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Singlet oxygen (¹O₂) is a prime cause of photo-damage of the photosynthetic apparatus. The chlorophyll molecules in the photosystem II reaction center and in the light-harvesting antenna complex are major sources of ¹O₂ generation. It has been thought that the generation of ¹O₂ mainly takes place in the appressed regions of the thylakoid membranes, namely, the grana core, where most of the active photosystem II complexes are localized. Apart from being a toxic molecule, new evidence suggests that ¹O₂ significantly contributes to chloroplast-to-nucleus retrograde signalling that primes acclimation and cell death responses. Interestingly, recent studies reveal that chloroplasts operate two distinct ¹O₂-triggered retrograde signalling pathways in which β-carotene and a nuclear-encoded chloroplast protein EXECUTER1 play essential roles as signalling mediators. The coexistence of these mediators raises several questions: their crosstalk, source(s) of ¹O₂, downstream signalling components, and the perception and reaction mechanism of these mediators towards ¹O₂. In this review, we mainly discuss the molecular genetic basis of the mode of action of these two putative ¹O₂ sensors and their corresponding retrograde signalling pathways. In addition, we also propose the possible existence of an alternative source of ¹O₂, which is spatially and functionally separated from the grana core.

We have identified hpm91, a Chlamydomonas mutant lacking Proton Gradient Regulation5 (PGR5) capable of producing hydrogen (H2 ) for 25 days with more than 30-fold yield increase compared to wild type. Thus, hpm91 displays a higher capacity of H2 production than a previously characterized pgr5 mutant. Physiological and biochemical characterization of hpm91 reveal that the prolonged H2 production is due to enhanced stability of PSII, which correlates with increased reactive oxygen species (ROS) scavenging capacity during sulfur deprivation. This anti-ROS response appears to protect the photosynthetic electron transport chain from photo-oxidative damage and thereby ensures electron supply to the hydrogenase.

The development of a repressible chloroplast gene expression system in Chlamydomonas reinhardtii has opened the door for studying the role of essential chloroplast genes. This approach has been used to analyze three chloroplast genes of this sort coding for the α subunit of RNA polymerase (rpoA), a ribosomal protein (rps12) and the catalytic subunit of the ATP-dependent ClpP protease (clpP1). Depletion of the three corresponding proteins leads to growth arrest and cell death. Shutdown of chloroplast transcription and translation increases the abundance of a set of plastid transcripts that includes mainly those involved in transcription, translation and proteolysis and reveals multiple regulatory feedback loops in the chloroplast gene circuitry. Depletion of ClpP profoundly affects plastid protein homeostasis and elicits an autophagy-like response with extensive cytoplasmic vacuolization of cells. It also triggers changes in chloroplast and nuclear gene expression resulting in increased abundance of chaperones, proteases, ubiquitin-related proteins and proteins involved in lipid trafficking and thylakoid biogenesis. These features are hallmarks of an unfolded protein response in the chloroplast and raise new questions on plastid protein homeostasis and plastid signaling. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Chloroplast transformation in microalgae offers great promise for the production of proteins of pharmaceutical interest or for the development of novel biofuels. For many applications, high level expression of transgenes is desirable. We have transformed the chloroplast of Chlamydomonas reinhardtii with two genes, acrV and vapA, which encode antigens from the fish pathogen Aeromonas salmonicida. The promoters and 5' untranslated regions of four chloroplast genes were compared for their ability to drive expression of the bacterial genes. The highest levels of expression were obtained when they were placed under the control of the cis-acting elements from the psaA-exon1 gene. The expression of these chimeric genes was further increased when a nuclear mutation that affects a factor involved in psaA splicing was introduced in the genetic background of the chloroplast transformants. Accumulation of both the chimeric mRNAs and the recombinant proteins was dramatically increased, indicating that negative feedback loops limit the expression of chloroplast transgenes. Our results demonstrate the potential of manipulating anterograde signalling to alter negative regulatory feedback loops in the chloroplast and improve transgene expression.

