Development of the photosynthetic apparatus during light-dependent greening of a mutant of Chlorella fusca

The formation of chlorophyll, cytochrome f, P-700, ribulose bisphosphate carboxylase as well as photosynthesis and Hill reaction activities were tested during the light-dependent greening process of the Chlorella fusca mutant G 10. Neither chlorophyll nor protochlorophyllide was detected in the darkgrown cells. When transferred to light the mutant cells developed chlorophyll and established its photosynthetic capacity after a short lag phase. In the in vivo absorption spectra a spectral shift of the red absorption peak position from 674 to 680 nm was indicated during the first 3 h of greening. Cytochrome f was already present in the dark-grown cells, but during the greening phase a threefold increase in the cytochrome f content could be seen. At the early stages of greening a characteristic primary oscillation in the content of cytochrome f was observed. P-700 was lacking in the dark and during the first 30 min of illumination. From the first to the second h of light a forced synthesis of P-700 took place and the time-course curve for the ratios of P-700/chlorophyll rose to a sharp maximum. The synthesis of P-700 started together with photosystem I activity and showed similar kinetics. We found the simultaneous appearance of photosystem II, photosystem I, and photosynthetic activities 30 min after the beginning of the illumination. Based on chlorophyll content they attained maximum activity after 2 h of light, but at this time photosystem I capacity proved to be remarkably higher than photosynthetic and photosystem II activities. Highest carboxylase activity existed in darkgrown cells. During the greening process the activity of the enzyme decreased continuously. After 2 h of illumination chlorophyll synthesis partially served to increase the size of the photosynthetic unit, which consequently led to a decrease in the light energy needed to saturate photosynthesis and also to a decrease of photosynthetic rate based on chlorophyll content.Abbreviations Chl chlorophyll - Cyt f cytochrome f - DPIP 2,6-dichlorophenolindophenol - EDTA ethylenediaminetetraacetic acid - GSH glutathione - LH light-harvesting - PS photosystem - RuBP ribulose bisphosphate     

On the functional organization of electron transport from plastoquinone to Photosystem I

Signal I, the EPR signal of P-700, induced by long flashes as well as the rate of linear electron transport are investigated at partial inhibition of electron transport in chloroplasts. Inhibition of plastoquinol oxidation by dibromothymoquinone and bathophenanthroline, inhibition of plastocyanin by KCN and HgCl₂, and inhibition by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide are used to study a possible electron exchange between electron-transport chains after plastoquinone. (1) At partial inhibition of plastocyanin the reduction kinetics of P-700⁺ show a fast component comparable to that in control chloroplasts and a new slow component. The slow component indicates P-700⁺ which is not accessible to residual active plastocyanin under these conditions. We conclude that P-700 is reduced via complexed plastocyanin. (2) The rate of linear electron transport at continuous illumination decreases immediately when increasing amounts of plastocyanin are inhibited by KCN incubation. This is not consistent with an oxidation of cytochrome f by a mobile pool of plastocyanin with respect to the reaction rates of plastocyanin being more than an order of magnitude faster than the rate-limiting step of linear electron transport. It is evidence for a complex between the cytochrome b₆ - f complex and plastocyanin. The number of these complexes with active plastocyanin is concluded to control the rate-limiting plastoquinol oxidation. (3) Partial inhibition of the electron transfer between plastoquinone and cytochrome f by dibromothymoquinone and bathophenanthroline causes decelerated monophasic reduction of total P-700⁺. The P-700 kinetics indicate an electron transfer from the cytochrome b₆ - f complex to more than ten Photosystem I reaction center complexes. This cooperation is concluded to occur by lateral diffusion of both complexes in the membrane. (4) The proposed functional organization of electron transport from plastoquinone to P-700 in situ is supported by further kinetic details and is discussed in terms of the spatial distribution of the electron carriers in the thylakoid membrane.     

