Despite recent elucidation of the three-dimensional structure of major photosynthetic complexes, our understanding of light energy conversion in plant chloroplasts and microalgae under physiological conditions requires exploring the dynamics of photosynthesis. The photosynthetic apparatus is a flexible molecular machine that can acclimate to metabolic and light fluctuations in a matter of seconds and minutes. On a longer time scale, changes in environmental cues trigger acclimation responses that elicit intracellular signaling between the nucleo-cytosol and chloroplast resulting in modification of the biogenesis of the photosynthetic machinery. Here we attempt to integrate well-established knowledge on the functional flexibility of light-harvesting and electron transfer processes, which has greatly benefited from genetic approaches, with data derived from the wealth of recent transcriptomic and proteomic studies of acclimation responses in photosynthetic eukaroytes.

The chloroplast-based photosynthetic apparatus of plants and algae associates various redox cofactors and pigments with~70 polypeptides to form ®ve major transmembrane protein complexes. Among these are two photosystems that have distinct light absorption properties but work in series to produce reducing equivalents aimed at the ®xation of atmospheric carbon. A short term chromatic adaptation known as`State transitions' was discovered thirty years ago that allows photosynthetic organisms to adapt to changes in light quality and intensity which would otherwise compromise the ef®ciency of photosynthetic energy conversion. A two-decade research effort has ®nally unraveled the major aspects of the molecular mechanism responsible for State transitions, and their physiological signi®cance has been revisited. This review describes how aÐstill elusiveÐ regulatory kinase senses the physiological state of the photosynthetic cell and triggers an extensive supramolecular reorganization of the photosynthetic membranes. The resulting picture of the photosynthetic apparatus is that of a highly¯exible energy convertor that adapts to the ever-changing intracellular demand for ATP and/or reducing power.

Absorption of light in excess of the capacity for photosynthetic electron transport is damaging to photosynthetic organisms. Several mechanisms exist to avoid photodamage, which are collectively referred to as nonphotochemical quenching. This term comprises at least two major processes. State transitions (qT) represent changes in the relative antenna sizes of photosystems II and I. High energy quenching (qE) is the increased thermal dissipation of light energy triggered by lumen acidification. To investigate the respective roles of qE and qT in photoprotection, a mutant (npq4 stt7-9) was generated in Chlamydomonas reinhardtii by crossing the state transition-deficient mutant (stt7-9) with a strain having a largely reduced qE capacity (npq4). The comparative phenotypic analysis of the wild type, single mutants, and double mutants reveals that both state transitions and qE are induced by high light. Moreover, the double mutant exhibits an increased photosensitivity with respect to the single mutants and the wild type. Therefore, we suggest that besides qE, state transitions also play a photoprotective role during high light acclimation of the cells, most likely by decreasing hydrogen peroxide production. These results are discussed in terms of the relative photoprotective benefit related to thermal dissipation of excess light and/ or to the physical displacement of antennas from photosystem II.

Photosynthesis is the biological process that feeds the biosphere with reduced carbon. The assimilation of CO2 requires the fine tuning of two co-existing functional modes: linear electron flow, which provides NADPH and ATP, and cyclic electron flow, which only sustains ATP synthesis. Although the importance of this fine tuning is appreciated, its mechanism remains equivocal. Here we show that cyclic electron flow as well as formation of supercomplexes, thought to contribute to the enhancement of cyclic electron flow, are promoted in reducing conditions with no correlation with the reorganization of the thylakoid membranes associated with the migration of antenna proteins towards Photosystems I or II, a process known as state transition. We show that cyclic electron flow is tuned by the redox power and this provides a mechanistic model applying to the entire green lineage including the vast majority of the cases in which state transition only involves a moderate fraction of the antenna.