Genetic analysis has revealed that the accumulation of several chloroplast mRNAs of the green alga Chlamydomonas reinhardtii requires specific nucleus-encoded functions. To gain insight into this process, we have cloned the nuclear gene encoding the Mbb1 factor by genomic rescue of a mutant specifically deficient in the accumulation of the mRNAs of the psbB/psbT/psbH chloroplast transcription unit. Mbb1 is a soluble protein in the stromal phase of the chloroplast. It consists of 662 amino acids with a putative chloroplast-transit peptide at its N-terminal end. A striking feature is the presence of 10 tandemly arranged tetratricopeptide-like repeats that account for half of the protein sequence and are thought to be involved in protein–protein interactions. The Mbb1 protein seems to have a homologue in higher plants and is part of a 300-kDa complex that is associated with RNA. This complex is most likely involved in psbB mRNA processing, stability, and/or translation.

Genetic analysis has revealed that the accumulation of several chloroplast mRNAs of the green alga Chlamydomonas reinhardtii requires specific nucleus-encoded functions. To gain insight into this process, we have cloned the nuclear gene encoding the Mbb1 factor by genomic rescue of a mutant specifically deficient in the accumulation of the mRNAs of the psbB/psbT/psbH chloroplast transcription unit. Mbb1 is a soluble protein in the stromal phase of the chloroplast. It consists of 662 amino acids with a putative chloroplast-transit peptide at its N-terminal end. A striking feature is the presence of 10 tandemly arranged tetratricopeptide-like repeats that account for half of the protein sequence and are thought to be involved in protein–protein interactions. The Mbb1 protein seems to have a homologue in higher plants and is part of a 300-kDa complex that is associated with RNA. This complex is most likely involved in psbB mRNA processing, stability, and/or translation.

Strains of Chlamydomonas reinhardtiilacking the PsaF gene or containing the mutation K23Q within the N-terminal part of PsaF are sensitive to high light (>400 μE m⁻² s⁻¹) under aerobic conditions.In vitro experiments indicate that the sensitivity to high light of the isolated photosystem I (PSI) complex from wild type and from PsaF mutants is similar. In vivo measurements of photochemical quenching and oxygen evolution show that impairment of the donor side of PSI in the PsaF mutants leads to a diminished linear electron transfer and/or a decrease of photosystem II (PSII) activity in high light. Thermoluminescence measurements indicate that the PSII reaction center is directly affected under photo-oxidative stress when the rate of electron transfer becomes limiting in the PsaF-deficient strain and in the PsaF mutant K23Q. We have isolated a high light-resistant PsaF-deficient suppressor strain that has a high chlorophyll a/b ratio and is affected in the assembly of light-harvesting complex. These results indicate that under high light a functionally intact donor side of PSI is essential for protection of C. reinhardtii against photo-oxidative damage when the photosystems are properly connected to their light-harvesting antennae.

The two [4Fe-4S] clusters FAand FB are the terminal electron acceptors of photosystem I (PSI) that are bound by the stromal subunit PsaC. Soluble ferredoxin (Fd) binds to PSI via electrostatic interactions and is reduced by the outermost iron-sulfur cluster of PsaC. We have generated six site-directed mutants of the green alga Chlamydomonas reinhardtii in which residues located close to the iron-sulfur clusters of PsaC are changed. The acidic residues Asp9 and Glu46, which are located one residue upstream of the first cysteine liganding cluster FB and FA, respectively, were changed to a neutral or a basic amino acid. Although Fd reduction is not affected by the E46Q and E46K mutations, a slight increase of Fd affinity (from 1.3- to 2-fold) was observed by flash absorption spectroscopy for the D9N and D9K mutant PSI complexes. In the FA2 triple mutant (V49I/K52T/R53Q), modification of residues located next to the FA cluster leads to partial destabilization of the PSI complex. The electron paramagnetic resonance properties of cluster FA are affected, and a 3-fold decrease of Fd affinity is observed. The introduction of positively charged residues close to the FB cluster in the FB1 triple mutant (I12V/T15K/Q16R) results in a 60-fold increase of Fd affinity as measured by flash absorption spectroscopy and a larger amount of PsaC-Fd cross-linking product. The first-order kinetics are similar to wild type kinetics (two phases witht Formula of