Characterization of Three Photosystem II Mutants in Zea mays L. Lacking a 32,000 Dalton Lamellar Polypeptide

Two fully blocked and one partially blocked photosystem II nuclear mutants have been selected in Zea mays. The fully blocked mutants lack photosystem II activity, variable fluorescence, the light-inducible C-550 signal, the high potential form of cytochrome b-559, and most or all of the low potential form of the cytochrome. The block in these mutants may primarily affect the reducing side of photosystem II, inasmuch as chloroplasts isolated from both mutants exhibit an elevated F695 fluorescence emission peak. The partially blocked mutant exhibits partial photosystem II activity and a reduction, but not the total loss of the variable fluorescence yield, the C-550 signal, and the high potential form of cytochrome b-559. Lamellae isolated from the fully blocked mutants are greatly deficient for a major lamellar polypeptide with an apparent molecular weight of 32,000 daltons, whereas lamellae from the partially blocked mutant show the partial loss of this same polypeptide, suggesting that the 32,000 dalton polypeptide is necessary for the proper function of photosystem II.     

The role of the high potential form of the cytochrome b559: Study of Thermosynechococcus elongatus mutants

Cytochrome b559 is an essential component of the photosystem II reaction center in photosynthetic oxygen-evolving organisms, but its function still remains unclear. The use of photosystem II preparations from Thermosynechococcus elongatus of high integrity and activity allowed us to measure for the first time the influence of cytochrome b559 mutations on its midpoint redox potential and on the reduction of the cytochrome b559 by the plastoquinone pool (or Q_B). In this work, five mutants having a mutation in the α-subunit (I14A, I14S, R18S, I27A and I27T) and one in the β-subunit (F32Y) of cytochrome b559 have been investigated. All the mutations led to a destabilization of the high potential form of the cytochrome b559. The midpoint redox potential of the high potential form was significantly altered in the αR18S and αI27T mutant strains. The αR18S strain also showed a high sensitivity to photoinhibitory illumination and an altered oxidase activity. This was suggested by measurements of light induced oxidation and dark re-reduction of the cytochrome b559 showing that under conditions of a non-functional water oxidation system, once the cytochrome is oxidized by P680⁺, the yield of its reduction by Q_B or the PQ pool was smaller and the kinetic slower in the αR18S mutant than in the wild-type strain. Thus, the extremely positive redox potential of the high potential form of cytochrome b559 could be necessary to ensure efficient oxidation of the PQ pool and to function as an electron reservoir replacing the water oxidation system when it is not operating.     

Thylakoid FtsH Protease Contributes to Photosystem II and Cytochrome b6f Remodeling in Chlamydomonas reinhardtii under Stress Conditions

FtsH is the major thylakoid membrane protease found in organisms performing oxygenic photosynthesis. Here, we show that FtsH from Chlamydomonas reinhardtii forms heterooligomers comprising two subunits, FtsH1 and FtsH2. We characterized this protease using FtsH mutants that we identified through a genetic suppressor approach that restored phototrophic growth of mutants originally defective for cytochrome b₆f accumulation. We thus extended the spectrum of FtsH substrates in the thylakoid membranes beyond photosystem II, showing the susceptibility of cytochrome b₆f complexes (and proteins involved in the c_i heme binding pathway to cytochrome b₆) to FtsH. We then show how FtsH is involved in the response of C. reinhardtii to macronutrient stress. Upon phosphorus starvation, photosynthesis inactivation results from an FtsH-sensitive photoinhibition process. In contrast, we identified an FtsH-dependent loss of photosystem II and cytochrome b₆f complexes in darkness upon sulfur deprivation. The D1 fragmentation pattern observed in the latter condition was similar to that observed in photoinhibitory conditions, which points to a similar degradation pathway in these two widely different environmental conditions. Our experiments thus provide extensive evidence that FtsH plays a major role in the quality control of thylakoid membrane proteins and in the response of C. reinhardtii to light and macronutrient stress.     

