Contrasting effects of plastocyanin on the photoreduction and photooxidation of cytochrome f in chloroplasts

Removal of plastocyanin from Photosystem I subchloroplast particles had no effect on the Photosystem I photooxidation of cytochrome f. Chloroplasts depleted of plastocyanin by sonication lost the ability to reduce cytochrome f in Photosystem II light. Addition of plastocyanin restored the photoreduction of cytochrome f. These results are consistent with a plastocyanin site on the reducing side of cytochrome f.     

Steady State of Photosynthetic Electron Transport in Cells of the Cyanophyti Synechocystis PCC 6714 Having Different Stoichiometry between PS I and PS II: Analysis of Flash-Induced Oxidation-Reduction of Cytochrome f and P700 under Steady State of Photosynthesis

The steady state of photosynthetic electron transport drivenby two photosystems was studied with cells of the cyanophyteSynechocystis PCC 6714 by analyzing the flash-induced oxidation-reductionof Cyt f and P700 under continuous background illumination.We first analyzed the spectra and the kinetics of flash-inducedabsorption changes in the 400 to 440 nm wavelength region anddefined the absorption changes due to oxidation-reduction ofCyt f and P700. Results indicated that the flash-induced absorptionchanges at 420 and 435 nm are due to the oxidation-reductionof Cyt f and P700, respectively. Determination of the steadystate of Cyt f (420 nm) and P700 (435 nm) was made for the cellsgrown under a weak orange light exciting mainly PS II (PS IIlight) and having a high ratio of PS I to PS II (PS I/PS II),and those grown under a weak red light exciting preferentiallyPS I (PS I light) and having a low PS I/PS II. The steady stateof electron transport in cells of the two types were comparedunder PS I and PS II lights. The results indicated that: (1)under the light conditions used for growth (both red and orangelight), the intermediate electron pool between the two photosystemsremained in a redox state so as to keep both photosystems inthe open state. (2) When shifted to PS I light, the intermediatepool and PS I in cells of high PS I/PS II became extremely electron-poor,and so most of the PS I reaction centers were closed. (3) Theintermediate pool in cells of low PS I/PS II became extremelyelectron-rich when shifted to PS II light, and most of the PSII reaction centers were closed. The electron transport stateis released from such biased states by regulation of PS I/PSII. Results supported our previously proposed hypothesis thatthe stoichiometry between PS I and PS II is regulated so asto keep the two photosystems in the open state. The relationshipbetween the steady state of electron transport and the regulationof PS I/PS II is discussed. (Received August 2, 1990; Accepted December 10, 1990)     

Light-induced oxidation-reduction reactions in a cell-free preparation from the blue-green alga Nostoc muscorum: The role of cytochrome f, cytochrome b558, C550, and P700 in noncyclic electron transport

1. Cytochrome f (λ_max = 554 nm, E_m = +0.35 V) and cytochrome b₅₅₈ (λ_max = 558 nm, E_m = +0.35 V) were photooxidized by Photosystem I and photoreduced by Photosystem II in a cell-free preparation from the blue-green alga Nostoc muscorum. The steady-state oxidation levels of both cytochromes were affected by noncyclic electron acceptors and by inhibitors of noncyclic electron transport. These results are consistent with the hypothesis that the mechanism of NADP reduction by water involves a Photosystem II and a Photosystem I light reaction operating in series and linked by a chain of electron carriers that includes cytochrome f and cytochrome b₅₅₈.2. Phosphorylation cofactors shifted the steady-state of cytochrome f to a more reduced level under conditions of noncyclic electron transport but had no effect on cytochrome b₅₅₈. These observations suggest that the noncyclic phosphorylation site lies before cytochrome f (on the Photosystem II side) and that cytochrome f is closer to this site than is cytochrome b₅₅₈.3. A Photosystem II photoreduction of C550 at 77 °K was observed, suggesting that in blue-green algae, as in other plants, C550 is closely associated with the primary electron acceptor for Photosystem II. A Photosystem I photooxidation of P700 at 77 °K was observed, consistent with P700 serving as the primary electron donor of Photosystem I.     