Plants have evolved a plethora of mechanisms to respond to the environment and modulate growth and reproduction. Abiotic signals provided by light, temperature and the availability of water, oxygen and nutrients are controlling factors. Many species possess strategies to endure abiotic assaults, and to respond to challenges from bacterial, fungal and viral pathogens. Although these responses are frequently regulated at the transcriptional level, there is extraordinary diversity in the post-transcriptional mechanisms that promote developmental plasticity and adaptation. The phenomenon of silencing of heterologous genes also involves transcriptional and post-transcriptional mechanisms. Current research links gene silencing to ordinary post-transcriptional mechanisms as well as defense of viral RNAs. An EMBO Workshop in Leysin, Switzerland, explored the mechanisms, implications and connections of post-transcriptional processes in plants. Several non-plant scientists also participated and presented an overview of post-transcriptional processes in mammals and yeast.

We have changed the potential phosphorylation site, a threonine residue at position 2 of the D2 polypeptide of the photosystem II complex of Chlamydomonas reinhardtii, to alanine, valine, aspartate, proline, glycine, or glutamate. Mutants with neutral amino acid changes did not display any phenotype with regard to photoautotrophic growth, light sensitivity, fluorescence transients, or photoinhibition. Pulse labeling of these mutants with 32P indicated that a phosphorylated protein of the same size as D2 is absent in these mutants, suggesting that threonine-2 is indeed the unique phosphorylation site of D2. In contrast, mutants in which threonine-2 has been replaced with acidic residues are deficient in photosystem II. Use of chimeric genes containing the psbD5′-untranslated region revealed that the initiation of translation was not affected in these mutants, but the mutations interfered with a later step of D2 synthesis and accumulation.

The major lesson from the 8th International Conference on the Cell and Molecular Biology of Chlamydomonas held at Lake Tahoe June 2-7, 1998, is that chis unicellular alga is emerging more and more as a powerful model system for studying a wide range of important biological processes. These processes include photosynthesis, phototaxis, flagellar function and assembly, gametogenesis, mating, cell wall synthesis, and chloroplast biogenesis. Among its many attractive features, Chlamydomonas is amenable to extensive genetic analysis, and many mutants affected in photosynthetic activity and flagellar movement have been isolated. Both flagellar and photosynthetic functions are dispensable in this alga under appropriate growth conditions, which greatly facilitate the use of genetic approaches. In addition, transformation systems for all three of the Chlamydomonas genomes - nuclear, chloroplast, and mitochondrial - have been developed; Chlamydomonas is the only organism for which this capacity presently exists.

The small chloroplast open reading frame ORF43 (ycf7) of the green unicellular alga Chlamydomonas reinhardtii is cotranscribed with the psaC gene and ORF58. While ORF58 has been found only in the chloroplast genome of C.reinhardtii, ycf7 has been conserved in land plants and its sequence suggests that its product is a hydrophobic protein with a single transmembrane alpha helix. We have disrupted ORF58 and ycf7 with the aadA expression cassette by particle-gun mediated chloroplast transformation. While the ORF58::aadA transformants are indistinguishable from wild type, photoautotrophic growth of the ycf7::aadA transformants is considerably impaired. In these mutant cells, the amount of cytochrome b₆f complex is reduced to 25-50% of wild-type level in mid-exponential phase, and the rate of transmembrane electron transfer per b₆f complex measured in vivo under saturating light is three to four times slower than in wild type. Under subsaturating light conditions, the rate of the electron transfer reactions within the b₆f complex is reduced more strongly in the mutant than in the wild type by the proton electrochemical gradient. The ycf7 product (Ycf7) is absent in mutants deficient in cytochrome b₆f complex and present in highly purified b₆f complex from the wild-type strain. Ycf7-less complexes appear more fragile than wild-type complexes and selectively lose the Rieske iron-sulfur protein during purification. These observations indicate that Ycf7 is an authentic subunit of the cytochrome b₆f complex, which is required for its stability, accumulation and optimal efficiency. We therefore propose to rename the ycf7 gene petL.

