Function-directed mutagenesis of the cytochrome b6f complex in Chlamydomonas reinhardtii: involvement of the cd loop of cytochrome b6 in quinol binding to the Q(o) site

The FUD2 mutant from the green alga Chlamydomonas reinhardtii expresses a cytochrome b6 variant of higher apparent molecular mass [Lemaire et al. (1986) Biochim. Biophys. Acta 851, 239-248]. Here, we show that the mutation corresponds to a 36 base pair duplication in the chloroplast petB gene, which corresponds to a 12 amino acid duplication in the cd loop of cytochrome b6. The resulting protein still binds its heme cofactors and assembles into cytochrome b6f complexes, which accumulate in wild type amounts in exponentially growing cells of FUD2. However, these cytochrome b6f complexes show loosened binding of the Rieske protein and are more prone to degradation in aging cells. Electron transfer through the cytochrome b6f complexes is about 8 times slower in FUD2 than in wild type cells. This is due to a slower oxidation of plastoquinol at the Q(o) site, the folding of which is most likely altered by the duplication. By varying the redox state of the plastoquinone pool in vivo, we show that there is a dramatic decrease in the affinity of the Q(o) site for plastoquinols, which is about 100 times lower in FUD2 than in wild type cells. Our results show that the value of the binding constant of plastoquinol to the Q(o) site (2 x 10(4) M(-1)) derived in [Kramer et al. (1994) Biochim. Biophys. Acta 1184, 251-262] may be extrapolated to in vivo conditions.

Mutations of cytochrome b6 in Chlamydomonas reinhardtii disclose the functional significance for a proline to leucine conversion by petB editing in maize and tobacco

We have introduced a proline codon in place of a leucine codon at position 204 of the petB gene of Chlamydomonas reinhardtii. This gene modification mimics the presence of proline codons at the same position in the petB genes of maize and tobacco, which are subsequently edited to leucine codons at the RNA level. Following transformation, we observed no editing at this position in C. reinhardtii, independent of the type of proline codon we have used: the CCA codon, edited in maize, or a CCT codon. Strains carrying the introduced mutation were non phototrophic and displayed a block in photosynthetic electron transfer, consistent with a lack of cytochrome b6f activity. Thus the presence of a proline residue at position 204 in cytochrome b6 is detrimental to photosynthesis. We show that the mutant phenotype arose from a defective assembly of cytochrome b6f complexes and not from altered electron transfer properties in the assembled protein complex. Biochemical comparison of the proline-containing transformants with a cytochrome b6 mutant deficient in heme-attachment indicates that their primary defect is at the level of assembly of apocytochrome b6 with the bh heme, thereby preventing assembly of the whole cytochrome b6f complex.

Nuclear mutants of Chlamydomonas reinhardtii defective in the biogenesis of the cytochrome b6f complex

The random integration of transforming DNA into the nuclear genome of Chlamydomonas has been employed as an insertional mutagen to generate a collection of photosynthetic mutants that display abnormal steady-state fluorescence levels and an acetate-requiring phenotype. Electron paramagnetic resonance spectroscopy was then used to identify those mutants that specifically lack a functional cytochrome b6f complex. Our analysis of RNA and protein synthesis in five of these mutants reveals four separate phenotypes. One mutant fails to accumulate transcript for cytochrome f, whilst a second displays a severely reduced accumulation of the cytochrome b6 transcript. Two other mutants appear to be affected in the insertion of the haem co-factor into cytochrome b6. The fifth mutant displays no detectable defect in the synthesis of any of the known subunits of the complex. Genetic analysis of the mutants demonstrates that in three cases, the mutant phenotype co-segregates with the introduced DNA. For the mutant affected in the accumulation of the cytochrome f transcript, we have used the introduced DNA as a tag to isolate the wild-type version of the affected gene.