Characterization of Nuclear Mutants of Maize Which Lack the Cytochrome f/b-563 Complex

Two high fluorescent, nuclear recessive mutants of maize (Zea mays L.), designated hcf-2 and hcf-6, are described which are missing the chloroplast cytochrome f/b-563 complex. Thylakoids from the mutants show a block in whole chain electron transport activity (H₂O to methyl viologen), while retaining activities associated with photosystem II (H₂O to phenylenediamine) and photosystem I (diaminodurene to methyl viologen). Chemically induced, optical difference spectra indicate a loss of cytochromes f and b-563. Cytochrome b-559 is present in both high and low potential forms. EPR analyses of thylakoid membranes of hcf-6 reveals the lack of a signal (g = 1.90) associated with the Rieske Fe-S center. Additionally, hcf-6 is lacking EPR signals at g = 6 (attributable to the high spin ferric heme of cytochrome b-563) and g = 2.5 (unidentified). The mutant retains signals at g = 2.9 (cytochrome b-559) and at g = 4.3 and 9 (both signals probably arising from a storage form of ferric iron).

Thylakoid polypeptides are examined using polyacrylamide gel electrophoresis. hcf-2 and hcf-6 have identical profiles, showing losses of polypeptides with apparent molecular masses of 33 (cytochrome f), 23 (cytochrome b-563), and 17.5 kilodaltons. The protein associated with the Rieske Fe-S center could not be determined from the gel profiles. Additionally, both mutants show an increase in a band with a molecular mass of 31 kilodaltons.

     

Kinetic analysis of P-700 photoconversion: Effect of secondary electron donation and plastocyanin inhibition

The kinetics of P-700 photoconversion under weak continuous actinic illumination were quantitatively analyzed to provide information on the relative absorption cross-section σ_PSI of the light-harvesting pigments associated with photosystem I and on the number of electrons stored between the two photosystems in dark-adapted chloroplasts. The theory of chemical kinetics for a system of monomolecular consecutive first-order reactions is reviewed briefly to provide support for the experimental approach taken. A complete inhibition of plastocyanin by cyanide eliminated all secondary electron donation to P-700⁺ and allowed the registration of the exponential (monomolecular) P-700 photoconversion at room temperature. The rate constant K_p-700 of the exponential kinetics was independent of the ionic (± Mg²⁺) and osmotic (± sucrose) strength of the chloroplast suspension medium, and of the oxidation-reduction state of photosystem II. The extent of plastocyanin inhibition in partially inhibited samples was greater under low ionic and low osmotic conditions. In dark-adapted chloroplast samples that were not cyanide treated, the number of electrons stored between the two photosystems was 3.9 ± 0.2 and independent of divalent cations. It is concluded that plastocyanin inhibition by cyanide is favored under low ionic and low osmotic conditions. The Mg²⁺ ion and redox state of photosystem II-independent photoconversion of P-700 does not support significant changes in the spillover of excitation from photosystem II to photosystem I in isolated chloroplasts.     

Electron transfer between the two photosystems. II. Equilibrium constants

(1) The equilibrium constants for the redox reactions occurring between Photosystem (PS) I donors were measured on chloroplasts, dark-adapted in the presence of sodium ascorbate and 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU) and then illuminated by d.c. light. The equilibrium constant for the electron transfer between plastocyanin and P-700 is close to 1 and the overall equilibrium constant between cytochrome f and P-700 is about 2.3. As these equilibrium constants do not depend upon the intensity of the d.c. beam, the low values we measured cannot be due to kinetic limitations. (2) The equilibrium constants were measured also in the absence of DCMU using chloroplasts in oxidizing conditions (ferricyanide or far red illumination) illuminated by a saturating flash. During the course of the reduction of PS I donors by plastoquinol molecules formed by the flash, the equilibrium constants are higher than in the preceding conditions: the value for plastocyanin to P-700 is close to 5, and that for cytochrome f to P-700 is about 25. (3) The variations of these equilibrium constants are tentatively interpreted as being due to mutual electrostatic interactions between cytochrome b and f which are included in the same complex. This model implies that the perturbation of the redox properties of cytochrome f by a positive charge located on cytochrome b is identical to the perturbation of the redox properties of cytochrome b by a positive charge located on cytochrome f.     