A genetic analysis of Chlamydomonas reinhardtii has revealed a large number of genes that are required for the expression of the chloroplast genome. These genes are surprisingly numerous and most of them are located in the nucleus. They are often specific for individual chloroplast genes and are required in post-transcriptional steps of gene expression. A demonstrative example is provided by the psaA gene, which is expressed from scattered exons by a mechanism that calls for two steps of RNA splicing in trans. This complex process requires the activity of more than 14 nuclear genes as well as one chloroplast locus. Our current understanding of this case of tight coupling between the nuclear and chloroplast genomes is reviewed here.

In this study we have isolated the chlorophyll a/b-binding proteins from a photosystem I preparation of the green alga Chlamydomonas reinhardtii and characterized them by N-terminal sequencing, fluorescence, and absorption spectroscopy and by immunochemical means. The results indicate that in this organism, the light-harvesting complex of photosystem I (LHCI) is composed of at least seven distinct polypeptides of which a minimum number of three are shown to bind chlorophyll a and b. Both sequence homology and immunological cross-reactivity with other chlorophyll-binding proteins suggest that all of the LHCI polypeptides bind pigments. Fractionation of LHCI by mildly denaturing methods showed that, in contrast to higher plants, the long wavelength fluorescence emission typical of LHCI (705 nm in C. reinhardtii) cannot be correlated with the presence of specific polypeptides, but rather with changes in the aggregation state of the LHCI components. Reconstitution of both high aggregation state and long wavelength fluorescence emission from components that do not show these characteristics confirm this hypothesis.

All chloroplast 23S ribosomal RNA genes of the unicellular alga Chlamydomonas reinhardtii contain an 888 bp group I intron with an internal open reading frame (ORF). A precursor RNA encompassing the intron with its 5' and 3' flanking sequences was shown to self-splice both during in vitro transcription and upon incubation of the isolated pre-RNA under self-splicing conditions. Expression of the internal ORF in Escherichia coli in the presence of a plasmid containing a cDNA corresponding to the intronless form of the 23S rRNA gene resulted in specific cleavage of the cDNA at or close to the exon junction sequence. To test whether this ORF-encoded double-strand DNA endonuclease is involved in intron mobility in vivo, the same ribosomal cDNA was stably integrated into the C. reinhardtii chloroplast genome using particle gun mediated transformation. All the transformants with the cDNA integrated at the expected site in the chloroplast genome had the intron precisely inserted at the artificial exon junction site. These experiments demonstrate that the chloroplast ribosomal intron of C. reinhardtii behaves as a ribozyme in vitro and also as a mobile genetic element in vivo provided a target site is present.

