Functional genomics of eukaryotic photosynthesis using insertional mutagenesis of Chlamydomonas reinhardtii[END_REF], and its characterization pointed to a defect in chloroplast ATP synthase. It fails to accumulate the chloroplast atpA mRNA (Fig. 1A), at variance with tda1 mutants, where the absence of the ATP synthase subunit α results from an impaired translation of the atpA mRNA [START_REF] Drapier | Evidence for nuclear control of the expression of the atpA and atpB chloroplast genes in Chlamydomonas[END_REF][START_REF] Eberhard | Dual functions of the nucleus-encoded factor TDA1 in trapping and translation activation of atpA transcripts in Chlamydomonas reinhardtii chloroplasts[END_REF]. Therefore, while TDA1 encodes an atpA mRNA translation factor, the MDA1 gene might encode a stability/maturation factor. Genetic analysis indicated that the mda1-1 mutation was nuclear but not linked to the ble resistance marker used for insertional mutagenesis. Therefore, to identify the mutation responsible for the phenotype, the strain was crossed to S1D2, a wild isolate showing a high level of sequence polymorphism with the reference genome [START_REF] Gross | Extensive restriction fragment length polymorphisms in a new isolate of Chlamydomonas reinhardtii[END_REF][START_REF] Rymarquis | Beyond complementation. Map-based cloning in Chlamydomonas reinhardtii[END_REF]. Upon genome sequencing of pooled wild-type and mutant progeny and mapping of the SNP frequency to the chromosomes, a 7 bp deletion in the third exon of the nuclear gene Cre16.g680850 (Fig. 1B) correlated with the defect in atpA mRNA accumulation. The Cre16.g680850 gene model is 4972 nucleotides long, encompasses 5 exons and 4 introns and encodes a predicted protein of 1207 residues (~120 kDa). The 7 bp deletion leads to a premature stop codon and translation abortion after 357 residues. Analysis with the FT-Rep software [START_REF] Rahire | Identification of an OPR protein involved in the translation initiation of the PsaB subunit of photosystem I[END_REF] of the predicted gene product identified it as an OPR protein with 13 OPR repeats (Suppl. Fig. S1), while a chloroplast targeting peptide was predicted at the Nterminus by the PredAlgo software [START_REF] Tardif | Predalgo: A new subcellular localization prediction tool dedicated to green algae[END_REF]. As other trans-acting factors [START_REF] Eberhard | Dual functions of the nucleus-encoded factor TDA1 in trapping and translation activation of atpA transcripts in Chlamydomonas reinhardtii chloroplasts[END_REF][START_REF] Loiselay | Molecular identification and function of cis-and trans-acting determinants for petA transcript stability in Chlamydomonas reinhardtii chloroplasts[END_REF][START_REF] Raynaud | Evidence for regulatory function of nucleus-encoded factors on mRNA stabilization and translation in the chloroplast[END_REF], MDA1 has orthologs in closely related Chlamydomonadaceae algae, but evolved rapidly with multiple species-specific insertions and its phylogenetic signature is rapidly lost in more distant organisms (Suppl. Fig. S2).

To confirm that this deletion was responsible for the mutant phenotype, we complemented the mda1-1 mutant with a BAC clone (clone 16F5 from the BAC library distributed by the Clemson Genomics Institute) spanning Cre16.g680850 and two other genes (Suppl. Fig. S3A). We recovered phototrophic transformants, which accumulated wild-type levels of CF1α, indicative of a restored accumulation of the atpA transcript (Suppl. Fig. S3B).

Genotyping with primers specific for the wild-type or mutated MDA1 alleles showed that both gene versions were present in transformants (Suppl. Fig. S3C).

Demonstration that Cre16.g680850 actually corresponds to MDA1 was obtained by complementation with a synthetic version of the gene (MDA1-Fl, see Fig. 1B), which contained the full-length CDS and the second intron, placed under the control of the AR promoter [START_REF] Schroda | Sequence elements within an HSP70 promoter counteract transcriptional transgene silencing in Chlamydomonas[END_REF] and of the rpl17 5'UTR [START_REF] Takahashi | peto interacts with other effectors of cyclic electron flow in Chlamydomonas[END_REF]. To detect the MDA1 protein by immunoblotting and perform pull-down experiments, we introduced a triple Flag tag (Fl) after residue 162, in a region that does not contain OPR repeats and where insertions are found in some MDA1 orthologs (Suppl. Fig. S2).

Seven phototrophic clones were analysed by immunoblot for the presence of MDA1-Fl and CF1α and by RNA blot for the recovery of the atpA mRNA (Fig. 1D). The MDA1 protein accumulated to contrasting levels in these strains, probably because of the random insertion of the transgene in the nuclear genome. None of the complemented strains however accumulated wild-type levels of the atpA mRNA, as the highest level of atpA mRNA accumulation, in clone #7, hereafter named MDA1-Fl and used for subsequent experiments, was only ~35% of the wild-type content. We suspect that the abundance of the tagged MDA1 expressed from the synthetic gene remained below that of the endogenous factor. As expected from the requirement of MDA1 for atpA mRNA accumulation and expression, we observed some correlation between the levels of atpA mRNA and those of MDA1 (Fig. 2B), the clones accumulating less MDA1 (#1, #2 and #4) also accumulating lower levels of atpA mRNA.

In parallel, we also constructed tagged versions of the TDA1 protein by inserting either a triple Flag tag after codon 1115 or a triple HA tag at position 210 (Fig. 1C; see Materials and Methods for details). We first used these tagged versions of the proteins to assess their intracellular localisation. To this end, we generated by crosses a double mutant strain, mt, lacking both MDA1 and TDA1. We then recovered strains expressing either MDA1-Fl in the absence of TDA1 (mtM), or TDA1-HA in the absence of MDA1 (mtT) by transformation of this double mutant with the appropriate plasmid and selection of transformants on paromomycin. Lastly, we transformed the mtM strain with the tagged version of TDA1 and selected mtMT double complemented transformants for the restoration of phototrophy.

After ultracentrifugation of whole cell extract from strain mtMT, both proteins were mostly found in the soluble fractions (Fig. 1E). We note, however a limited but significant amount of TDA1 in the membrane, cushion and pellet fractions, suggesting that this factor either belong to very high molecular weight complexes or tends to form aggregates upon chloroplast disruption.

M and T factors characterized so far act on the 5'UTR of their target transcript , with M factors interacting more specifically with the very 5'end of the mature transcript (reviewed in [START_REF] Choquet | Translational regulations as specific traits of chloroplast gene expression[END_REF][START_REF] Stern | Chloroplast RNA metabolism[END_REF][START_REF] Barkan | Expression of Plastid Genes: Organelle-Specific Elaborations on a Prokaryotic Scaffold[END_REF][START_REF] Germain | RNA processing and decay in plastids[END_REF]. To determine if MDA1 also acts on the atpA 5'UTR, we transformed the mda1-1 and MDA1-Fl strains with the chimeric construct 5'dAf [START_REF] Drapier | Intertwined translational regulations set uneven stoichiometry of chloroplast ATP synthase subunits[END_REF][START_REF] Boulouis | The nucleus-encoded trans -acting factor MCA1 plays a critical role in the regulation of cytochrome f synthesis in Chlamydomonas chloroplasts[END_REF], where the 5'UTR of the endogenous petA gene is replaced by that of atpA (Fig. 2A). RNA blots showed that the 5'atpA-petA chimeric transcript behaves as the endogenous atpA mRNA, failing to accumulate in the mda1-1 nuclear context while being stabilized in the complemented strain (Fig. 2B). The atpA 5'UTR is thus sufficient to confer an MDA1-dependent stability to a downstream coding sequence. Using the same chimera, we previously showed that the atpA 5'UTR was also the target of the translation activator TDA1 [START_REF] Eberhard | Dual functions of the nucleus-encoded factor TDA1 in trapping and translation activation of atpA transcripts in Chlamydomonas reinhardtii chloroplasts[END_REF].