Maturation of pre-apocytochrome f in vivo. A site-directed mutagenesis study in Chlamydomonas reinhardtii

The biosynthesis of cytochrome f is a multistep process which requires processing of the precursor protein and covalent ligation of a c-heme upon membrane insertion of the protein. The crystal structure of a soluble form of cytochrome f has revealed that one axial ligand of the c-heme is provided by the alpha-amino group of Tyr1 generated upon cleavage of the signal sequence from the precursor protein (Martinez S. E., Huang D., Szczepaniak A., Cramer W.A., and Smith J. L. (1994) Structure 2, 95-105). We therefore investigated, by site-directed mutagenesis, the possible interplay between protein processing and heme attachment to cytochrome f in Chlamydomonas reinhardtii. These modifications were performed by chloroplast transformation using a petA gene encoding the full-length precursor protein and also a truncated version lacking the C-terminal membrane anchor. We first substituted the two cysteinyl residues responsible for covalent ligation of the c-heme, by a valine and a leucine, and showed that heme binding is not a prerequisite for cytochrome f processing. In another series of experiments, we replaced the consensus cleavage site for the thylakoid processing peptidase, AQA, by an LQL sequence. The resulting transformants were nonphototrophic and displayed delayed processing of the precursor form of cytochrome f, but nonetheless both the precursor and processed forms showed heme binding and assembled in cytochrome b6f complexes. Thus, pre-apocytochrome f adopts a suitable conformation for the cysteinyl residues to be substrates of the heme lyase and pre-holocytochrome f folds in an assembly-competent conformation. In the last series of experiments, we compared the rates of synthesis and degradation of the various forms of cytochrome f in the four types of transformants under study: (i) the C terminus membrane anchor apparently down-regulates the rate of synthesis of cytochrome f and (ii) degradation of misfolded forms of cytochrome f occurs by a proteolytic system intimately associated with the thylakoid membranes.

Chloroplasts can accommodate inclusion bodies. Evidence from a mutant of Chlamydomonas reinhardtii defective in the assembly of the chloroplast ATP synthase

We identified two neighboring missense mutations in the chloroplast atpA gene which are responsible for the defect of ATP synthase assembly in the FUD16 mutant from Chlamydomonas reinhardtii. The two corresponding amino acid substitutions, Ile184-->Asn and Asn186-->Tyr, occurred at strictly conserved sites among the alpha and beta subunits of (C)F1 complexes from bacteria, mitochondria, and chloroplasts. The altered region in the alpha polypeptide chain is located 7 amino acids downstream of the P-loop, which forms most of the conserved nucleotide binding site. Although the resulting chloroplast mutant fails to accumulate most of the ATP synthase subunits, it displays an increased intracellular content in both the alpha and beta subunits. We demonstrate that the two subunits do not bind to the thylakoid membranes but associate and overaccumulate in the chloroplast stroma as inclusion bodies. Increased rates of synthesis of the two subunits in the mutant point to an early interaction between the two subunits during their biogenesis.

Conversion of cytochrome f to a soluble form in vivo in Chlamydomonas reinhardtii

We introduced a stop codon in place of the ATT codon encoding Ile283 (numbered from the Met initiation codon) in the petA gene from Chlamydomonas reinhardtii. The resulting protein was expected to be truncated on its carboxy-terminus end, lacking the last 35 amino acids. This region of the polypeptide sequence encompasses a hydrophobic stretch assumed to anchor the protein in the thylakoid membrane. Once introduced in whole cells of C. reinhardtii by chloroplast transformation, the modified petA gene expressed a truncated apoprotein which was efficiently converted to a truncated holocytochrome f. This protein accumulated in the lumen of the thylakoids in a soluble form. Thus the conversion of preapocytochrome f to holocytochrome f does not require an interaction with the membrane through its C-terminus anchor. We show that the rest of the cytochrome b6f complex failed to accumulate in the transformants, most probably because of a lack of interaction between soluble cytochrome f and the other cytochrome b6f subunits. However, soluble cytochrome f was still able to donate electrons to photosystem I, which is indicative of its ability to maintain interactions with plastocyanin. The control of the rate of synthesis of cytochrome f by the neighboring subunit, suIV (Kuras & Wollman (1994) EMBO J. 13, 1019-1027), was not observed with the truncated cytochrome f. This observation suggests that either the transmembrane anchor of cytochrome f contains a target for the regulation of cytochrome f translation by suIV or there is a transient form of membrane-bound cytochrome f which is highly sensitive to proteolysis at an early post-translational stage.