Studies on photosystem I. II. Involvement of ferredoxin in cyclic electron flow

The effects of ferredoxin (Fd) and ferredoxin-NADP reductase on the light-induced spectral changes of cytochrome f (cyt f) were investigated with specific reference to their possible involvement in the cyclic electron transfort pathway of photosystem I (PS I). The steady-state level of photooxidation of reduced cytochrome f is decreased by ferredoxin but unaffected by either ferredoxin-NADP reductase alone or ferredoxin plus ferredoxin-NADP reductase when present in equimolar concentrations. These data are taken as evidence for a cyclic electron transport pathway of: PS I → “X” → Fd → (cyt f) → PC → PS I. The reduced ferredoxin could either reduce directly plastocyanin (PC) or via cytochrome f; the data do not allow differentiation between these two possibilities. However, neither ferredoxin-NADP reductase nor cytochrome b564 appear to serve as electron carriers in this pathway.     

The relationship between changes in the redox state of plastoquinone and control of excitation energy distribution in photosynthesis

The hypothesis that excitation energy distribution between PS I and PS II is controlled by the redox state of the plastoquinone pool between the two photosystems was investigated using the green alga Chlorella vulgaris. Changes in the redox state of the pool were monitored by measurement of the area above the fluorescence induction curve on exposure to high-intensity light. In agreement with the hypothesis, exposure of state I adapted cells to light preferentially absorbed by PS II led to a reduction of the plastoquinone pool whilst exposure of State II adapted cells to light preferentially absorbed by PS I resulted in its oxidation. However, the limits within which these fluctuations occurred were much narrower than anticipated. The reasons for this are discussed in terms of the possible involvement of changes in the redox state of more specialised molecules associated with the main plastoquinone pool and the postulated role of plastoquinone as an electron shuttle between the two photosystems.     

Characterization of synchronized cultures of Bumilleriopsis filiformis

Activity of the photosynthetic apparatus of synchronized cultures was studied with the xanthophycean alga Bumilleriopsis filiformis, following the kinetics of fluorescence induction and photooxidation of cytochrome f (= cytochrome c-553) of intact cells. During the beginning of the cell-division phase, minimum cellular photosynthetic activity is observed and a maximum after its completion, which is accompanied by corresponding changes in Hill reaction activity and re-reduction of cytochrome f by photosystem II light. At minimum activity, the level of steady state fluorescence was higher than at the maximum. This is due, at least in part, to the diminished electron flow between the two photosystems seemingly caused by decreased photosystem I activity. This explanation was suported by the kinetics of cytochrome-f photooxidation.Thus, electron transport activity of both photosystems appears to vary during the cell cycle.Abbreviations pBQ p-benzoquinone - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DCIP dichlorophenolindophenol - MV methylviologen (paraquat) - Q fluorescence quencher (in photosystem II)     

ApcD is necessary for efficient energy transfer from phycobilisomes to photosystem I and helps to prevent photoinhibition in the cyanobacterium Synechococcus sp. PCC 7002

Phycobilisomes (PBS) are the major light-harvesting, protein-pigment complexes in cyanobacteria and red algae. PBS absorb and transfer light energy to photosystem (PS) II as well as PS I, and the distribution of light energy from PBS to the two photosystems is regulated by light conditions through a mechanism known as state transitions. In this study the quantum efficiency of excitation energy transfer from PBS to PS I in the cyanobacterium Synechococcus sp. PCC 7002 was determined, and the results showed that energy transfer from PBS to PS I is extremely efficient. The results further demonstrated that energy transfer from PBS to PS I occurred directly and that efficient energy transfer was dependent upon the allophycocyanin-B alpha subunit, ApcD. In the absence of ApcD, cells were unable to perform state transitions and were trapped in state 1. Action spectra showed that light energy transfer from PBS to PS I was severely impaired in the absence of ApcD. An apcD mutant grew more slowly than the wild type in light preferentially absorbed by phycobiliproteins and was more sensitive to high light intensity. On the other hand, a mutant lacking ApcF, which is required for efficient energy transfer from PBS to PS II, showed greater resistance to high light treatment. Therefore, state transitions in cyanobacteria have two roles: (1) they regulate light energy distribution between the two photosystems; and (2) they help to protect cells from the effects of light energy excess at high light intensities.     