In several instances, trans-acting factors have been shown to physically interact with their target transcript [START_REF] Dauvillée | Tab2 is a novel conserved RNA binding protein required for translation of the chloroplast psaB mRNA[END_REF][START_REF] Pfalz | Site-specific binding of a PPR protein defines and stabilizes 5′ and 3′ mRNA termini in chloroplasts[END_REF][START_REF] Rahire | Identification of an OPR protein involved in the translation initiation of the PsaB subunit of photosystem I[END_REF]. To determine whether MDA1 and TDA1 also interact with the atpA 5'UTR, we immuno-precipitated Flagtagged versions of the two proteins (in strains MDA1-Fl and TDA1-Fl, respectively) and performed a similar experiment on a wild-type cellular extract, hence devoid of Flag-tagged proteins, as a negative control (Fig. 2C). Using a probe covering most of the atpA 5'UTR or an unrelated probe derived from the petD chloroplast gene as a control, we analysed the RNA extracted from the immuno-precipitated pellets (P) by dot blot. Samples from both MDA1-Fl and TDA1-Fl strains, but not from the wild-type strain, were specifically enriched in atpA 5'UTR, but not in petD transcript, demonstrating the specific binding of the two proteins to the atpA 5'UTR in vivo. To further define the region of interaction, we repeated this experiment using a series of partially overlapping probes about 100 nucleotides long (Fig. 2D), that together span the whole atpA 5'UTR from the transcription start site (TSS) up to the beginning of the coding sequence. A probe covering the atpA 3'UTR was used as a control. Unexpectedly, for both MDA1-Fl and TDA1-Fl, the strongest enrichment was detected with the probe 5'atpA-3 (Fig. 2E), that covers nucleotides 135-249 of the 5'UTR. Thus, MDA1 and TDA1 interact with the same region in the middle of the atpA 5'UTR.

The MDA1-TDA1-atpA ternary complex.

The latter observation prompted us to assess their possible association in a same oligomeric complex. We thus performed RNA and protein co-immuno-precipitation experiments in soluble cellular extract from strain mtMT (Fig. 3). This input cellular extract was either untreated or RNase-treated, to investigate the possible dependence of a putative interaction between MDA1 and TDA1 on the presence of an intact atpA 5'UTR. Treated and untreated extracts were immuno-precipitated using either α-Flag or α-HA magnetic beads and the recovered RNA analysed by dot blot. Hybridization with the 5'atpA-3 probe confirmed the absence of RNA in RNase-treated pulled-down (P) fractions (Fig. 3A). The RNase-treated total extract (E) fraction saved contains partially digested atpA mRNAs that could be detected by dot blot using the 5'atpA-3 probe. Immunoblot analysis of total extract (E), supernatant (S) and pulled-down (P) fractions showed that a large fraction of MDA1 co-precipitated with TDA1 (Fig. 3B). However, only a minor -but significant-fraction of TDA1 was pulled down with MDA1. This asymmetrical behaviour suggests that the accumulation level of TDA1 might be higher than that of MDA1 in the double-complemented strain mtMT. An RNase treatment prevented most if not all of these co-IP signals. Therefore, we conclude that MDA1 and TDA1 can assemble in a ternary complex on the atpA 5'UTR. Moreover, the binding of MDA1 to the atpA 5'UTR does not depend on the presence of TDA1, as shown by RIP experiments performed on the mtM strain, the double mda1-1/tda1-1 mutant complemented with the tagged version of MDA1 only, that lacks TDA1 (Suppl. Fig. S4). Whether TDA1 interacts with its target in absence of MDA1 could not be assessed, since the atpA mRNA does not accumulate in such a situation.

We then performed size exclusion chromatography using untreated and RNase-treated mtMT soluble extracts as well as untreated extracts from the mtM and mtT strains (Fig. 4). In untreated mtMT, MDA1 was mostly detected in fractions 3 to 8, with a peak in fractions 5 to 7 corresponding to a complex with an apparent molecular mass of ~550-670 kDa. The distribution of TDA1 partially overlapped that of MDA1, but most of it was found in a complex of lower molecular mass (~450-600 kDa, fractions 6-9). Upon RNase treatment the distribution of MDA1 shifted to lower molecular weight complexes (fractions 6-10), with another population now found in higher molecular weight (fraction 2), possibly corresponding to protein aggregates. The minor fraction of TDA1 found in fractions 3-5 was lost upon RNAse treatment, releasing an enrichment of TDA1 in fractions 8-9, in the ~450-520 kDa region. In the mtM sample the distribution of MDA1 was bimodal with a broad repartition in fractions 3-4 similar to that in the untreated sample and a sharp peak in fractions 6-7. In the mtT strain, where neither MDA1 nor the atpA mRNA are present, TDA1 was mainly found in the 520 kDa region (fractions 7-8). As a control, the distribution of the RuBisCO holoenzyme (~520 kDa) remained the same in all samples (lower panel). Taken together, the co-immuno-precipitation and sizeexclusion chromatography experiments indicate that MDA1 and TDA1 interact with the same region in the middle of the atpA 5'UTR to form high molecular weight (HMW) complexes of about 670 kDa and above. Owing to the uneven expression of TDA1 and MDA1 in mtMT, which results from the random insertion of their genes in the double mt mutant, the excess of TDA1 remains MDA1-free in a lower molecular weight complex. Note that in these experiments no special care was taken to preserve the integrity of RNAs: RNAs retained in ternary complexes were probably restricted to fragments closely surrounding the MDA1 or TDA1 binding sites, as shown in RIP experiments by the weak signal against probes 5'atpA-2 and -4 despite a strong interaction with probe 5'atpA-3. RNA should thus account for only a minor increase in molecular mass.

MDA1, but not TDA1, is required for the accumulation of the 5'end processed

atpA-mRNA

In C. reinhardtii, most chloroplast mRNAs are processed after transcription. The atpA gene is transcribed as a tetracistronic primary transcript with a tri-phosphorylated 5'end [START_REF] Cavaiuolo | Small RNA profiling in Chlamydomonas: Insights into chloroplast RNA metabolism[END_REF]. It is then processed to position +36 relative to the TSS to a monophosphorylated monocistronic mRNA [START_REF] Drapier | The chloroplast atpA gene cluster in Chlamydomonas reinhardtii. Functional analysis of a polycistronic transcription unit[END_REF][START_REF] Cavaiuolo | Small RNA profiling in Chlamydomonas: Insights into chloroplast RNA metabolism[END_REF]. Because MDA1 interacts with a region located in the middle of the atpA 5'UTR, we investigated whether it had any role in this 5' processing event. We used circular RT-PCR (cRT-PCR) on total RNA samples from wild-type and mda1-1 strains, as well as the complemented MDA1-Fl strain (Fig. 5). To assess if 5'end maturation also depends on the presence of the translation activator TDA1, we also included the tda1Δ mutant [START_REF] Eberhard | Dual functions of the nucleus-encoded factor TDA1 in trapping and translation activation of atpA transcripts in Chlamydomonas reinhardtii chloroplasts[END_REF].

Prior to RNA self-ligation, half of the RNA was treated with RNA 5'polyphosphatase (RPP) which converts tri-phosphorylated 5'ends into mono-phosphorylated forms. Comparison between RPP and mock samples allows distinguishing primary transcripts from processed mRNA, as only RNAs with mono-phosphorylated 5'ends can be self-ligated. Circular RNA was retro-transcribed with an atpA 5'UTR-specific primer (A-5' 2 RV) and the single-stranded cDNA was used as a template to amplify the junction between transcript 5' and 3'ends using outward directed primers to the atpA 3' (A-3' FW) and 5' (A-5' 1 RV) UTRs. We observed two major products: A1 with the expected size of 240 bp and A2 with a size of ~450 bp (Fig. 5A andB). The A1 amplicon was detected in all, but not in the mock-treated mda1-1, samples. The A2 amplicon, readily detectable in the RPP-treated sample from strain mda1-1 and in both RPPand mock-treated samples from strain MDA1-Fl was hardly detectable in wild-type and tda1Δ samples. Gel-purified amplicons A1 and A2 were sequenced with the primers used for the amplification to map the major 5' and 3'end of the atpA transcript (Suppl. Table ST2). In the wild-type and tda1Δ samples, either mock-or RPP-treated, only the mature 5'end could be detected, while the 3'end was found at the position (+162), identified in [START_REF] Drapier | The chloroplast atpA gene cluster in Chlamydomonas reinhardtii. Functional analysis of a polycistronic transcription unit[END_REF] for the mono-cistronic atpA mRNA. In contrast, the mda1-1 strain showed only the primary 5'end, as suggested by the absence of A1 amplicon in the mock-treated sample. In the complemented MDA1-Fl strain the correctly processed 5'end was found in the mock sample but the primary 5'end predominated in the RPP-treated sample, in both the A2 and A1 bands.