The petD gene is transcribed by functionally redundant promoters in Chlamydomonas reinhardtii chloroplasts

FUD6, a nonphotosynthetic mutant of Chlamydomonas reinhardtii, was previously found to be deficient in the synthesis of subunit IV of the cytochrome b6/f complex, the chloroplast petD gene product (C. Lemaire, J. Girard-Bascou, F.-A. Wollman, and P. Bennoun, Biochim. Biophys. Acta 851:229-238, 1986). The lesion in FUD6 is a 236-bp deletion between two 11-bp direct repeats in the chloroplast genome. It extends from 82 to 72 bp upstream of the 5' end of wild-type petD mRNA to 156 to 166 bp downstream of the 5' end. Thus, the deletion extends into the putative promoter and 5' untranslated region of petD. No petD mRNA of the normal size can be detected in FUD6 cells, but a low level of a dicistronic message accumulates, which contains the coding regions for subunit IV and cytochrome f, the product of the upstream petA gene. petD transcriptional activity in FUD6 is not significantly altered from the wild-type level. This transcriptional activity was eliminated by petA promoter disruptions, suggesting that it originates at the petA promoter. We conclude that the petD-coding portion of most cotranscripts is rapidly degraded in FUD6, possibly following processing events that generate the 3' end of petA mRNA. A chloroplast transformant was constructed in which only the sequence from -81 to -2 relative to the major 5' end of the petD transcript was deleted. Although this deletion eliminates all detectable petD promoter activity, the transformant grows phototrophically and accumulates high levels of monocistronic petD mRNA. We conclude that the petD gene can be transcribed by functionally redundant promoters. In the absence of a functional petD promoter, a lack of transcription termination allows the downstream petD gene to be cotranscribed with the petA coding region and thereby expressed efficiently.

The assembly of cytochrome b6/f complexes: an approach using genetic transformation of the green alga Chlamydomonas reinhardtii

As an approach to the study of the biogenesis of the cytochrome b6/f complex, we characterized the behaviour of its constitutive subunits in mutant strains of Chlamydomonas reinhardtii bearing well-defined mutations. To this end, we have constructed three deletion mutant strains, each lacking one of the major chloroplast pet genes: the delta petA, delta petB and delta petD strains were unable to synthesize cyt f, cyt b6 and subunit IV (suIV) respectively. Western blotting analysis, pulse-labelling and pulse-chase experiments allowed us to compare the cellular accumulation, the rates of synthesis and the turnover of the cyt b6/f subunits remaining in the various strains. We show that the rates of synthesis of cyt b6 and suIV are independent of the presence of the other subunits of the complex but that their stabilization in the thylakoid membranes is a concerted process, with a marked dependence of suIV stability on the presence of cyt b6. In contrast, mature cyt f was stable in the absence of either suIV or cyt b6 but its rate of synthesis was severely decreased in these conditions. We conclude that the stoichiometric accumulation of the chloroplast-encoded subunits of the cyt b6/f complex results from two regulation processes: a post-translational regulation leading to the proteolytic disposal of unassembled cyt b6 and suIV and a co-translational (or early post-translational) regulation which ensures the production of cyt f next to its site of assembly.

Extensive accumulation of an extracellular L-amino-acid oxidase during gametogenesis of Chlamydomonas reinhardtii

In a previous study [Bulté, L. & Wollman, F.-A. (1992) Eur. J. Biochem. 204, 327-336], we identified a novel gamete-specific polypeptide of Chlamydomonas reinhardtii, M alpha. This 66-kDa polypeptide reacts with antibodies to cytochrome f and accumulates in gametes only in conditions that promote destabilisation of the cytochrome b6/f complex. Here, we show that M alpha is not a modification product of cytochrome f, but is part of protein M, a high-molecular-mass L-amino-acid oxidase located in the periplasm. It catalyzes oxidation of all L-amino acids tested, except cysteine. Using phenylalanine as a substrate, saturation of the enzymatic rate is reached at 2 microM. These characteristics suggest that protein M may operate in vivo as an efficient scavanger of ammonium from extracellular amino acids. The enzyme contains non-covalently bound FAD. It exists in two forms with essentially similar enzymatic properties, of 1.2-1.3 MDa and 0.9-1.0 MDa, respectively. The lighter form is an oligomer of M alpha, while the heavier form contains, in addition to M alpha, a second polypeptide of 135 kDa, M beta, in a molar ratio of 3-4 M alpha/M beta. Both polypeptides are glycosylated.