The chloroplast ribosomal unit of Chlamydomonas reinhardii displays two features which are not shared by other chloroplast ribosomal units. These include the presence of an intron in the 23 S ribosomal RNA gene and of two small genes coding for 3 S and 7 S rRNA in the spacer between the 16 S and 23 S rRNA genes (Rochaix & Malnoë, 1978). Sequencing of the 7 S and 3 S rRNAs as well as their genes and neighbouring regions has shown that: (1) the 7 S and 3 S rRNA genes are 282 and 47 base-pairs long, respectively, and are separated by a 23 base-pair A + T-rich spacer. (2) A sequence microheterogeneity exists within the 3 S RNA genes. (3) The sequences of the 7 S and 3 S rRNAs are homologous to the 5′ termini of prokaryotic and other chloroplast 23 S rRNAs, indicating that the C. reinhardii counterparts of 23 S rRNA have a composite structure. (4) The sequences of the 7 S and 3 S rRNAs are related to that of cytoplasmic 5.8 S rRNA, suggesting that these RNAs may perform similar functions in the ribosome. (5) Partial nucleotide sequence complementarity is observed between the 5′ ends of the 7 S and 3 S RNAs on one hand and the 23 S rRNA sequences which flank the ribosomal intron on the other. These data are compatible with the idea that these small rRNAs may play a role in the processing of the 23 S rRNA precursor.

The green unicellular alga Chlamydomonas reinhardtii offers interesting possibilities for studying the function, synthesis and assembly of the photosynthetic apparatus. Chloroplast and nuclear genes coding for subunits of photosystem II, a major multimolecular membrane-associated photosynthetic complex, have been isolated and characterized. Examination of these genes and of their expression in wild-type cells and in chloroplast and nuclear photosystem II mutants has revealed a complex regulatory circuitry between chloroplast and nucleocytoplasmic compartments. One of the photosystem II subunits which is involved in herbicide binding has been found to have distinct single amino acid substitutions in several herbicide resistant mutants. Some of these mutations do not appreciably affect photosynthetic yield and may therefore be of agronomic interest.

Two chloroplast-encoded proteins of photosystem II (PSII) have recently been the object of considerable interest. One is a rapidly synthesized 32-kD protein (Bottomley et al. 1974; Eaglesham and Ellis 1974; Hottman-Falk et al. 1982), also identified as a herbicide-binding protein (Pfister et al. 1981) and designated the QB protein (Hirschberg et al. 1984) because of its proposed role in a protein-quinone complex, which serves as the second stable electron acceptor of PSII (Arntzen et al. 1982). The other is a protein of similar size, called D2, that has several regions of homology with the QB protein (Rochaix et al. 1984). In this paper, we briefly review what is known about the primary structure of these two proteins and their genes and the mutations in the gene for the QB protein that affect herbicide resistance and cross-resistance.

A physical map of the chloroplast DNA of Chlamydomonas reinhardii has been established which includes EcoRI, BamHI and BglII restriction fragments. The map is circular with a size of 190 kb. The two ribosanal RNA coding regions are contained within two 19.5 kb inverted repeats whose ends are 72 and 78 kb apart on the genome. DNA from a cloned 1.5 kb fragment, mapping adjacent to the ribosanal unit, can hybridize to most of the BamHI chloroplast restriction fragments, indicating the presence of repetitive sequences in the chloroplast genane. The 4S RNA coding regions are interspersed throughout the genome although not randomly. The gene for the large RudP-carboxylase subunit is located on the 5.6 kb EcoRI fragment Rl5 which is not adjacent to the restriction fragments carrying the ribosomal RNA genes. Preliminary evidence suggests that regions of the chloroplast DNA, which include also non ribosomal DNA fragments are preferentially transcribed and/or that the corresponding transcripts are stable.