Sequencing the PCR products from the 3'ends showed that A2 bands extend variously into psbI.

We also analysed the 5'-3' junctions of the atpA-psbI and atpA-psbI-cemA di-and tri-cistronic mRNAs and found the same dependency on the presence of MDA1 for 5'end maturation (Suppl. Fig. S5 and Table ST3). Altogether, these results demonstrate that the atpA-psbI-cemA-atpH gene cluster is transcribed in the mda1-1 mutant, the lack of atpA mRNA in this strain being due to a stability defect: atpA transcripts are still produced even if most of them are rapidly degraded.

Many M factors generate "footprints" on the target mRNA. A sRNA of 34-nt mapping the 5'end of the processed atpA transcript has been identified in the sRNA sequencing datasets of the wild-type strain [START_REF] Cavaiuolo | Small RNA profiling in Chlamydomonas: Insights into chloroplast RNA metabolism[END_REF]. We similarly performed sRNA-seq on the mda1-1 and tda1Δ mutant strains (Fig. 5C). The atpA 5'end-associated sRNA was missing in the mda1-1 mutant and hardly detectable in strain tda1Δ. However, since MDA1 and TDA1 bind at a distance from this 5'end, we cannot exclude that this footprint is generated by another protein, whose stable binding to the atpA 5'end would require the presence of MDA1. Thus, MDA1, even though not binding the 5'end of the atpA mRNA, is required for the stabilization of the atpA mRNA and for the maturation of its 5'end.

Once translated, the atpA mRNA is targeted for degradation

To assess if MDA1 and TDA1 remain associated with the atpA mRNA once translation has started, we analysed their localisation in polysomes gradients (Fig. 6A). None of the two proteins were found in polysomal fractions 1-5, where atpA mRNA are steadily detected. Even if we cannot exclude that the affinity of MDA1 or TDA1 for their mRNA target is too low to resist our polysome purification procedure, this observation suggests a release of both factors from their 5'UTR target once the transcript associates with the translation machinery. A similar situation has been described for other trans-acting factors in Chlamydomonas chloroplasts [START_REF] Boudreau | The Nac2 gene of Chlamydomonas encodes a chloroplast TPR-like protein involved in psbD mRNA stability[END_REF][START_REF] Schwarz | Synthesis of the D2 protein of photosystem II in Chlamydomonas is controlled by a high molecular mass complex containing the RNA stabilization factor Nac2 and the translational activator RBP40[END_REF][START_REF] Dauvillée | Tab2 is a novel conserved RNA binding protein required for translation of the chloroplast psaB mRNA[END_REF]. It should be noted that no ribosomal components could be immuno-precipitated with either MDA1 or TDA1 (Fig. 3B), even in presence of Chloramphenicol, which stalls the ribosomes on transcripts, thereby stabilising polysomes (Fig. 6B andC).

We looked for a possible post-translational degradation of the atpA mRNA, that could result from the absence of rebinding of MDA1, or prevent its re-binding. Indeed, the MDA1 and TDA1 factors are stable proteins with a half-life longer than 6 hr, as shown by immunochase experiments (Suppl. Fig. S6). Thus, they should remain potentially available for rebinding the atpA transcript, once translated. To study the stability of the atpA mRNA upon translation, we inhibited chloroplast transcription with rifampicin, in absence or presence of lincomycin, an inhibitor of chloroplast translation initiation (Fig. 6D). A control treatment with lincomycin alone did not show any effect on atpA mRNA accumulation, as previously reported [START_REF] Boulouis | Spontaneous dominant mutations in Chlamydomonas highlight ongoing evolution by gene diversification[END_REF][START_REF] Cavaiuolo | Small RNA profiling in Chlamydomonas: Insights into chloroplast RNA metabolism[END_REF] and in agreement with the phenotype of the tda1Δ strain that does not show a decreased accumulation of the atpA transcript despite its impaired translation.

Upon transcription inhibition, the half-life of the atpA mRNA was of about 3 hours, but it increased considerably (over 6 hours) when chloroplast translation was also inhibited, which shows that translation promotes its degradation.

DISCUSSION

MDA1 is an OPR protein required for the accumulation of the atpA mRNA

In the chloroplast of C. reinhardtii, the post-transcriptional control of atpA gene expression was previously shown to rely on the trans-acting protein factor TDA1, a nucleusencoded OPR protein which targets the atpA 5'UTR to activate its translation [START_REF] Drapier | Evidence for nuclear control of the expression of the atpA and atpB chloroplast genes in Chlamydomonas[END_REF][START_REF] Eberhard | Dual functions of the nucleus-encoded factor TDA1 in trapping and translation activation of atpA transcripts in Chlamydomonas reinhardtii chloroplasts[END_REF]. Here, we identified another trans-acting factor, MDA1, specifically controlling the maturation and stability of the atpA mRNA. MDA1 is also an OPR protein, which further illustrates how, in C. reinhardtii, this family of nucleus-encoded helical repeat proteins is extensively involved in the post-transcriptional control of chloroplast gene expression. By complementing an mda1 mutant with a tagged version of the protein, we retrieved a series of independent phototrophic clones expressing various levels of MDA1. As observed with other M factor, in mutants complemented with a tagged version of the mutated gene none of the complemented strains restored wild-type levels of the atpA mRNAs. The expression level of transgenes in Chlamydomonas rarely reaches that of resident genes, as observed here with our MDA1-transformants. We cannot exclude that the tag, although not abolishing the function of the protein, decreases its affinity for the atpA mRNA.

MDA1 and TDA1 physically interact with the atpA 5'UTR in vivo.

MDA1 and TDA1 genetically target the atpA 5'UTR, as shown using chimeric constructs (this work and Eberhard et al, 2011, respectively). Our RIP experiments provided further evidence for a physical interaction of MDA1 and TDA1 with the atpA 5'UTR in vivo.

Whether these interactions are direct or involve additional and still unidentified protein factors remains to be determined, but OPR proteins typically interact directly with RNA [START_REF] Rahire | Identification of an OPR protein involved in the translation initiation of the PsaB subunit of photosystem I[END_REF][START_REF] Kleinknecht | RAP, the sole octotricopeptide repeat protein in Arabidopsis, is required for chloroplast 16S rRNA maturation[END_REF]. Unexpectedly, a fine mapping of these interactions showed that both MDA1 and TDA1 target the middle (+136 to +250 from the TSS) of the 425 nucleotides long atpA 5'UTR. This situation is unprecedented and implies that MDA1 does not bind the very 5'end of the atpA mRNA, but stabilises it from within. TDA1 also binds the atpA 5'UTR at least 170 nucleotides away from the start codon. The molecular mechanism by which it activates translation remains to be studied and could involve the remodelling of cis-acting structural elements in the atpA 5'UTR, an interaction with additional trans-acting factors or ribosome recruitment.

Neither MDA1 nor TDA1, generate footprints in the middle of the atpA 5'UTR, even though their binding to the target RNA is stable enough for the interaction to resist our experimental conditions for RIP analysis. Remarkably, the sRNA-Seq analysis reveals a 34 nt long sRNA, which maps at the very 5'end of the mature atpA transcript and is no longer observed in absence of MDA1. It is unlikely to represent the footprint of MDA1, which binds in the middle of the atpA 5'UTR. We cannot exclude, however, that the most upstream OPR repeats of MDA1 can bind the 5'end of the transcript and would generate the footprint in our wild-type strain. In the MDA1-Fl complemented strain, this interaction may be weakened by the insertion of the triple Flag tag, preventing us to observe the footprint in our RIP experiments.

But this would not explain the absence of the footprint in the TDA1-Fl complemented strain, which expresses a wild-type MDA1 protein. We rather consider that this footprint results from a conformational change of the atpA mRNA upon MDA1 binding to its distant target or is the footprint of another "canonical", yet unknown, M factor (MDA2?) bound to the 5'end of the transcript, whose association to the atpA mRNA would depend on the presence of MDA1.

While the binding of the hypothetical MDA2 factor would be cooperative with that of MDA1, it should be independent of the presence of TDA1, the atpA transcript being stable in tda1 mutants. Strikingly, however, this sRNAs is also almost absent in the absence of TDA1 (see below).