Further identification of the exoplasmic face particles on the freeze-fractured thylakoid membranes: a study using double and triple mutants from Chlamydomonas reinhardtii lacking various photosystem II subunits and the cytochrome b6/f complex

About 20% of the exoplasmic face (EF) particles present in the freeze-fractured thylakoid membranes of the wild type strain of Chlamydomonas reinhardtii remain in mutants lacking photosystem II (PSII) because of the absence of either one of the two PSII subcomplexes CP43 or D1/D2/CP47. We show that about half of these residual EF particles can be accounted for by PSII subcomplexes still present in such mutants, and by cytochrome (cyt) b6/f complexes. Analysis of double mutants lacking both types of protein complexes points to an association of cyt b6/f complexes with PSII subcomplexes in some of these EF particles and to a requirement in cyt b6/f complexes for the translocation of each of the two PSII subcomplexes (the CP43 subunit and the D1/D2/CP47 subcomplex) from the unstacked to the stacked regions of the thylakoid membranes.

Evidence for a selective destabilization of an integral membrane protein, the cytochrome b6/f complex, during gametogenesis in Chlamydomonas reinhardtii

We studied the process of photosynthetic inactivation during gametogenesis of the unicellular green alga Chlamydomonas reinhardtii. We show that it is caused by the selective destabilization of a single transmembrane protein complex, the cytochrome b6/f complex, which is initially accumulated in the thylakoid membranes of vegetative cells. This protein destabilization is controlled by the intracellular energy sources available in the gametes, i.e. the coupled electron flow in the mitochondria and the amount of starch accumulated in the chloroplast. It nevertheless requires the expression of gamete-specific proteins. The loss of cytochrome b6/f complexes during gametogenesis is prevented by the addition of cycloheximide, but is chloramphenicol insensitive. Therefore, it is likely to involve some translation product of nuclear origin, specifically expressed during gametogenesis. Among the new polypeptides specifically found in the gametes, we detected a soluble polypeptide M alpha (approximate molecular mass of 63 kDa), which shared common epitopes with cytochrome f. Its synthesis displays an antibiotic sensitivity typical of a nuclear-encoded polypeptide and is controlled by the same intracellular signals which control the destabilization of the cytochrome b6/f complexes in the thylakoid membranes.

Lateral redistribution of cytochrome b6/f complexes along thylakoid membranes upon state transitions

The cytochrome b6/f complex operates in photosynthetic electron transfer either in linear electron flow from photosystem II to photosystem I or in cyclic flow around photosystem I. Using membrane fractionation and immunocytochemistry, we show a change in lateral distribution of cytochrome b6/f complexes along the thylakoid membranes during state transitions. This change is seen in maize as well as in the green algae Chlamydomonas reinhardtii. When either of the two organisms were adapted to state II in vivo, the proportion of cytochrome b6/f complexes found in the photosystem I-enriched stroma lamellae regions was significantly larger than after adaptation to state I. A similar observation was made upon state I to state II transitions done in vitro by illuminating, in the presence of ATP, broken maize chloroplasts prepared from dark-adapted leaves. This reorganization of the electron-transfer chain is concurrent with the change in light-energy distribution between the two photosystems, which requires lateral displacement of light-harvesting complex II. That the changes in lateral distribution of both cytochrome b6/f and light-harvesting II complexes seen upon state transition in vitro similarly required addition of exogenous ATP, suggests that the change in cytochrome b6/f organization also depends on kinase activity. The increased concentration of cytochrome b6/f complexes in the vicinity of photosystem I in state II is discussed in terms of an increase in cyclic electron flow, thus favoring ATP production. Because transition to state II can be triggered in vivo by ATP depletion, we conclude that state transitions should be regarded not only as a light-adaptation mechanism but also as a rerouting of photosynthetic electron flow, enabling photosynthetic organisms to adapt to changes in the cell demand for ATP.

Evidence for thylakoid membrane fusion during zygote formation in Chlamydomonas reinhardtii

To understand whether fusions of thylakoid membranes from the parental chloroplasts occurred during zygote formation in Chlamydomonas reinhardtii, we performed an ultrastructural analysis of the zygotes produced by crossing mutants lacking photosystem I or II protein complexes, in the absence of de novo chloroplast protein synthesis. Thylakoid membranes from each parent could be distinguished on thin sections due to their organization in "supergrana" in mutants lacking photosystem I centers, by freeze-fracturing due to the absence of most of the exoplasmic-face (EF) particles in mutants lacking photosystem II centers, by immunocytochemistry using antibodies directed against photosystem II subunits. We demonstrate that a fusion of the thylakoid membranes occurred during zygote formation approximately 15 h after mating. These fusions allowed a lateral redistribution of the thylakoid membrane proteins. These observations provide the structural basis for the restoration of photosynthetic electron flow in the mature zygote that we observed in fluorescence induction experiments.