Studies on the structure, function and regulation of genes coding for chloroplast proteins are important for understanding the biosynthesis of the photosynthetic apparatus and the integration of chloroplasts within plant cells. Chlamydomonas reinhardii is particularly well suited for solving these problems because this green unicellular alga can be manipulated with ease both at the biochemical and genetic level. Several genes have been identified on the physical map of the chloroplast genome. They include genes coding for ribosomal RNA, tRNA and several proteins including the large subunit of ribulose 1,5 bisphosphate carboxylase (RubisCo) and several thylakoid polypeptides. The nuclear gene for the small subunit of RubisCo has also been cloned. Because chloroplast DNA recombination occurs in C. reinhardii, a rare property among plants, chloroplast genes can be analyzed by genetic means. Numerous chloroplast photosynthetic mutations have been isolated and several of them have been shown to be part of a single linkage group (Gillham, 1978). We have reached the stage where the genetic and biochemical approaches can be coupled efficiently in C. reinhardii; in particular, it has been possible to correlate the physical and genetic chloroplast DNA maps at a few sites. A nuclear transformation system has been developed for C. reinhardii by using a cell wall, deficient arginine auxotroph which can be complemented with a plasmid containing the yeast ARG4 locus. Transformation vectors have been constructed by inserting random nuclear and chloroplast DNA fragments into a plasmid containing the yeast ARG4 locus and by testing the recombinant plasmids for their ability to promote autonomous replication in yeast (ARS sites) and C. reinhardii (ARC sites). Several plasmids have been recovered that act as shuttle vectors between E.coli, C. reinhardii and yeast. Four ARS sites and four ARC sites have been mapped on the chloroplast genome of C. reinhardii. One plasmid replicates both in C. reinhardii and yeast. Because C. reinhardii cells contain a single large chloroplast they offer interesting possibilities for attempts of chloroplast transformation by microinjection. Since appropriate selective markers and transformation vectors are available, this approach can now be explored.

We have isolated a 5.7-kbp dispersed moderately repeated DNA sequence (TOC1) from the mutant OEE1 gene of the Chlamydomonas reinhardtii strain FUD44. The copy number (2 to over 30) and genomic locations of TOC1 elements vary widely in different C. reinhardtii strains. Our standard laboratory photosynthetic strain exhibits a high degree of TOC1 instability during short periods of mitotic growth. TOC1 appears to be a retrotransposon: it contains LTRs and an oligonucleotide stretch that corresponds to a conserved pentapeptide of reverse transcriptase. TOC1 is an unusual retrotransposon: it is not flanked by a target site duplication in the OEE1 gene, the left end of TOC1 only contains a fraction of the LTR the remainder of which is present at its right end and TOC1 does not start with a 5' TG and end with a 3' CA. In most cases, TOC1 excision leaves behind a complete solo LTR sequence (577 bp) and in one case a deleted solo LTR sequence (191 bp). Solo LTR sequences form a separate family of repeated sequences in most of the strains tested.

Chloroplast biogenesis requires coordination of gene expression between the organellar and nuclear genomes. Sequencing of chloroplast genomes from a large collection of green organisms has revealed the existence of approximately one hundred genes in this organelle present in multiple copies. In the green alga Chlamydomonas reinhardtii, many of these genes have been characterized thanks to its dispensable photo-autotrophic growth in acetate-containing medium and the successful development of plastid transformation methods based on its highly efficient chloroplast homologous recombination system. However, there are still a number of genes for which it has never been possible to disrupt all of their copies. These genes are believed to encode essential factors for growth and survival of the cell. The aim of this thesis was to establish an inducible/repressible expression system for chloroplast genes in Chlamydomonas reinhardtii and to explore its potential for elucidating the role of essential chloroplast genes.

Photosynthesis provides the primary source of energy for life by producing sugar compounds. However, photosynthetic organisms have to cope with changes in light quality and intensity to optimize their photosynthetic yield. To achieve this, they have evolved different mechanisms. When the incident light energy exceeds the absorption capacity of the photosynthetic apparatus, green photosynthetic organisms are able to dissipate the excess energy thereby preventing the production of compounds harmful for the cell. On the other hand, when incident light becomes limiting, state transitions allow optimizing the photosynthetic performance by a rearrangement of the light-harvesting antenna. This conserved process occurs through the reversible phosphorylation of light-harvesting proteins mediated by chloroplast kinases and phosphatases. During my thesis I studied the thylakoid-associated kinases Stt7/STN7 and Stl1/STN8 involved in light adaptation in Chlamydomonas reinhardtii and Arabidopsis thaliana. The biochemical characterization in alga and the physiological role of these kinases in plants have been investigated.