MDA1 and TDA1 form high molecular weight complexes

Many trans-acting factors for chloroplast gene expression form high molecular weight mRNA-protein complexes (HMW-RNP: [START_REF] Auchincloss | Characterization of Tbc2, a nucleus-encoded factor specifically required for translation of the chloroplast psbC mRNA in Chlamydomonas reinhardtii[END_REF][START_REF] Boudreau | The Nac2 gene of Chlamydomonas encodes a chloroplast TPR-like protein involved in psbD mRNA stability[END_REF][START_REF] Boulouis | The nucleus-encoded trans -acting factor MCA1 plays a critical role in the regulation of cytochrome f synthesis in Chlamydomonas chloroplasts[END_REF][START_REF] Dauvillée | Tab2 is a novel conserved RNA binding protein required for translation of the chloroplast psaB mRNA[END_REF][START_REF] Johnson | MRL1, a conserved Pentatricopeptide repeat protein, is required for stabilization of rbcL mRNA in Chlamydomonas and Arabidopsis[END_REF][START_REF] Perron | A multiprotein complex involved in chloroplast group II intron splicing[END_REF][START_REF] Schwarz | Synthesis of the D2 protein of photosystem II in Chlamydomonas is controlled by a high molecular mass complex containing the RNA stabilization factor Nac2 and the translational activator RBP40[END_REF][START_REF] Stoppel | Recruitment of a ribosomal release factor for light-and stress-dependent regulation of petB transcript stability in Arabidopsis chloroplasts[END_REF][START_REF] Vaistij | Characterization of Mbb1, a nucleus-encoded tetratricopeptide-like repeat protein required for expression of the chloroplast psbB/psbT/psbH gene cluster in Chlamydomonas reinhardtii[END_REF]. MDA1 and TDA1 also both belong to HMW-RNP. Here, we observed that a noticeable proportion of MDA1 co-precipitated with TDA1, while only a limited fraction of TDA1 co-precipitated with MDA1. However this experiment was performed on an MDA1/TDA1 double complemented strain that does not express a wildtype amount of the two proteins. Thus, owing to the co-IP profiles, this strain is likely to express TDA1 in excess of MDA1. This is consistent with the size-exclusion chromatography experiments where MDA1 was mostly found, when the atpA mRNA was present, in a complex of ~550-670 kDa, where a significant but minor fraction of TDA1 is also detected. Indeed, the bulk of TDA1 belonged to a lower MW complex in the 450-500 kDa region, where MDA1 was almost absent. The fractions that contain both proteins likely correspond to a ternary complex: i) they are the most affected by an RNase treatment ii) the presence of MDA1 in the 670 kDa region is decreased in the absence of TDA1 and iii) TDA1 is not found any more in these fractions when MDA1, and therefore the atpA mRNA, are not present. The apparent size of this ternary complex, though, does not allow estimating the factor stoichiometry, mostly because we cannot exclude the presence of additional components.

Maturation of the atpA mRNA requires MDA1

In C. reinhardtii, atpA is the first cistron of a polycistronic unit also comprising psbI, cemA and atpH. In chloroplasts, polycistronic transcripts are often processed to monocistronic mRNAs by intercistronic cleavages followed by 5' and 3' exonucleolytic trimming. Stem loop structures in 3'UTRs protect the mRNA from 3'→5' exonucleases [START_REF] Stern | 3' end maturation of the Chlamydomonas reinhardtii chloroplast atpB mRNA is a two-step process[END_REF][START_REF] Goldschmidt-Clermont | Redundant cis-acting determinants of 3′ processing and RNA stability in the chloroplast rbcL mRNA of Chlamydomonas[END_REF][START_REF] Blowers | Functional in vivo analyses of the 3′ flanking sequences of the Chlamydomonas chloroplast rbcL and psaB genes[END_REF], while binding of protein factors defines the position of the mature 5'ends by hampering 5'→3' exonuclease activity [START_REF] Drager | In vivo evidence for 5'-3' exoribonuclease degradation of an unstable chloroplast mRNA[END_REF]reviewed in Barkan, 2011). Primary mRNAs possess tri-phosphorylated 5' ends, while their processed forms have mono-phosphorylated 5' ends. In agreement, the atpA gene is transcribed as a tri-phosphorylated tetracistronic mRNA and processed to a monocistronic mRNA with a mature mono-phosphorylated 5'end at position +36 with respect to the TSS [START_REF] Cavaiuolo | Small RNA profiling in Chlamydomonas: Insights into chloroplast RNA metabolism[END_REF][START_REF] Drapier | The chloroplast atpA gene cluster in Chlamydomonas reinhardtii. Functional analysis of a polycistronic transcription unit[END_REF].

Although MDA1 does not bind the atpA mRNA 5'end, it is required for its stabilisation and processing to its mature 5'end, as shown by circular RT-PCR experiments. Compared to primer extension analysis, cRT-PCR allows selecting a specific 5'-3' junction and it also allows assessing specifically the 5' maturation of each of the processing intermediates of the primary transcript. In the mda1-1 mutant, the only 5'end detected for the mono-, di-and tri-cistronic transcripts is the tri-phosphorylated TSS. This TSS was not detected in the wild-type and tda1Δ strains, where only the mature monophosphorylated 5'end was observed. Therefore, while MDA1 is required for the proper processing and stable accumulation of the atpA transcript, TDA1 is not, as the mature atpA mRNA accumulates to wild-type levels in the tda1Δ mutant.

The MDA1-complemented strain shows both the primary and processed ends, respectively in the RPP and mock-treated samples. In that strain, atpA transcript accumulates to only 35% of the wild-type levels and hence at least 65% of the newly transcribed atpA mRNA would fail to bind MDA1 and undergo degradation. Interestingly, the tri-phosphorylated atpA transcripts detected in the mda1-1 and MDA1-Fl strains only (A2 amplicon) show heterogeneous 3'ends, extending into the downstream genes, which suggests that this technique captures atpA transcripts very early after transcription, which are too short-lived to be properly processed, before their 5' and 3' maturation is completed and before binding of MDA1. These processing intermediates should also exist in the wild-type and tda1Δ strains, but would only represent a tiny and undetectable fraction of the total transcripts. Thus 3' processing and 5'end maturation events are not mechanistically coupled or even temporally ordered, but rather happen stochastically until the final mature mono-cistronic form is produced.

We conclude that, although not directly binding the mature atpA 5'end, MDA1 appears pivotal in its formation, by recruiting or assisting additional factor(s) that would protect the mature 5'end.

The subset of translated atpA mRNAs is subsequently targeted for degradation

The interaction of MDA1 and TDA1 with the atpA transcript appears stable enough to be detected in co-IP and RIP experiments. However, their interaction with the atpA transcript is disrupted upon translation initiation: neither MDA1 nor TDA1 were found in polysomal fractions, and co-IPs failed to show a stable interaction with ribosomal components. Similarly, the NAC2 (MBD1) factor, required for psbD mRNA stabilisation [START_REF] Nickelsen | Determinants for stability of the chloroplast psbD RNA are located within its short leader region in Chlamydomonas reinhardtii[END_REF][START_REF] Boudreau | The Nac2 gene of Chlamydomonas encodes a chloroplast TPR-like protein involved in psbD mRNA stability[END_REF], recruits the psbD translational activator, RBP40, with which it forms a HMW-complex. This complex is not associated with polysomes, suggesting that the proteins dissociate from their target mRNA once it is engaged in translation [START_REF] Schwarz | Synthesis of the D2 protein of photosystem II in Chlamydomonas is controlled by a high molecular mass complex containing the RNA stabilization factor Nac2 and the translational activator RBP40[END_REF]. Also the translation activators of the psbC and psaB genes, respectively TBC2 and TAB2, were not found in polysomal fractions [START_REF] Auchincloss | Characterization of Tbc2, a nucleus-encoded factor specifically required for translation of the chloroplast psbC mRNA in Chlamydomonas reinhardtii[END_REF][START_REF] Dauvillée | Tab2 is a novel conserved RNA binding protein required for translation of the chloroplast psaB mRNA[END_REF].