A new mechanism for adaptation to changes in light intensity and quality in the red alga,Porphyra perforata. I. Relation to State 1—State 2 transitions

Time courses of chlorophyll fluorescence and fluorescence spectra at 77 K after various light treatments were measured in the red alga, Porphyra perforata. Photosystem (PS) I or II light (light 1 or 2) induced differences in the fluorescence spectra at 77 K. Light 2 decreased the two PS II fluorescence bands (F-685 and F-695) in parallel, while light 1 preferentially increased F-695. Light 1 and 2 also produced different effects on the activities of PS I and II. Preillumination with light 1 increased PS II activity and decreased PS I activity. However, preillumination with light 2 decreased PS II activity with no effect on PS I activity. These results show that there are at least two mechanisms that can alter the transfer of light energy in P. perforata. The dark state in this alga was found to be State 2 and light 1 induced a State 2-State 1 transition which retarded the transfer of light energy from PS II to PS I. Light 2 induced another change (which we have called a State 2-State 3 transition) that was accompanied by a change only in PS II activity.     

High Yield Non-detergent Isolation of Photosystem I-Light-harvesting Chlorophyll II Membranes from Spinach Thylakoids: IMPLICATIONS FOR THE ORGANIZATION OF THE PS I ANTENNAE IN HIGHER PLANTS*

Styrene-maleic acid copolymer was used to effect a non-detergent partial solubilization of thylakoids from spinach. A high density membrane fraction, which was not solubilized by the copolymer, was isolated and was highly enriched in the Photosystem (PS) I-light-harvesting chlorophyll (LHC) II supercomplex and depleted of PS II, the cytochrome b₆/f complex, and ATP synthase. The LHC II associated with the supercomplex appeared to be energetically coupled to PS I based on 77 K fluorescence, P₇₀₀ photooxidation, and PS I electron transport light saturation experiments. The chlorophyll (Chl) a/b ratio of the PS I-LHC II membranes was 3.2 ± 0.9, indicating that on average, three LHC II trimers may associate with each PS I. The implication of these findings within the context of higher plant PS I antenna organization is discussed.     

Vanadium in photosynthesis of Chlorella fusca and higher plants

The influence of vanadium compounds (vanadate, vanadyl citrate) on photosynthesis in Chlorella fusca and in algal and spinach chloroplasts has been investigated. It was found that: 1. At moderately high concentrations (at least 0.1 mM) both vanadate and vanadyl citrate enhance photosynthetic O₂ production in intact C. fusca cells. At lower V concentration (about 2 μM) only vanadate stimulates photosynthesis. The increase is dependent on culture conditions and on light intensity. 2. Up to 1 mM V, neither vanadium compound influences PS II activity, either in intact cells or in algal or spinach chloroplasts. 3. The PS I reaction in algal and spinach chloroplasts is maximally enhanced (3-fold) in presence of vanadium (20 μM). The increase is independent of light intensity. 4. Cr(VI), Mo(VI), and W(VI) (1 mM) stimulate photosynthesis in intact C. fusca cells, but do not influence the photosystems of isolated chloroplasts. Vanadium is suggested to act as a redox catalyst in the electron transport from PS II to PS I.     

Two electrogenic mechanisms contributing to the 560 nm absorption changes in intact Bryopsis chloroplasts