Dissection of respiratory and cyclic electron transport in Synechocystis sp. PCC 6803

Cyclic electron transport (CET) is an attractive hypothesis for regulating photosynthetic electron transport and producing the additional ATP in oxygenic phototrophs. The concept of CET has been established in the last decades, and it is proposed to function in the progenitor of oxygenic photosynthesis, cyanobacteria. The in vivo activity of CET is frequently evaluated either from the redox state of the reaction center chlorophyll in photosystem (PS) I, P700, in the absence of PSII activity or by comparing PSI and PSII activities through the P700 redox state and chlorophyll fluorescence, respectively. The evaluation of CET activity, however, is complicated especially in cyanobacteria, where CET shares the intersystem chain, including plastoquinone, cytochrome b₆/f complex, plastocyanin, and cytochrome c₆, with photosynthetic linear electron transport (LET) and respiratory electron transport (RET). Here we sought to distinguish the in vivo electron transport rates in RET and CET in the cyanobacterium Synechocystis sp. PCC 6803. The reduction rate of oxidized P700 (P700⁺) decreased to less than 10% when PSII was inhibited, indicating that PSII is the dominant electron source to PSI but P700⁺ is also reduced by electrons derived from other sources. The oxidative pentose phosphate (OPP) pathway functions as the dominant electron source for RET, which was found to be inhibited by glycolaldehyde (GA). In the condition where the OPP pathway and respiratory terminal oxidases were inhibited by GA and KCN, the P700⁺ reduction rate was less than 1% of that without any inhibitors. This study indicate that the electron transport to PSI when PSII is inhibited is dominantly derived from the OPP pathway in Synechocystis sp. PCC 6803.

     

Functional Characterization of the Small Regulatory Subunit PetP from the Cytochrome b6f Complex in Thermosynechococcus elongatus

The cyanobacterial cytochrome b₆f complex is central for the coordination of photosynthetic and respiratory electron transport and also for the balance between linear and cyclic electron transport. The development of a purification strategy for a highly active dimeric b₆f complex from the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1 enabled characterization of the structural and functional role of the small subunit PetP in this complex. Moreover, the efficient transformability of this strain allowed the generation of a ΔpetP mutant. Analysis on the whole-cell level by growth curves, photosystem II light saturation curves, and P700⁺ reduction kinetics indicate a strong decrease in the linear electron transport in the mutant strain versus the wild type, while the cyclic electron transport via photosystem I and cytochrome b₆f is largely unaffected. This reduction in linear electron transport is accompanied by a strongly decreased stability and activity of the isolated ΔpetP complex in comparison with the dimeric wild-type complex, which binds two PetP subunits. The distinct behavior of linear and cyclic electron transport may suggest the presence of two distinguishable pools of cytochrome b₆f complexes with different functions that might be correlated with supercomplex formation.     

Studies on electron-transport reactions of photosynthesis in plastome mutants of oenothera

Fluorescence characteristics and light-induced absorbance changes of 5 plastome mutants of Oenothera, all having a defect in photosynthesis, were investigated to localize the site of the block in their photosynthetic mechanism and to relate mutational changes in the plastome to specific biochemical events in photosynthesis. In 4 of the mutants examined photosystem 2 was largely, or completely, nonfunctional. Excitation of system 2 did not cause reduction of oxidized cytochrome f in these mutants. The system-2 dependent absorbance change at 518 mμ seen in normal leaves was absent in the mutants. Moreover, the mutants had a high initial fluorescence in the presence and in the absence of 3- (3,4-dichlorophenyl)-1,1-dimethylurea, which did not change during illumination, indicating that the reaction centers of system 2 were affected by the mutations. Photosystem 1 functioned normally.