The dissociation of both TDA1 and MDA1 from the atpA 5'UTR upon translation initiation should deliver an unprotected transcript that would be prone to degradation. However, mutations in T factors, here the deletion of TDA1 [START_REF] Eberhard | Dual functions of the nucleus-encoded factor TDA1 in trapping and translation activation of atpA transcripts in Chlamydomonas reinhardtii chloroplasts[END_REF], do not affect mRNA accumulation [START_REF] Drapier | Evidence for nuclear control of the expression of the atpA and atpB chloroplast genes in Chlamydomonas[END_REF][START_REF] Stampacchia | A nuclear-encoded function essential for translation of the chloroplast psaB mRNA in Chlamydomonas[END_REF][START_REF] Schwarz | Synthesis of the D2 protein of photosystem II in Chlamydomonas is controlled by a high molecular mass complex containing the RNA stabilization factor Nac2 and the translational activator RBP40[END_REF]Dauvillee et al, 2003;[START_REF] Rahire | Identification of an OPR protein involved in the translation initiation of the PsaB subunit of photosystem I[END_REF][START_REF] Girard-Bascou | A nuclear mutation affects the synthesis of the chloroplast psbA gene production Chlamydomonas reinhardtii[END_REF]. Similarly, inhibition of translation initiation by lincomycin does not significantly modify the steady-state levels of most chloroplast mRNAs, including atpA [START_REF] Boulouis | Spontaneous dominant mutations in Chlamydomonas highlight ongoing evolution by gene diversification[END_REF][START_REF] Cavaiuolo | Small RNA profiling in Chlamydomonas: Insights into chloroplast RNA metabolism[END_REF]. Either translation does not interfere with mRNA degradation pathways or, if it does, on-going chloroplast transcription compensates for degradation. Here we addressed this question by examining the effect of translation inhibition by lincomycin on mRNA decay upon rifampicin treatment. We showed that the atpA mRNA is degraded faster when translated: in absence of de novo transcription, its half-life of ~3 hr approximately doubles when translation is also prevented. This is probably because MDA1 remains bound to its target, thus protecting it from degradation. In physiological conditions, transcription compensates for transcript degradation and the size of the pool of atpA transcript is determined by the amount of MDA1. Translationinduced degradation of the atpA mRNA is consistent with its over-accumulation in tda1 mutants [START_REF] Drapier | Evidence for nuclear control of the expression of the atpA and atpB chloroplast genes in Chlamydomonas[END_REF] and with its high instability in cells grown in autotrophic conditions under high light, i.e. when an active translation of chloroplast genes is required [START_REF] Eberhard | Searching limiting steps in the expression of chloroplast-encoded proteins: Relations between gene copy number, transcription, transcript abundance and translation rate in the chloroplast of Chlamydomonas reinhardtii[END_REF].

One may argue that in the presence of rifampicin alone, MDA1 and TDA1 dissociated from the atpA mRNA upon translation remain available -both being long-lived enough (Suppl. Fig. S6) -to rebind and stabilise atpA transcripts once released from ribosomes. However we showed in this study that such is not the case. They do not rebind at a rate high enough to prevent the posttranslational degradation of the atpA transcript. This observation can be interpreted in two ways (Fig. 7). Translation of the atpA mRNA, prone to translational pauses [START_REF] Stollar | Ribosomes pause during the expression of the large ATP synthase gene cluster in spinach chloroplasts[END_REF], may last long enough to allow some co-translational degradation of the atpA 5'end by the 5'  3'exonucleases particularly active in the chloroplast, thereby preventing the transcript to be rebound by an hypothetical 5' MDA2 factor or antisense sRNA. Assuming that its binding is cooperative with that of MDA1, as suggested by the loss of the atpA mRNA in mda1 mutant, the two factors will be directed to de novo transcribed mRNA with intact 5'ends, rather than to already translated and 5' trimmed atpA transcripts, explaining why newly transcribed atpA mRNAs are favoured over those that have already been translated.

The increased stability of the atpA mRNA upon translation inhibition may also explain the puzzling observation of the loss of the 5' sRNA footprint in strain tda1Δ, despite a preserved or even increased stability of the atpA transcript. Accumulation levels of footprint sRNA probably result from a complex interplay between the abundance of the mRNA complexed to its cognate M factor, the rate of endonucleolytic cleavages followed by 3'  5' trimming up to the region protected by the M factor and the rate of dissociation of the M factor and its footprint.

These levels appear extremely variable, even when comparing genes with similar accumulation: the footprint at the 5'end of the rbcL mRNA is almost below detection threshold, while that at the 5'end of the atpH mRNA is highly abundant [START_REF] Cavaiuolo | Small RNA profiling in Chlamydomonas: Insights into chloroplast RNA metabolism[END_REF]Ozawa et al, 2018).

The increased stability of atpA transcripts upon inhibition of translation, whether by antibiotics treatment or deletion of TDA1, may also be due to a lower rate of endonucleolytic cleavages associated with ribosome stalling or pausing [START_REF] Stollar | Ribosomes pause during the expression of the large ATP synthase gene cluster in spinach chloroplasts[END_REF]. The rate of production of the footprint may be lowered up to a level where its dissociation from the M factor and its subsequent degradation take over.

The selective advantage of such a "translate and destroy" mechanism, compared to a less wasteful "translate and recycle" strategy remains to be studied. atpA is one of those transcripts that accumulate in large excess over what is required for translation, since as little as 5% of the wild-type atpA mRNA levels still ensure a 50% accumulation of the α subunit [START_REF] Boulouis | Spontaneous dominant mutations in Chlamydomonas highlight ongoing evolution by gene diversification[END_REF][START_REF] Drapier | A dominant nuclear mutation in Chlamydomonas identifies a factor controlling chloroplast mRNA stability by acting on the coding region of the atpA transcript[END_REF], while less than 50 % of the transcript are loaded on polysomes at a given time [START_REF] Eberhard | Dual functions of the nucleus-encoded factor TDA1 in trapping and translation activation of atpA transcripts in Chlamydomonas reinhardtii chloroplasts[END_REF]. For many genes, transcription rates exceed what can be stabilised by M factors [START_REF] Loiselay | Molecular identification and function of cis-and trans-acting determinants for petA transcript stability in Chlamydomonas reinhardtii chloroplasts[END_REF], while transcript accumulation exceeds what is required for translation [START_REF] Eberhard | Searching limiting steps in the expression of chloroplast-encoded proteins: Relations between gene copy number, transcription, transcript abundance and translation rate in the chloroplast of Chlamydomonas reinhardtii[END_REF][START_REF] Boulouis | Spontaneous dominant mutations in Chlamydomonas highlight ongoing evolution by gene diversification[END_REF]. As a consequence, the rate of production of a protein can be adjusted simply by modulating translation initiation. Would biosynthetic processes being adjusted at the level of downstream steps, more complex regulatory events would be required to also increase mRNA accumulation. In bioenergetics organelles, the key to evolutionary success probably resides more in fast responses to environmental changes than in minimizing energy expenditure. The physiological conditions in which the "translate and destroy" system would provide an improved fitness remain to be determined. We note however that translation of atpA is subject to CES regulation, being stimulated by unassembled CF1β subunits, whose level in turn depends on CF1α availability [START_REF] Drapier | Intertwined translational regulations set uneven stoichiometry of chloroplast ATP synthase subunits[END_REF]. Furthermore, we performed the present experiments in acetatesupplemented medium under continuous light and we cannot rule out that a recycling of atpA mRNA after translation would be favoured under photoautotrophic, circadian or high-light stress conditions, where the levels of the atpA OTAFs may drastically change.

MATERIALS AND METHODS

Strains and growth conditions

Wild-type (t222+: CC5001, derived from 137c: nit1 nit2 mt+; [START_REF] Gallaher | Chlamydomonas genome resource for laboratory strains reveals a mosaic of sequence variation, identifies true strain histories, and enables strain-specific studies[END_REF], mutant, and transformed strains of C. reinhardtii were grown at 25°C in Tris-acetate-phosphate (TAP) medium (pH 7.2) [START_REF] Harris | The Chlamydomonas source book: a comprehensive guide to biology and laboratory use[END_REF] under continuous low light (5 to 10 µE m -2 s -1 ). The original mda1 mutant strain CAL018.01.04, obtained by insertional mutagenesis [START_REF] Dent | Functional genomics of eukaryotic photosynthesis using insertional mutagenesis of Chlamydomonas reinhardtii[END_REF] was crossed twice with our reference strain t222+ to generate the zeocin-sensitive strain mda1zeo S mt+ (hereafter called mda1-1) used in that study. The nuclear mutants tda1Δ and tda1-F54 have been described in [START_REF] Eberhard | Dual functions of the nucleus-encoded factor TDA1 in trapping and translation activation of atpA transcripts in Chlamydomonas reinhardtii chloroplasts[END_REF]. The mt double mutant was obtained in a cross between strains mda1-1 and tda1-1, performed according to [START_REF] Harris | The Chlamydomonas source book: a comprehensive guide to biology and laboratory use[END_REF].