Light -induced absorbance changes at 560 nm in dark-adapted intact chloroplasts of the green alga, Bryopsis maxima were studied in the time range of 200 ms. The initial rise of the 560 nm signals constists of two major components which are both electrochromic absorbance changes of the carotenoids, sipnonein and/or siphonaxanthin, but different in mechanisms of the field formation.The first component (component S) is related to electron transport since it was sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) and showed a light-intensity dependence similar to that of electron transport in chloroplasts. In the presence of DCMU, component S could be restored on addition of proton-transporting electron donors such as reduced 2,6-dichlorophenol indophenol and phenazine methosulfate, but not on addition of N,N,N′,N′-tetramethyl-p-phenylenediamine which does not carry protons with electrons (Trebst, A. (1974) Annu. Rev. Plant Physiol. 25, 423–458). We propose that component S is due to the electric field set up by the proton translocation across the thylakoid membrane.The second component (component R) was resistant to DCMU and DBMIB. The light-intensity dependency of component R was similar to that of cytochrome f photooxidation which showed saturation at a relatively low light intensity. The magnitude of component R was markedly reduced by phenylmercuric acetate, suggesting the participation of ferredoxin and ferredoxin-NADP oxidoreductase in the mechanism of the field formation responsible for this component. In the presence of DCMU and phenylmercuric acetate, time courses of the 560 nm changes paralleled those of cytochrome f changes. These results indicate that component R is due to the electric field formed between oxidized cytochrome f and other intersystem electron carriers located in the inner part of the thylakoid membrane and reduced electron acceptors of Photosystem I situated on the membrane surface.The complex natures of the 560 nm changes, as well as the contributions of Photosystems I and II to the absorbance changes, are explained in terms of the two electrogenic mechanisms.     

State transitions,photosystem stoichiometry adjustment and non-photochemical quenching in cyanobacterial cells acclimated to light absorbed by photosystem I or photosystem II

Cells of the cyanobacterium Synechococcus 6301 were grown in yellow light absorbed primarily by the phycobilisome (PBS) light-harvesting antenna of photosystem II (PS II), and in red light absorbed primarily by chlorophyll and, therefore, by photosystem I (PS I). Chromatic acclimation of the cells produced a higher phycocyanin/chlorophyll ratio and higher PBS-PS II/PS I ratio in cells grown under PS I-light. State 1-state 2 transitions were demonstrated as changes in the yield of chlorophyll fluorescence in both cell types. The amplitude of state transitions was substantially lower in the PS II-light grown cells, suggesting a specific attenuation of fluorescence yield by a superimposed non-photochemical quenching of excitation. 77 K fluorescence emission spectra of each cell type in state 1 and in state 2 suggested that state transitions regulate excitation energy transfer from the phycobilisome antenna to the reaction centre of PS II and are distinct from photosystem stoichiometry adjustments. The kinetics of photosystem stoichiometry adjustment and the kinetics of the appearance of the non-photochemical quenching process were measured upon switching PS I-light grown cells to PS II-light, and vice versa. Photosystem stoichiometry adjustment was complete within about 48 h, while the non-photochemical quenching occurred within about 25 h. It is proposed that there are at least three distinct phenomena exerting specific effects on the rate of light absorption and light utilization by the two photoreactions: state transitions; photosystem stoichiometry adjustment; and non-photochemical excitation quenching. The relationship between these three distinct processes is discussed.Abbreviations Chl chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F relative fluorescence intensity at emission wavelength nm - F _o fluorescence intensity when all PS II traps are open - light 1 light absorbed preferentially by PS I - light 2 light absorbed preferentially by PS II - PBS phycobilisome - PS photosystem     

State 1-state 2 transition in leaves and its association with ATP-induced chlorophyll fluorescence quenching

By using chlorophyll fluorescence, a study has been made of changes in spillover of excitation energy from Photosystem (PS) II to PS I associated with the State 1–State 2 transition in intact pea and barley leaves and in isolated envelope-free chloroplasts treated with ATP. (1) In pea leaves, illumination with light preferentially absorbed by PS II (Light 2) led to a condition of maximum spillover (state 2) while light preferentially absorbed by PS I induced minimum spillover condition (State 1) as judged from the redox state of Q and low-temperature emission spectra. The State 1–State 2 transitions took several minutes to occur, with the time increasing when the temperature was lowered from 19 to 6°C. (2) In contrast to the wild type, leaves of a chlorophyll b-less mutant barley did not exhibit a State 1–State 2 transition, suggesting the involvement of the light-harvesting chlorophyll $\text{a}\text{b-}\text{protein}$ complex in spillover changes in higher plants. (3) Spillover in isolated pea chloroplasts was increased by treatment with ATP either (a) in Light 2 in the absence of an electron acceptor or (b) in the dark in the presence of NADPH and ferredoxin. These observations can be interpreted in terms of the model that a more reduced state of plastoquinone activates the protein kinase which catalyzes phosphorylation of the light-harvesting chlorophyll $\text{a}\text{b-}\text{protein}$ complex (Allen, J.F., Bennett, J., Steinback, K.E. and Arntzen, C.J. (1981). Nature 291, 25–29). This process was found to be very temperature sensitive. (4) Pea chloroplasts illuminated in the presence of ATP seemed to exhibit a slight decrease in the degree of thylakoid stacking, and an increased intermixing of the two photosystems. (5) The possible mechanism by which protein phosphorylation regulates the State 1–State 2 changes in intact leaves is presented in terms of changes in the spatial relationship of two photosystems resulting from alteration in membrane organization.     