A fifth mutant had an impaired photosystem 1. Even high intensity far-red light did not lead to an accumulation of oxidized cytochrome f as was seen in normal plants. Photosystem 2 was functioning, as evidenced by the fast reduction of the primary system-2 oxidant, and by the characteristics of the 518-mμ absorbance change.

Because 1 of the 2 photosystems is functional in all mutants, and because they all have the enzymes of the photosynthetic carbon cycle, it appears that the effect of the mutation is specific. The results suggest that the plastome controls reactions within the electron-transport chain of photosynthesis.

     

Mutational analysis of photosystem I of Synechocystis sp. PCC 6803: the role of four conserved aromatic residues in the j-helix of PsaB

Photosystem I is the light-driven plastocyanin-ferredoxin oxidoreductase in the photosynthetic electron transfer of cyanobacteria and plants. Two histidyl residues in the symmetric transmembrane helices A-j and B-j provide ligands for the P700 chlorophyll molecules of the reaction center of photosystem I. To determine the role of conserved aromatic residues adjacent to the histidyl molecule in the helix of B-j, we generated six site-directed mutants of the psaB gene in Synechocystis sp. PCC 6803. Three mutant strains with W645C, W643C/A644I and S641C/V642I substitutions could grow photoautotrophically and showed no obvious reduction in the photosystem I activity. Kinetics of P700 re-reduction by plastocyanin remained unaltered in these mutants. In contrast, the strains with H651C/L652M, F649C/G650I and F647C substitutions could not grow under photoautotrophic conditions because those mutants had low photosystem I activity, possibly due to low levels of proteins. A procedure to select spontaneous revertants from the mutants that are incapable to photoautotrophic growth resulted in three revertants that were used in this study. The molecular analysis of the spontaneous revertants suggested that an aromatic residue at F647 and a small residue at G650 may be necessary for maintaining the structural integrity of photosystem I. The (P700⁺ - P700) steady-state absorption difference spectrum of the revertant F647Y has a ∼5 nm narrower peak than the recovered wild-type, suggesting that additional hydroxyl group of this revertant may participate in the interaction with the special pair while the photosystem I complexes of the F649C/G650T and H651Q mutants closely resemble the wild-type spectrum. The results presented here demonstrate that the highly conserved residues W645, W643 and F649 are not critical for maintaining the integrity and in mediating electron transport from plastocyanin to photosystem I. Our data suggest that an aromatic residue is required at position of 647 for structural integrity and/or function of photosystem I.     

In vivo electron donation from plastocyanin and cytochrome c6 to PSI in Synechocystis sp. PCC6803

Many cyanobacteria species can use both plastocyanin and cytochrome c₆ as lumenal electron carriers to shuttle electrons from the cytochrome b₆f to either photosystem I or the respiratory cytochrome c oxidase. In Synechocystis sp. PCC6803 placed in darkness, about 60% of the active PSI centres are bound to a reduced electron donor which is responsible for the fast re-reduction of P₇₀₀ in vivo after a single charge separation. Here, we show that both cytochrome c₆ and plastocyanin can bind to PSI in the dark and participate to the fast phase of P₇₀₀ reduction, but the fraction of pre-bound PSI is smaller in the case of cytochrome c₆ than with plastocyanin. Because of the inter-connection of respiration and photosynthesis in cyanobacteria, the inhibition of the cytochrome c oxidase results in the over-reduction of the photosynthetic electron transfer chain in the dark that translates into a lag in the kinetics of P₇₀₀ oxidation at the onset of light. We show that this is true both with plastocyanin and cytochrome c₆, indicating that the partitioning of electron transport between respiration and photosynthesis is regulated in the same way independently of which of the two lumenal electron carriers is present, although the mechanisms of such regulation are yet to be understood.     