Mapping of the mda1-1 mutation

Strain mda1-1 was crossed to S1D2 (CC-2290, mt-) and 32 individual zygotes were dissected, from which acetate requiring (ac-) and acetate independent (ac+) progeny were isolated and grown individually in 15 ml cultures. DNA was isolated (DNeasy Plant maxi kit, Qiagen) from cell pellets of pooled cultures of ac+ or ac-progeny. The two DNA samples were sequenced using Illumina technology (PE125) by Eurofins Genomics, in parallel with DNA from S1D2 and t222+. SHOREmap [START_REF] Schneeberger | SHOREmap: Simultaneous mapping and mutation identification by deep sequencing[END_REF]) identified a region around Chromosome_16:5,300,000 (genome version 5.5) where the ac-pool showed the lowest proportion of S1D2 polymorphisms, and the ac+ pool the highest proportion (Fig. S7). Within this region, SHOREmap 'annotate' identified candidate mutations that were individually examined. A 7-bp deletion (pos. 5478830-5478836) in gene Cre16.g680850 was identified as possible cause of the phenotype.

Construction of plasmids:

Standard nucleic acid manipulations were performed according to [START_REF] Sambrook | Molecular Cloning[END_REF]. Primers used in this study are listed in Supplemental Table ST1. All DNA constructs were sequenced before transformation in Chlamydomonas. i) pMDA1-Fl was purchased from the GenScript Company. The MDA1 coding sequence containing the first intron and the 3xFlag tag was generated by DNA synthesis (Fig. 1B), digested with EcoRI and BamHI and cloned into the pPEARL plasmid [START_REF] Takahashi | peto interacts with other effectors of cyclic electron flow in Chlamydomonas[END_REF] digested with the same enzymes.

ii) To construct pTDA1-Fl, the first 2359 bp of the TDA1 genomic sequence were amplified using the primers NTDA1 FW and NTDA1 RV and cloned into the pPEARL plasmid into restriction sites EcoRI and BamHI. A 3xFlag tag was inserted by overlapping PCR [START_REF] Higuchi | Recombinant PCR. In: PCR protocols: a guide to methods and application[END_REF] in the last 4448 bp of the TDA1 genomic sequence, using the external primers CTDA1 FW and CTDA1 RV and the mutagenic primers TDA1-Flag-Rev and TDA1-Flag-Cod.

The obtained fragment was then cloned into the previous construct using the restriction sites SgrAI and NdeI.

Transformation experiments

Chloroplast transformations were performed by tungsten particle bombardment [START_REF] Boynton | Chloroplast transformation in Chlamydomonas[END_REF] as described by [START_REF] Kuras | The assembly of cytochrome b6/f complexes: an approach using genetic transformation of the green alga Chlamydomonas reinhardtii[END_REF]. Transformants were selected on TAP medium supplemented with spectinomycin (100 µg ml -1 ) under low light (5 to 10 µE m -2 s -1 ) and further sub-cloned under darkness on selective medium until they reached homoplasmy, assessed by RFLP analysis of specific PCR fragments. At least three independent transformants were analysed and proved identical.

Nuclear transformations were performed by electroporation, as described by [START_REF] Shimogawara | High-efficiency transformation of Chlamydomonas reinhardtii by electroporation[END_REF] and [START_REF] Raynaud | Evidence for regulatory function of nucleus-encoded factors on mRNA stabilization and translation in the chloroplast[END_REF], using the following parameters: 10 µF/1000 to 1200 V cm -1 , depending on the recipient strains. Plasmids were linearized with PsiI prior to transformation. Transformants were either selected for phototrophy under high light (200 µE m -2 s -1 ) on minimum medium [START_REF] Harris | The Chlamydomonas source book: a comprehensive guide to biology and laboratory use[END_REF] or for resistance to paromomycin (10 µg ml -1 ) under low light on TAP medium.

Protein isolation and analysis

Protein isolation, separation, and immunoblot analyses were performed on exponentially growing cells (2x10 -6 cells.ml -1 ) as described by [START_REF] Kuras | The assembly of cytochrome b6/f complexes: an approach using genetic transformation of the green alga Chlamydomonas reinhardtii[END_REF]. Cell extracts were loaded on an equal chlorophyll basis. Protein samples from co-immunoprecipitation, size-exclusion chromatography and polysome gradients were loaded on an equal volume basis. At least three biological replicas were performed for each experiment. Proteins were detected by ECL using the ChemiDoc Imager (BioRad) and quantified, as described in [START_REF] Choquet | Cytochrome f translation in Chlamydomonas chloroplast is autoregulated by its carboxyl-terminal domain[END_REF], using the ImageLab software (BioRad). Primary antibodies, diluted 100,000-fold (cyt. f), 50,000-fold (CF1β, RuBisCO), 10,000-fold (CF1α, RPS12), were revealed by horseradish peroxidase-conjugated antibodies against rabbit IgG (Promega).

Antibodies against α and β subunits of CF1, and cytochrome f have been previously described [START_REF] Lemaire | The chloroplast ATP synthase in Chlamydomonas reinhardtii[END_REF][START_REF] Kuras | The assembly of cytochrome b6/f complexes: an approach using genetic transformation of the green alga Chlamydomonas reinhardtii[END_REF]. HA-and Flag-tagged proteins were detected by ECL using monoclonal antibodies anti HA.11 (Covance) and anti-Flag M2 (Sigma-Aldrich), respectively, and revealed with horseradish peroxidase-conjugated antibody against mouse IgG (Promega).

Subcellular protein localisation was assessed as described in [START_REF] Boulouis | The nucleus-encoded trans -acting factor MCA1 plays a critical role in the regulation of cytochrome f synthesis in Chlamydomonas chloroplasts[END_REF].

RNA isolation and analysis

Total RNAs were extracted and analysed as described in [START_REF] Drapier | The chloroplast atpA gene cluster in Chlamydomonas reinhardtii. Functional analysis of a polycistronic transcription unit[END_REF] except that probes amplified with primers listed in Supplemental Table ST1 were Digoxigeninlabelled, using DIG-dUTP, the anti-Digoxigenin Fab fragment and CDP Star reagent (Roche).

Signal was aquired in ChemiDoc Imager (BioRad) and analysed with the ImageLab software (BioRad). Ribosomal RNAs were visualized by Methylene blue staining of filters. Circular RT-PCR was performed as described in [START_REF] Pfalz | Site-specific binding of a PPR protein defines and stabilizes 5′ and 3′ mRNA termini in chloroplasts[END_REF]. PCR products were gel-extracted and sequenced. sRNA-Seq datasets (SRA BioProject PRJNA379963) were produced and analysed as described in [START_REF] Cavaiuolo | Small RNA profiling in Chlamydomonas: Insights into chloroplast RNA metabolism[END_REF].

RNA chase was performed on exponentially growing cells in TAP medium, supplemented at t = 0 with rifampicin (350 µg ml -1 ), lincomycin (500 µg ml -1 ), or both. At the indicated time points, aliquots were removed for RNA isolation.

RNA and protein co-immuno-precipitations

Immuno-precipitations and RNase treatment of soluble extracts were done as described by [START_REF] Boulouis | Spontaneous dominant mutations in Chlamydomonas highlight ongoing evolution by gene diversification[END_REF], with minor modifications since cells were broken by sonication and the immuno-precipitation was performed on 1mL fraction at 4°C for 2 hr. For RNA immunoprecipitation (RIP), beads, resuspended in 250 µL AE buffer (50 mM Na-acetate pH 5.2, 10 mM EDTA), were extracted once with phenol/ chloroform/IsoAmyl Alcohol (25:24:1) before ethanol precipitation in the presence of 2 µL GlycoBlue (Invitrogen, USA). For CAP treatment, chloramphenicol (100 µg.mL -1 ) was added to the cells 10 mn before sonication.