Analysis of the Chloroplast Protein Kinase Stt7 during State Transitions

State transitions allow for the balancing of the light excitation energy between photosystem I and photosystem II and for optimal photosynthetic activity when photosynthetic organisms are subjected to changing light conditions. This process is regulated by the redox state of the plastoquinone pool through the Stt7/STN7 protein kinase required for phosphorylation of the light-harvesting complex LHCII and for the reversible displacement of the mobile LHCII between the photosystems. We show that Stt7 is associated with photosynthetic complexes including LHCII, photosystem I, and the cytochrome b₆f complex. Our data reveal that Stt7 acts in catalytic amounts. We also provide evidence that Stt7 contains a transmembrane region that separates its catalytic kinase domain on the stromal side from its N-terminal end in the thylakoid lumen with two conserved Cys that are critical for its activity and state transitions. On the basis of these data, we propose that the activity of Stt7 is regulated through its transmembrane domain and that a disulfide bond between the two lumen Cys is essential for its activity. The high-light–induced reduction of this bond may occur through a transthylakoid thiol–reducing pathway driven by the ferredoxin-thioredoxin system which is also required for cytochrome b₆f assembly and heme biogenesis.     

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.     

Photoreactions of cytochrome b-559 and cyclic electron flow in Photosystem II of intact chloroplasts

The high potential cytochrome b-559 of intact spinach chloroplasts was photooxidized by red light with a high quantum efficiency and by far-red light with a very low quantum efficiency, when electron flow from water to Photosystem II was inhibited by a carbonyl cyanide phenylhydrazone (FCCP or CCCP). Dithiothreitol, which reacts with FCCP or CCCP, reversed the photooxidation of cytochrome b-559 and restored the capability of the chloroplasts to photoreduce CO₂ showing that the FCCP/CCCP effects were reversible. The quantum efficiency of cytochrome b-559 photooxidation by red or far-red light in the presence of FCCP was increased by 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone which blocks oxidation of reduced plastoquinone by Photosystem I. When the inhibition of water oxidation by FCCP or CCCP was decreased by increased light intensities, previously photooxidized cytochrome b-559 was reduced. Red light was much more effective in photoreducing oxidized high potential cytochrome b-559 than far-red light. The red/far-red antagonism in the redox state of cytochrome b-559 is a consequence of the different sensitivity of the cytochrome to red and far-red light and does not indicate that the cytochrome is in the main path of electrons from water to NADP. Rather, cytochrome b-559 acts as a carrier of electrons in a cyclic path around Photosystem II. The redox state of the cytochrome was shifted to the oxidized side when electron transport from water became rate-limiting, while oxidation of water and reduction of plastoquinone resulted in its shifting to the reduced side.     

Changes in the antenna size of Photosystem I and Photosystem II in Synechococcus sp. strain PCC 7942 grown in the presence of SANDOZ 9785 — a Photosystem II inhibitor