Effect of Supra-High Irradiation on the Photosynthetic System of the Cyanophyte Synechocystis PCC 6714

Stability of thylakoid components under supra-high irradiancewas studied with the cyanophyte Synechocystis PCC 6714. Theactivity of overall photosynthesis was quickly inactivated (T_1/2=20min) under supra-high irradiance (300 W m^–2, white light).In parallel with the inactivation of photosynthesis, QA in PSII was also inactivated. Both inactivations were acceleratedby chloramphenicol (CAP) addition. The reactivation of PS IIrequired weak irradiation and was suppressed by CAP. However,PS I measured as P700 was very stable. The level of PS I measuredas P700 was not significantly reduced by the irradiation for12 h even in the presence of CAP while the level of Cyt b₅₅₉,component of PS II, was decreased markedly. The function ofPS I before and after supra-high irradiation with CAP was examinedby comparing sizes of P700 oxidation induced by a short flash,by a continuous light, and by determination of O₂-and ferredoxin-reduction.No difference was observed in PS I actions before and afterthe irradiation treatment. These results indicate that the PSI complex is very tolerant of supra-high irradiation. However,the cells grown under supra-high irradiance contained much fewerPS I and PS II complexes than Cyt b₆–f complexes. Theformer levels were reduced to a half to one fourth of thosebefore growth while the level of Cyt b₆–f complex wasnot reduced so much. A possible mechanism for changes in thylakoidcomposition under supra-high irradiation was discussed. (Received February 16, 1991; Accepted June 12, 1991)     

Electron transport from cytochrome b6-f complexes to Photosystem I reaction center complexes in Synechococcus sp. Is cytochrome c-553 a mobile electron carrier?

Numbers of the Photosystem I reaction center complexes and the cytochrome b₆-f complexes with which a cytochrome c-553 molecule can interact within the limiting time of photosynthetic electron transport were examined by measuring flash-induced absorption changes of P-700, cytochrome c-553 and cytochrome f in the thermophilic cyanobacterium Synechococcus sp. The addition of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) did not affect the common 2 ms half-time of P-700, cytochrome c-553 and cytochrome f reduction, which is ascribed to electron transfer from the plastoquinone pool. The inhibitor decreased, however, amounts of the three electron carriers which underwent the 2 ms reduction in the order of cytochrome f, cytochrome c-553 and P-700. On excitation with weak flashes which oxidized only a small fraction of cytochrome c-553 molecules present in cells, P-700 remained in the oxidized state after the flashes was reduced with electrons from the Rieske center or plastoquinone but not from cytochrome c-553. The ratios of cytochrome c-553 to cytochrome f oxidized at various flash intensities were constant and similar to the ratio of the two cytochromes present in cells. It is concluded that cytochrome c-553 cannot exchange electrons with large numbers of the Photosystem I reaction center complexes and the cytochrome b₆-f complexes in the limiting time, but has a mobility sufficient to mediate electron transfer between the two complexes, which are present at an unbalanced ratio in Synechococcus cells.     

Effect of light quality on the composition and function of thylakoid membranes in atriplex triangularis

Light quality was shown to exert well-coordinated regulatory effects on the composition and function of the thylakoid membranes as well as on the photosynthetic rates of intact leaves from Atriplex triangularis grown in continuous blue, white and red lights (50 μE · m^?2 · s^?1). The higher photosynthetic rates in plants grown in blue light, as compared to those in white and red lights, resulted from marked changes in both light-harvesting complexes and electron carriers. The concentrations of electron carriers such as atrazine binding sites, plastoquinone, cytochromes b and f and P-700 on a chlorophyll basis were markedly increased in Atriplex grown in blue light; and the apparent light-harvesting antenna unit sizes of Photosystems I and II were greatly reduced. Consequently, the electron transport capacities of Photosystems I and II were also increased as was the coupling factor CF₁ activity. Atriplex grown in red light had lower photosynthetic rates than those grown in blue or white light by incorporating changes in the composition and function of the thylakoids in a direction opposite to those caused by growth in blue light. When these regulatory effects of light quality were compared with those of light quantity [6,7], it is clear that $\text{Chl}\text{a}\text{Chl}\text{b}$ ratios, electron transport capacities of Photosystems I and II, concentrations of plastoquinone, atrazine binding sites, coupling factor CF₁ activity and the apparent antenna unit size of Photosystem II are more affected by light quantity, whereas light quality has a greater influence on the concentration of P-700, the apparent antenna unit size of Photosystem I and the overall photosynthetic rates of intact leaves.     