Size-exclusion chromatography

Size-exclusion chromatography was done according to [START_REF] Boulouis | Spontaneous dominant mutations in Chlamydomonas highlight ongoing evolution by gene diversification[END_REF], with minor modifications since cells were broken by sonication and protein separation was performed on Superose 6 Increase 10/300 GL column (GE Healthcare) column, with elution performed at 4°C at a rate of 0.2 ml•min -1 .

Polysome analysis

Polysomes were purified as described by [START_REF] Minai | Chloroplast biogenesis of photosystem II cores involves a series of assembly-controlled steps that regulate translation[END_REF] under MgCl2 conditions. Initial solutions to form the gradients were prepared at 15% or 55% w/v sucrose, but the real sucrose concentration of the collected fractions was determined by refractrometry. Both RNA and proteins were purified from each collected fraction. C) RNA immuno-precipitation with anti-Flag magnetic beads identifying RNAs associated with TDA1-Fl and MDA1-Fl in the strains respectively expressing them (wild-type used as control strain). Dot blot of input (E) and beads pellet (P) RNA samples for each strain was hybridised against atpA-5' and petD (control) probes. A) RNA immuno-precipitation with anti-Flag and anti-HA magnetic beads identifying RNAs associated with MDA1-Fl and TDA1-HA in untreated and RNAse-treated mtMT samples. Dot blot of input (E) and beads pellet (P) RNA fractions for each sample analysed with 5'atpA-3 (shown in Figure 3D) and petD (control) probes.

D)

B)

Immuno-detection of MDA1-Fl and TDA1-HA in the input (E), supernatant (S) and pellet (P) samples from the same immuno-precipitations as in (A). The antibody against the ribosomal protein Rps12 was used as control. C) Distribution of sRNAs in mutant strains mda1-1 (red) and tda1Δ (blue) along the atpA 5'UTR compared to that in the wild-type (black). Coverage is normalized as Reads Per Million (RPM) and averaged over two biological replicates. The lower scheme depicts the atpA 5'UTR region (same convention than in Fig. 2D).

D)

Wild-type cells were treated with rifampicin (Rif, 350 µg ml -1 ), lincomycin (Lin, 500 µg ml -1 ) or both (Rif+Lin). RNA was extracted just before or 3 and 6 hr after inhibitors addition.

Accumulation of atpA and Cβlp2 (nuclear control) were analysed by RNA blot, ribosomal

RNAs were detected by methylene blue staining of the filters. For each condition, the percentage of atpA mRNA amounts relative to time 0 is shown on the right (n=3). Whole cell extracts (E) from the mtMT strain, overlaid on a 1.5 M sucrose cushion, were separated by ultracentrifugation into the supernatant (S), membrane (M), cushion (C) and pellet (P) fractions. Equal volumes of each fraction were loaded on gel and probed with the indicated antibodies. Hsp70 and cytochrome f provide markers of the soluble and membrane fractions, respectively. C) Distribution of sRNAs in mutant strains mda1-1 (red) and tda1Δ (blue) along the atpA 5'UTR compared to that in the wild-type (black). Coverage is normalized as Reads Per Million (RPM) and averaged over two biological replicates. The lower scheme depicts the atpA 5'UTR region (same convention than in Fig. 2D). Cas 1 --------------------------------------------------------------------------------Csp 1 -----------------------------------------------------------------QVQLVWTAPQPRRVS Gpe 1 -----------------------------------------------------MLPMWRPLGRCLQVQLWSEGTRHRQHL Tso 1 ---------MLLEVFSALARASTAQPDVTDTRASRAGQARDVWKQLQPQQQLQQQGKQHQQQQQQRRRLFPQDGVPPPEV Vca 1 ---------MRTLGRHLSRRSRPHFPQACPFSHYRGRRRGLAILAGAPPLPSVSSGVDNPPRRARQVQLVWNASSQQPRW Esp 1 ---------MRTQGRQAARRGRPHAAQVCPF-HGRGRRGRLAILAGAPPVNSYASGVDTPPARTRSVPLVWSGEYAQPNG Cde 1 --------------------------------------------------------------------------------Cre 1 MNLQALGRPAALGARRSLSRQAARKALIVSARLAKPEGLHGSIAASRVPQVVQGDQPGGQLESPYQVRPARHQQHQHQHG Yun 1 -----------------------------------------------------------------QVQLVWNQPQHQRR-

  iii) To construct pTDA1-HA, a 3xHA tag was first inserted by overlapping PCR in the first 2359 bp of the TDA1 genomic sequence, using primers NTDA1 FW and NTDA1 RV (external) and TDA1-HA-Rev and TDA1-HA-Cod (internal). The amplified fragment was cloned into pPAR in restriction sites EcoRI and BamHI. The last 4448 bp of the TDA1 genomic sequence were then amplified using primers CTDA1 FW and CTDA1 RV and cloned into the previous construct into SgrAI and NdeI sites.

Figure 1 .

 1 Figure legends:

Figure 2 .

 2 Figure 2. Both MDA1 and TDA1 physically interact with the atpA 5'UTR. A) Schematic representation of the 5'dAf chimera, used to replace the petA endogenous gene in the mda1-1 {5'dAF} and MDA1-Fl {5'dAF} strains. B) Transcript accumulation detected using probes specific for the atpA 5'UTR (shown in panel D), petA coding sequence and Cβlp2 (nuclear control) in mda1-1 and MDA1-Fl recipient strains and in the respective transformants containing the 5'dAf chimeric construct (indicated by the stars).

  Schematic representation of the atpA gene model with a zoom on the 5'UTR showing the positions of the probes (black lines) used in dot blot experiments. The atpA transcription start site (TSS) and the mature 5'end are indicated. The black rectangle symbolises a sRNA mapping at the 5'end of the mature transcript (see below).E) Immuno-precipitated RNA samples from panel C were probed with the partially overlapping probes 5'atpA 1 to 5 with the control probe atpA-3'.

Figure 3 .

 3 Figure 3. The atpA 5'UTR mediates the in-vivo interaction of MDA1 and TDA1.

Figure 4 .

 4 Figure 4. Interaction of MDA1, TDA1 and atpA mRNA probed by size-exclusion chromatography. Soluble cell extract from strains mtMT, either untreated or RNase-treated, mtM or mtT, respectively expressing both MDA1-Fl and MDA1-HA, MDA1-Fl alone or MDA1-HA alone were fractionated on a Superose 6 column. After gel electrophoresis, fractions were immunodecorated with anti-Flag or HA antibodies. Molecular masses of the complexes found in each fraction were estimated by comparison with protein standards of the MHW gel filtration calibration kit and with the elution profile of the RuBisCO holoenzyme (represented by the large subunit, LSU).

Figure 5 .

 5 Figure 5. Mapping of atpA mRNA 5'ends and sRNA accumulation in presence or absence of MDA1 and TDA1.

Figure 6 .

 6 Figure 6. MDA1 and TDA1 are not associated with the atpA mRNAs engaged in translation, which are destabilized. A) Solubilized whole-cell extracts (T) from TDA1-HA and MDA1-Fl complemented strains collected in presence of chloramphenicol (CAP, 100 µg ml -1 ) were loaded on sucrose gradients. After ultracentrifugation, ten fractions were collected and used for both RNA and protein extraction. The distribution of atpA along the gradient was analysed by RNA blotting (ribosomal RNAs were detected by methylene blue staining of the filters). The distributions of TDA1, MDA1 and of ribosomal protein Rps12 were analysed by immunoblotting. Sucrose concentrations are indicated at the bottom. B) RNA co-immuno-precipitated with TDA1-HA and MDA1-Fl in untreated and CAP-pretreated mtMT samples analysed by dot-blot with 16S rRNA and petD (control) probes. C) MDA1, TDA1 and ribosomal protein Rps12 immunodetection in the input (E), supernatant (S) and pellet (P) fractions from the CAP-treated samples shown in B.

Figure 7 :Figure 1 .

 71 Figure 7: Working model for the post-transcriptional control of the atpA gene expression.De novo transcribed atpA mRNA molecules (represented in the monocistronic form for sake of simplicity), possessing a primary 5'end (in blue), are stabilized by the binding of MDA1 (M) to a region located in the middle of the 5'UTR. The binding of MDA1 is likely cooperative

Figure 2 .Figure 3 .Figure 4 .Figure 5 .