Le sujet de cette thèse est la régulation de la synthèse de la chlorophylle dans l'algue unicellulaire Chlamydomonas reinhardtii. En particulier, je me suis concentré sur les protéines GUN4 et FLP. GUN4 stimule l'activité de la MG-chelatase, la première enzyme impliquée dans la synthèse de la chlorophylle. FLP est un régulateur négatif de la synthèse de la chlorophylle et l'expression de cette protéine est régulée par un signal chloroplastique dépendant de l'accumulation des précurseurs de la chlorophylle. Nous avons montré que l'absence de GUN4 limite l'accumulation de la chlorophylle dans la cellule et qu'un niveau réduit de FLP provoque l'accumulation de niveaux élevés de différents précurseurs de la chlorophylle. Nous avons également identifié deux régulateurs putatifs de l'épissage alternatif de FLP.

La synthèse des protéines est un processus complexe impliquant de nombreux mécanismes biologiques. En particulier, la régulation de la traduction des ARNm permet aux cellules de répondre rapidement à un stimulus extracellulaire ou à un changement dans la composition de leur environnement. Ces signaux peuvent être transmis par l'intermédiaire des récepteurs tyrosine kinases, d'intégrines, ou indirectement, via l'entrée de calcium ou de la disponibilité d'acides aminés. Il est maintenant bien connu que la plupart des voies de signalisation sont impliquées dans le contrôle de la traduction. Elles peuvent induire la phosphorylation de facteurs comme eIF4E, 4EBP1, eIF4B et la protéine ribosomale S6 et ainsi avoir un impact sur l'initiation de la traduction. Il est maintenant largement admis que la région non traduite avant (5'UTR) et après (3'UTR) la région codante, sont impliquées dans la modulation de l'expression des protéines. En effet, des éléments structurels, des sites de liaison pour des protéines ou des ARNs, ainsi que des AUGs dans le 5'UTR peuvent influencer la traduction. En outre, d'autres modes d'initiation (par exemple, les IRES) permettent d'assurer la traduction des ARNm lors de contextes cellulaires particuliers. En ce qui concerne ce dernier point, nous avons développé une technique permettant de suivre le re-recrutement polysomal suite à une courte exposition à une drogue...

All eukaryotic organisms face the challenge to coordinate the activity of different genomes and to ensure the stochiometric assembly of multiprotein complexes whose subunits have their genetic orginin in the nucleus, the mitochondria or, in the case of plants and algae, in the chloroplasts. This thesis work bears on one aspect of the coordination between genomes, namely how the nucleus influences gene expression in the chloroplast in the model organism Chlamydomonas reinhardtti. Many nucleus-encoded proteins have been identified with crucial functions in chloroplast biogenesis and I have concentrated here on two proteins, Tab2 and Mab1, which turn out to be both required for the synthesis of PsaB and therefore for the generation of the Photosystem I.

Ce travail de thèse concerne l'étude de l'expression des gènes chloroplastiques "psbB/psbT/psbH" chez "Chlamydomonas reinhardtii". Une première série de huits mutations successives dans l'extrémité 5' non traduite de l'ARN "psbB" a permis de caractériser des séquences impliquées dans la maturation, la stabilisation et la traduction de ces transcrits. Pour tester si certaines de ces séquences également présentes sur le 5' de l'ARN "psbH", une deuxième série de mutations a été initiée sur ce gène. Ces travaux ont permis de révéler que l'une des séquences, présente en aval du site de maturation de ces deux ARNs, est requise pour leur accumulation probablement grâce à un facteur commun. Un troisième aspect de ce travail a été l'étude des complexes ribonucléoprotéiques contenant MBB1. Il apparaît que la formation des grands complexes MBB1 dépend de l'ARN "psbB" et que la maturation des trasncrits "psbB"/psbT/psbH" dépend de l'état d'assemblage du photosystème II.