Abstract. We studied the assembly of photosystem II (PSII) in several mutants from Chlamydomonas reinhardtii which were unable to synthesize either one PSII core subunit (P6 [43 kD], D1, or D2) or one oxygen-evolving enhancer (OEE1 or OEE2) subunit. Synthesis of the PSII subunits was analyzed on electrophoretograms of cells pulse labeled with [14C]acetate. Their accumulation in thylakoid membranes was studied on immunoblots, their chlorophyll-binding ability on nondenaturating gels, their assembly by detergent fractionation, their stability by pulse-chase experiments and determination of in vitro protease sensitivity, and their localization by immunocytochemistry.In Chlamydomonas, the PSII core subunits P5 (47 kD), D1, and D2 are synthesized in a concerted manner while P6 synthesis is independent. P5 and P6 accumulate independently of each other in the stacked membranes. They bind chlorophyll soon after, or concomitantly with, their synthesis and independently of the presence of the other PSII subunits. Resistance to degradation increases step by step: beginning with assembly of P5, D1, and D2, then with binding of P6, and, finally, with binding of the OEE subunits on two independent high affinity sites (one for OEE1 and another for OEE2 to which OEE3 binds). In the absence of PSII cores, the OEE subunits accumulate independently in the thylakoid lumen and bind loosely to the membranes; OEE1 was found on stacked membranes, but OEE2 was found on either stacked or unstacked membranes depending on whether or not P6 was synthesized.p HOTOSYSTEM IX (PSII) ~ is a major protein complex of the photosynthetic apparatus in oxygen-evolving species. Light-harvesting chlorophyll-protein complexes (LHCs) transfer excitons to PSII cores where primary photochemistry occurs. PSII complexes (PSII cores with oxygen-evolving enhancer [OEE] subunits) are able to carry out the oxidation of water.The PSII core comprises five main intrinsic chloroplastencoded subunits P5, P6, D1, D2, and cytochrome b559 (59). Their molecular masses vary slightly from one species to another. Two subunits of 4%50 and 43-47 kD, called P5 and P6 in Chlamydomonas reinhardtii or by their molecular mass in higher plants (respectively encoded by psbB and psbC genes), bind most of the PSII core chlorophylls (58) and form the core antenna (9, 44). The chlorophyll-P5 and chlorophyll-P6 complexes-called, respectively, CPIII and CPIV in C. reinhardtii or CP47 and CP43 in higher plantscan be separated by electrophoresis at 4°C (13, 21). D1 and D2 of 32-35 kD (encoded, respectively, by psbA and psbD genes [18,48,64]) cooperate in the binding of the primary 1. Abbreviations used in this paper: LHC, light-harvesting complex; OEE, oxygen-evolving enhancer; PSII, photosystem II; WT, wild type. reactants (44) and show sequence homologies with the subunits L and M of the reaction center from purple bacteria (39, 53). Three extrinsic polypeptides encoded by nuclear genes (12, 60) are involved in oxygen evolution; OEE1 (29-33 kD) stabilizes the association of manganese ions ...

We created a Qo pocket mutant by site-directed mutagenesis of the chloroplast petD gene in Chlamydomonas reinhardtii. We mutated the conserved PEWY sequence in the EF loop of subunit IV into PWYE. The pwye mutant did not grow in phototrophic conditions although it assembled wild-type levels of cytochrome b 6 f complexes. We demonstrated a complete block in electron transfer through the cytochrome b 6 f complex and a loss of plastoquinol binding at Qo. The accumulation of cytochrome b 6 f complexes lacking affinity for plastoquinol enabled us to investigate the role of plastoquinol binding at Qo in the activation of the light-harvesting complex II (LHCII) kinase during state transitions. We detected no fluorescence quenching at room temperature in state II conditions relative to that in state I. The quantum yield spectrum of photosystem I charge separation in the two state conditions displayed a trough in the absorption region of the major chlorophyll a/b proteins, demonstrating that the cells remained locked in state I. 33 P i labeling of the phosphoproteins in vivo demonstrated that the antenna proteins remained poorly phosphorylated in both state conditions. Thus, the absence of state transitions in the pwye mutant demonstrates directly that plastoquinol binding in the Qo pocket is required for LHCII kinase activation.

SummaryWe performed a systematic investigation of the quantitative relationship between genome copy number, transcription, transcript abundance and synthesis of photosynthetic proteins in the chloroplast of the green algae Chlamydomonas reinhardtii grown either in mixotrophic or phototrophic conditions. The chloroplast gene copy number is lower in the latter condition and the half-life and accumulation levels of most chloroplast transcripts are signi®cantly reduced, although the relative rates of protein synthesis remain similar. Our study shows that, in most instances, chloroplast protein synthesis is poorly sensitive to changes in gene copy number or transcript abundance in the chloroplast. Treatment with 5-¯uoro-2¢-deoxyuridine, that inhibits chloroplast DNA replication and decreases extensively the number of copies of the chloroplast genome, had limited effects on the abundance of most chloroplast transcripts and little if any effect on the rates of protein synthesis. When using rifampicin, that selectively inhibits chloroplast transcription, we found no direct correlation between the level of transcripts remaining in the chloroplast and the rates of chloroplast protein synthesis. For two chloroplast genes, a 90% decrease in the amount of transcript did not cause a drop in the rate of synthesis of the corresponding protein product. Overall, our results demonstrate that there is no gene dosage effect in the chloroplast and that transcript abundance is not limiting in the expression of chloroplast-encoded protein.

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