Nucleotide sequences of the continuous and separated petA, petB and petD chloroplast genes in Chlamydomonas reinhardtii

We have mapped and sequenced the petA (cytf), petB (cytb6) and petD (subunit IV) genes on the chloroplast genome of Chlamydomonas reinhardtii. At variance with the pet genes in higher plant chloroplasts, the petB and petD genes are continuous, not adjacent and not located next to the psbB gene. The corresponding polypeptide sequences are highly conserved when compared with their counterparts from other sources but have a few features specific of algal cytb6/f complexes. In particular the transit sequence of cytf displays unique characteristics when compared with those previously described for cytf in higher plants.

The chlorophyll-a/b proteins of photosystem II in Chlamydomonas reinhardtii : Isolation, characterization and immunological cross-reactivity to higher-plant polypeptides

We have adapted the procedure for the isolation of PSII membranes from higher plants (D.A. Berthold et al., 1981, FEBS Lett. 134, 231-234) to the green algae Chlamydomonas reinhardtii. The chlorophyll (Chl)-binding proteins from this PSII preparation have been further separated into single Chl-binding polypeptides and characterized spectroscopically. Seven single polypeptides were shown to bind Chl a and Chl b. In particular, we demonstrate that polypeptides p9, p10 and p22, which had not been previously shown to bind Chl a and b, have characteristics similar to those of CP29, CP26 and CP24 from higher plants. We note, however, that p9 and p10 are phosphorylatable in C. reinhardtii, at variance with CP29 and CP26 from higher plants. Our data support the notion that the PSII antenna systems in C. reinhardtii and in higher plants are very similar. Therefore, studies on the organization and regulation of light-harvesting processes in C. reinhardtii may provide information of general relevance for both green algae and higher plants.

Posttranslational events leading to the assembly of photosystem II protein complex: a study using photosynthesis mutants from Chlamydomonas reinhardtii

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.

The chloroplast ATP synthase in Chlamydomonas reinhardtii. II. Biochemical studies on its biogenesis using mutants defective in photophosphorylation

We have carried out an analysis of the synthesis, cellular accumulation, and membrane binding of the chloroplast-encoded subunits of the ATP synthase (alpha, beta, epsilon, I, III, and IV) in several mutants of Chlamydomonas reinhardtii defective in photophosphorylation. These data gave some insight on the putative genetic lesion in each mutant and allowed some characterization of the assembly and stabilization of the ATP synthase complex in the thylakoid membranes. Four chloroplast mutants are likely to be altered in chloroplast structural genes coding for coupling factor (CF) 1 subunits beta and epsilon and for CF0 subunits I and IV. A fifth chloroplast mutant and three nuclear mutants were altered in genes regulating either transcription or translation of chloroplast genes coding for CF1 subunits alpha and beta and CF0 subunits III and IV. Evidence is presented (i) for a control of the rate of synthesis of subunit beta by subunit alpha in the absence of ATP synthase assembly and (ii) for an interaction between alpha and beta subunits in the stroma of the chloroplast which protects alpha subunits from proteolytic degradation. The role of several chloroplast-encoded subunits of CF0 and CF1 in the stabilization of partially assembled ATP synthase is discussed. We conclude that in the absence of ATP synthase assembly, CF0 cannot accumulate in the thylakoid membranes, whereas alpha and beta subunits, presumably engaged in soluble CF1, can accumulate in the stroma of the chloroplast.

The chloroplast ATP synthase in Chlamydomonas reinhardtii. I. Characterization of its nine constitutive subunits

We have characterized the subunit composition of the chloroplast ATP synthase from Chlamydomonas reinhardtii by means of a comparison of the polypeptide deficiencies in a mutant defective in photophosphorylation, with the polypeptide content in purified coupling factor (CF)1 and CF1.CF0 complexes. We could distinguish nine subunits in the enzyme, four of which were CF0 subunits. Further characterization of these subunits was undertaken by immunoblotting experiments, [14C]dicyclohexylcarbodiimide binding and analysis of their site of translation. In particular, we were able to show the presence of an as yet unidentified delta subunit in CF1 from C. reinhardtii. We have identified a 70-kDa peripheral membrane protein in the thylakoid membranes of C. reinhardtii, which is immunologically related to the beta subunit of CF1. We discuss its conceivable ATPase function with respect to the Ca2+-dependent ATPase activity previously reported in the thylakoid membranes from C. reinhardtii.