SANDOZ 9785, also known as BASF 13.338, is a pyridazinone derivative that inhibits Photosystem II (PS II) activity leading to an imbalance in the rate of electron transport through the photosystems. Synechococcus sp. strain PCC 7942 cells grown in the presence of sublethal concentration of SANDOZ 9785 (SAN 9785) for 48 hours exhibited a 20% decrease in Chl a per cell. However, no changes were observed in the content of phycocyanin per cell, the size of the phycobilisomes or in the PS II:PS I ratio. From an estimate of PS II electron transport rate under varying light intensities and spectral qualities and analysis of room temperature Chl a fluorescence induction, it was deduced that growth of Synechococcus PCC 7942 in the presence of SAN 9785 leads to a redistribution of excitation energy in favour of PS II. Though the redistribution appears to be primarily caused by changes affecting the Chl a antenna of PS II, the extent of energetic coupling between phycobilisomes and PS II is also enhanced in SAN 9785 grown Synechococcus PCC 7942 cells. There was a reduction in the effective size of PS I antenna based on measurement of P700 photooxidation kinetics. These results indicate that when PS II is partially inhibited, the structure of photosynthetic apparatus alters to redistribute the excitation energy in favour of PS II so that the efficiency of utilization of light energy by the two photosystems is optimized. Our results suggest that under the conditions used, drastic structural changes are not essential for redistribution of excitation energy between the photosystems.Abbreviations APC Allophycocyanin - Chl a chlorophyll a - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU 3-(3,4-dichlorophyenyl)-1,1-dimethyl urea - DCIP 2,6-dichlorophenolindophenol - F_o fluorescence when all the reaction centres are open - f_m fluorescence yield when all the reaction centres are closed - F_v variable chlorophyll fluorescence - HEPES N-2-Hydroxyethylpiperazine-N-2-ethanesulphonic Acid - I₅₀ concentration that causes 50% inhibition in activity - MV methyl viologen - pBQ para benzoquinone - PBS phycobilisome - PC phycocyanin - PS I, PS II Photosystem I, Photosystem II - P700 reaction centre Chl a of PS I - SAN 9785 SANDOZ 9785 i.e. 4-chloro-5-dimethylamino-2-phenyl-3 (2H) pyridazinone, also known as BASF 13.338     

Inhibition of photosynthetic electron transport and formation of inactive chlorophyll in winter stressed Pinus silvestris

Kinetics of fluorescence at room temperature, electron transport and photooxidation of P700 and cytochrome f have been studied in chloroplasts isolated from active and winter stressed Pinus silvestris. The winter stress induced block in the electron transport chain between the two photosystems is close to the site of plastoquinone, since winter stress and DCMU caused the same type of inhibition of the reoxidation of the primary electron acceptor Q of photosystem II. No winter inhibition of the electron transport between cytochrome f and P700 was observed. Time course studies of P700 photooxidation in chloroplasts of active and winter stressed pine have shown that the photosynthetic unit size must be about equal in the two types of chloroplasts. An apparent increase of the photosynthetic unit size was induced by winter stress, as revealed by the high chlorophyll/P700 ratio of winter stressed pine. The phenomenon is explained by the formation of photosynthetically inactive chlorophyll. Low-temperature fluorescence emission spectra were recorded when either chlorophyll a (433 nm) or chlorophyll b (477 nm) were preferentially excited. Winter stress induced the formation of a chlorophyll a fraction emitting at 673 nm. This chlorophyll is most likely derived from the chlorophyll a antennae of the two photosystems, and it probably contributes to the photosynthetically inactive pool of chlorophyll in winter stressed pine. The light harvesting chlorophyll a/b complex is relatively resistant to winter stress.     

Cytochrome f and plastocyanin kinetics in Chlorella pyrenoidosa. I. Oxidation kinetics after a flash

P-700, plastocyanin and cytochrome f redox kinetics were measured after one flash, using dark-adapted Chlorella in the presence of hydroxylamine and 3(3,4-dichlorophenyl)-1,1-dimethylurea. Plastocyanin becomes increasingly oxidized with a half-time of 70 μs, then undergoes reduction with a half-time of 7 ms. Cytochrome f oxidation has a sigmoidal time-course and a half-time of 100 μs. Its reduction exhibits a half-time of 4 ms. These results are interpreted in a linear scheme:

An equilibrium constant of 2 between cytochrome f and plastocyanin (PC), which contrasts with the large equilibrium constant between PC and P-700 is computed.The presence of cytochrome b₆ in a cyclic path around Photosystem I is confirmed under these conditions.