Electron Transport Reactions in Grana Preparations from Spinach Chloroplasts

Fraction 2 (grana-stack) particles prepared with the French press showed absorbance changes, at room temperature and with sodium ascorbate and methyl-viologen, that were produced by the oxidation of cytochrome b-559. This oxidation was inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and sensitized by system II of photosynthesis. The oxidation is too slow to account for the rates of the Hill reaction that have been observed with nicotinamide-adenine dinucleotide phosphate (NADP⁺). It appears that this cytochrome is not functioning in the main pathway of electron transport. In the presence of 2,3,5,6-tetramethyl-p-phenylene-diamine (DAD) and ascorbate, light-induced oxidation of cytochrome f took place within 3 msec (or faster) in the grana-stack particles. Treatment with the detergent Triton X-100 disrupted this rapid cytochrome f oxidation as well as the oxidation of cytochrome b-559. Subsequent plastocyanin addition did not restore the rapid oxidation of cytochrome f (nor of cytochrome b-559) but only slow changes of cytochrome f. In view of the fact that these particles contain almost no plastocyanin, it is unlikely that plastocyanin functions in electron transport between cytochrome f and P-700 in the particles derived from the grana-stack regions of the chloroplast.     

Involvement of state transitions in the switch between linear and cyclic electron flow in Chlamydomonas reinhardtii

The energetic metabolism of photosynthetic organisms is profoundly influenced by state transitions and cyclic electron flow around photosystem I. The former involve a reversible redistribution of the light-harvesting antenna between photosystem I and photosystem II and optimize light energy utilization in photosynthesis whereas the latter process modulates the photosynthetic yield. We have used the wild-type and three mutant strains of the green alga Chlamydomonas reinhardtii—locked in state I (stt7), lacking the photosystem II outer antennae (bf4) or accumulating low amounts of cytochrome b₆f complex (A-AUU)—and measured electron flow though the cytochrome b₆f complex, oxygen evolution rates and fluorescence emission during state transitions. The results demonstrate that the transition from state 1 to state 2 induces a switch from linear to cyclic electron flow in this alga and reveal a strict cause–effect relationship between the redistribution of antenna complexes during state transitions and the onset of cyclic electron flow.     

Recherche du cytochrome b-563 et du p700 chez trois mutants non photosynthétiques de Chlamydomonas reinhardti

Studies of cytochrome b-563 and P 700 in three non-photosynthetic mutants of Chlamydomonas reinhardAn investigation into the presence of cytochrome b-563 and of P700 in three non-photosynthetic mutants (Fl 5, Fl 9, Fl 15) of Chlamydomonas reinhardti was carried out. These three mutants exhibit several functional anomalies (described elsewhere), which indicate that the electron transport chain between the two photoreactions is blocked. In addition, Fl 5 is unable to carry out any reaction related to System I.Mutants Fl 9 and Fl 15 had less than 19% of the cytochrome b-563 content found in the wild type (which was about 0.27 mole per 100 moles chlorophyll); mutant Fl 5 had more than 80% of this content. The deficiencies (only traces) in bound cytochrome c-553, previously observed with mutants Fl 9 and Fl 15, but not Fl 5, were confirmed (in the wild type, there is about 0.20 mole bound cytochrome per 100 moles chlorophyll).Photosystem I particles, prepared from wild type and mutants Fl 9 and Fl 15 chloroplast fragments, had about 2 (Fl 9, Fl 15) and 3 (wild type) moles P700 per 100 moles chlorophyll. Mutant Fl 5 particles showed neither P700 spectroscopic characteristics nor photooxidation activity; their chlorophyll a/b ratio was lower by a factor of 2 and protein/chlorophyll ratio about 8 times higher than in the wild type particles. This mutant appears to lack P700.