Restoration of phototrophic growth in a mutant of Chlamydomonas reinhardtii in which the chloroplast atpB gene of the ATP synthase has a deletion: an example of mitochondria-dependent photosynthesis

The Chlamydomonas reinhardtii mutant FUD50 has a deletion in the atpB gene of the chloroplast ATP synthase [Woessner, J. P., Masson, A., Harris, E. H., Bennoun, P., Gillham, N. W., and Boynton, J. E. (1984) Plant Mol. Biol. 3, 177-190]. We have isolated a suppressed strain (FUD50su) that can grow under phototrophic conditions, although it still showed no synthesis of the beta subunit of coupling factor 1. Thylakoid membranes of the FUD50su strain were similar to those of the original FUD50 strain, in that they both lacked all the subunits making up the chloroplast ATP synthase complex. We show that photosynthesis in FUD50su is sensitive to inhibitors such as antimycin, specific for mitochondrial electron transport. This observation indicates that photosynthesis in the FUD50su strain is achieved through an unusual interaction between mitochondria and chloroplast. Exportation of light-induced reduced compounds from the chloroplast to the mitochondria elicits ATP formation in the latter, and ATP is subsequently imported to the chloroplast. The activation of such an ATP shuttle coupled to an NADPH shuttle would thus provide the reducing power and the free energy needed for carbon assimilation in a chloroplast that lacks chloroplast ATP synthase.

Protein rotational mobility in thylakoid membranes of different polypeptide composition in the wild type and mutant strains of Chlamydomonas reinhardtii

The rotational mobility of thylakoid membrane proteins labeled with a paramagnetic analog of N-ethylmaleimide was investigated by saturation transfer electron spin resonance. In the wild type strain of Chlamydomonas reinhardtii two polypeptides are prominently labeled. They correspond to the 19-kDa subunit of the reaction center I protein and to the 30-kDa subunit of the light harvesting complex. Several polypeptides, most of which are either trypsin or alkaline sensitive, are also labeled. In order to circumvent the lack of specificity during the labeling, we have compared the rotational mobilities of labeled proteins in thylakoid membranes from several mutant strains which lack in photosystem I., ATPase or light harvesting complexes. Comparison of the saturation transfer electron spin resonance spectra obtained with these mutant membranes as well as with trypsin- and alkaline-treated membranes allowed us to characterize the rotational contribution of some of the labeled proteins to the overall protein dynamics observed in the wild type strain. The reaction center I protein undergoes slow rotation as compared to the other labeled proteins. The rotational characteristics of the labeled light harvesting complexes are those of a peptide fragment in the complex which is in rapid motion in unstacked membranes. Stacking of the thylakoid membranes upon Mg2+ addition is accompanied by a marked change in shape of the saturation transfer spectra, and corresponds to the appearance of highly immobilized nitroxides. We interpret these changes as arising mainly from the hindrance upon membrane appression, of the labeled fragment of the light harvesting complexes which protrude at the thylakoid outer surface.

Correlation between changes in light energy distribution and changes in thylakoid membrane polypeptide phosphorylation in Chlamydomonas reinhardtii

We have used a new method to extensively modify the redox state of the plastoquinone pool in Chlamydomonas reinhardtii intact cells. This was achieved by an anaerobic treatment that inhibits the chlororespiratory pathway recently described by P. Bennoun (Proc. Natl. Acad. Sci. USA, 1982, 79:4352-4356). A state I (plus 3,4-dichlorophenyl-1,1-dimethylurea) leads to anaerobic state transition induced a decrease in the maximal fluorescence yield at room temperature and in the FPSII/FPSI ratio at 77 degrees K, which was three times larger than in a classical state I leads to state II transition. The fluorescence changes observed in vivo were similar in amplitude to those observed in vitro upon transfer to the light of dark-adapted, broken chloroplasts incubated in the presence of ATP. We then compared the phosphorylation pattern of thylakoid polypeptides in C. reinhardtii in vitro and in vivo using gamma-[32P]ATP and [32P]orthophosphate labeling, respectively. The same set of polypeptides, mainly light-harvesting complex polypeptides, was phosphorylated in both cases. We observed that this phosphorylation process is reversible and is mediated by the redox state of the plastoquinone pool in vivo as well as in vitro. Similar changes of even larger amplitude were observed with the F34 mutant intact cells lacking in photosystem II centers. The presence of the photosystem II centers is then not required for the occurrence of the plastoquinone-mediated phosphorylation of light-harvesting complex polypeptides.