 2345 Figure 2. Both MDA1 and TDA1 physically interact with the atpA 5'UTR. A) Schematic representation of the 5'dAf chimera, used to replace the petA endogenous gene in the mda1-1 {5'dAF} and MDA1-Fl {5'dAF} strains. B) Transcript accumulation detected using probes specific for the atpA 5'UTR (shown in panel D), petA coding sequence and Cβlp2 (nuclear control) in mda1-1 and MDA1-Fl recipient strains and in the respective transformants containing the 5'dAf chimeric construct (indicated by the stars).C) RNA immuno-precipitation with anti-Flag magnetic beads identifying RNAs associated with TDA1-Fl and MDA1-Fl in the strains respectively expressing them (wild-type used as control strain). Dot blot of input (E) and beads pellet (P) RNA samples for each strain was hybridised against atpA-5' and petD (control) probes. D) Schematic representation of the atpA gene model with a zoom on the 5'UTR showing the positions of the probes (black lines) used in dot blot experiments. The atpA transcription start site (TSS) and the mature 5'end are indicated. The black rectangle symbolises a sRNA mapping at the 5'end of the mature transcript (see below). E) Immuno-precipitated RNA samples from panel C were probed with the partially overlapping probes 5'atpA 1 to 5 with the control probe atpA-3'.

Figure 6 .

 6 Figure 6. MDA1 and TDA1 are not associated with the atpA mRNAs engaged in translation, which are destabilized. A) Solubilized whole-cell extracts (T) from TDA1-HA and MDA1-Fl complemented strains collected in presence of CAP (100 µg ml -1 ) were loaded on sucrose gradients. After ultracentrifugation, ten fractions were collected and used for both RNA and protein extraction. The distribution of atpA along the gradient was analysed by RNA blotting (ribosomal RNAs were detected by methylene blue staining of the filters). The distributions of TDA1, MDA1 and of ribosomal protein Rps12 were analysed by immunoblotting. Sucrose concentrations are indicated at the bottom. B) RNA co-immuno-precipitated with TDA1-HA and MDA1-Fl in untreated and CAP-pre-treated mtMT samples by dot-blot with 16S rRNA and petD (control) probes. C) MDA1, TDA1 and ribosomal protein Rps12 immunodetection in the input (E), supernatant (S) and pellet (P) fractions from the CAP-treated samples shown in B.D)Wild-type cells were treated with rifampicin (Rif, 350 µg ml -1 ), lincomycin (Lin, 500 µg ml -1 ) or both (Rif+Lin). RNA was extracted just before or 3 and 6 hr after inhibitors addition. Accumulation of atpA and Cβlp2 (nuclear control) were analysed by RNA blot, ribosomal RNAs were detected by methylene blue staining of the filters. For each condition, the percentage of atpA mRNA amounts relative to time 0 is shown on the right (n=3).

  ---------------------------------------------LTTLIKQCNTWQELAGFLARHEWQLNRIHASAL Csp 16 HSQPQRQPPAWRDHEPSRSSGGCAD-------------ADG-------LGVGGSRGGGGYA-----LHPDAPSPRRGHHS-APSWAVDHASSAS-----ARPS Cre149 VTHMAQLHASSSSGSGRG-------DGLVRRR----LE-----LSRPPQ----GHHI-ALGGRDSYNSSKP-----GKPA Yun 80 VTHMAQLHTSSSS-----SSNSGPS-DVWRGGVGRDRGIYDHDHHDAAP----GHVLNSLGRSDAASTTT-----TQSPP consensus 161 vTHmAQLh ssss s r

AUTHOR FOR CORRESPONDENCE: Yves Choquet (choquet@ibpc.fr) Phone: +33 1 5841 5075 Fax +33 1 5841 50200

ACKNOWLEDGEMENTS

We thank Dr Rachel Dent and Dr Silvia Raimundo for their kind gifts of the strain CAL018.01.04 (mda1-1) and of the antibody against Rps12, respectively, Nicolas Tourasse for setting up the pipeline used to identify the mda1-1 mutation and all members of UMR7141 for stimulating discussions.

This work was supported by Unité Mixte de Recherche 7141, CNRS/Sorbonne Université, by the Agence Nationale de la Recherche (ChloroRNP: ANR-13-BSV7-0001-01), and by the "Initiative d'Excellence" program (Grant "DYNAMO," ANR-11-LABX-0011-01).

No conflict of interest declared.

FUNDING

This work was supported by Unité Mixte de Recherche 7141, CNRS/Sorbonne Université, by the Agence Nationale de la Recherche (ChloroRNP: ANR-13-BSV7-0001-01), and by the "Initiative d'Excellence" program (Grant "DYNAMO," ANR-11-LABX-0011-01).

No conflict of interest declared.

Accession numbers: Sequence data from this article can be found in the UniProtKB/Swiss-Prot or GenBank databases under the following accession numbers: cytochrome f: CAA51422.1; RbcL: J01399.1; Rps12: AAC16329 ; AtpA: P26526 ; cblp2: X53574 ; petA: X72919 ; TDA1 : CCA62914/FR850045 ; MDA1: Cre16.g680850.t1.1 (https://phytozome.jgi.doe.gov/pz/portal.html#!gene?search=1&detail=1&method=4614&sear chText=transcriptid:30777176

with that of another factor (question mark) that binds the 5'end of the transcript, protecting it from exonucleases and defining the maturation site. MDA1 can bind atpA in absence of TDA1 and its level determines the size of the stable atpA mRNA pool. TDA1 (T) interacts with the 5'UTR of a fraction of this pool in close proximity with MDA1. The binding of TDA1 activates the translation of the atpA mRNAs. When translation is initiated, MDA1 and TDA1 dissociate from the 5'UTR of ribosome-bound atpA mRNAs, leaving them unprotected. After or during translation, the mRNAs are degraded by nuclease and are not re-bound by MDA1, which is recycled to bind newly transcribed atpA mRNAs; TDA1 is probably also recycled. ST1: Oligonucleotides used in this study. ST2: Mapping 5' and 3'ends of the monocistronic atpA mRNA in presence or absence of MDA1 and TDA1. ST3: Mapping 5'ends of the di-and tri-cistronic atpA mRNAs in presence or absence of MDA1.

Suppl. Table

Suppl. Table

Suppl. Table

SUPPLEMENTAL DATA:

Suppl. Figure S1: Annotated sequence of the MDA1 protein. A) Amino acid sequence of MDA1. The predicted chloroplast targeting peptide is written in red, OPR repeats are alternatively boxed or grey-highlighted. Intron locations (downward triangles) are also shown, while the site of insertion of the 3xFlag tag is highlighted in blue and boxed. The two lines below the sequence respectively depict the predicted secondary structure of the protein (H: α-helical region written in red and highlighted in yellow; E: extended strand; T: turn confirmation, C: the rest) and the predicted disorder (O: ordered; D: disordered). MDA1 orthologues were retrieved from the GenBank database by BLAST searches, aligned with the MUSCLE software using default option followed by manual editing to improve the alignment. OPR repeats shared by the different proteins are shown on top of sequences. Additional species-specific OPR repeats are highlighted in grey. Residues conserved in more than half of the sequences are written in red, while conservative substitutions are written in blue. The residue after which the 3xtag is inserted is written in green, boxed and highlighted in blue. De novo transcribed atpA mRNA molecules (represented in the monocistronic form for sake of simplicity), possessing a primary 5'end (in blue), are stabilized by the binding of MDA1 (M) to a region located in the middle of the 5' UTR. The binding of MDA1 is likely cooperative with that of another factor (question mark) that binds the 5'end of the transcript, protecting it from exonucleases and defining the maturation site. MDA1 can bind atpA in absence of TDA1 and its level determines the size of the stable atpA mRNA pool. TDA1 (T) interacts with the 5'UTR of a fraction of this pool in close proximity with MDA1. The binding of TDA1 activates the translation of the atpA mRNAs. When translation is initiated, MDA1 and TDA1 dissociate from the 5'UTR of ribosome-bound atpA mRNAs, leaving them unprotected. Upon translation initiation, the mRNAs might be spatially re-located inside the chloroplast. After or during translation, the mRNAs are degraded by nuclease and are not re-bound by MDA1, which is recycled to bind newly transcribed atpA mRNAs; TDA1 is probably also recycled.

Abbreviations of species names