Oxygen reduction in a plastoquinone pool of isolated pea thylakoids

Oxygen uptake in isolated pea thylakoids in the presence of an inhibitor of plastoquinol oxidation by b ₆/f-complex dinitrophenylether of 2-iodo-4-nitrothymol (DNP-INT) was studied. The rate of oxygen uptake in the absence of DNP-INT had a distinct maximum at pH 5.0 followed by a decline to pH 6.5 and posterior slow rise, while in the presence of an inhibitor it increased at an increasing pH from 4.5 to 6.5 and then kept close to the rate in its absence up to pH 8.5. Gramicidin D substantially affected the oxygen uptake rate in the absence of DNP-INT, and only slightly in its presence. Such differences pointed to the presence of special oxygen reduction site(s) in photosynthetic electron transport chain `before' cytochrome complex. Oxygen uptake in membrane fragments of Photosystem II (BBY-particles) was low and did not depend on pH. This did not support the participation of Q_B in oxygen reduction in DNP-INT-treated thylakoids. Oxygen uptake in thylakoids in the presence of DNP-INT was inhibited by DCMU as well as by catalase in whole pH range. The catalase effect indicated that oxygen uptake was the result of dioxygen reduction by electrons derived from water, and that H₂O₂ was a final product of this reduction. Photoreduction of Cyt c in the presence of DNP-INT was partly inhibited by superoxide dismutase (SOD), and this pointed to superoxide formation. The latter was confirmed by a rise of the oxygen uptake rate in the presence of ascorbate and by suppression of this rise by SOD. Both tests showed that the detectable superoxide radicals averaged 20–25% of potentially formed superoxide radicals the quantity of which was calculated from the oxygen uptake rate. The obtained data implies that the oxygen reduction takes place in a plastoquinone pool and occurs mainly inside the membrane, where superoxide can be consumed in concomitant reactions. A scheme for oxygen reduction in a plastoquinone pool in thylakoid membranes is proposed. This revised version was published online in June 2006 with corrections to the Cover Date.     

The origin of photosystem-I-mediated electron transport stimulation in heat-stressed chloroplasts

Exposure of isolated chloroplasts of pea (Pisum sativum L.) to temperatures above 35° C leads to a stimulation of photosystem-I-mediated electron transport from dichlorophenolindophenol to methyl viologen. The threshold temperature for this stimulation coincides closely with that for heat-induced inhibition of photosystem-II activity in such chloroplasts. This coincidence is explained in terms of a rearrangement of the thylakoid membrane resulting in the exposure of a new set of donor sites for dichlorophenolindophenol within the cytochrome f/b ₆ complex of the electron-transport chain linking the two photosystems.Abbreviations cyt cytochrome - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCPIP (H₂) 2,6-dichlorophenolindophenol - EDAC ethyldimethylaminopropyl-carbodiimide - MV methyl viologen - PSI, II photosystem I, II - PCy plastocyanin - PQ(H₂) plastoquinone     

Cyclic electron flow around photosystem I in unicellular green algae

Cyclic electron flow around PSI, or cyclic photophosphorylation, is the photosynthetic process which recycles the reducing equivalents produced by photosystem I in the stroma towards the plastoquinone pool. Through the activity of cytochrome b ₆ f, which also transfers protons across the membrane, it promotes the synthesis of ATP. The literature dealing with cyclic electron flow in unicellular algae is far less abundant than it is for plants. However, in the chloroplast of algae such as Chlorella or Chlamydomonas, an efficient carbohydrate catabolism renders the redox poise much more reducing than in plant chloroplasts. It is therefore worthwhile highlighting the specific properties of unicellular algae because cyclic electron flow is highly dependent upon the accumulation of these stromal reducing equivalents. Such an increase of reducing power in the stroma stimulates the reduction of plastoquinones, which is the limiting step of cyclic electron flow. In anaerobic conditions in the dark, this reaction can lead to a fully reduced plastoquinone pool and induce state transitions, the migration of 80% of light harvesting complexes II and 20% of cytochrome b ₆ f complex from the PSII-enriched grana to the PSI-enriched lamella. These ultrastructural changes have been proposed to further enhance cyclic electron flow by increasing PSI antenna size, and forming PSI-cyt b ₆ f supercomplexes. These hypotheses are discussed in light of recently published data.     

Effects of winter stress on photosynthetic electron transport and energy distribution between the two photosystems of pine as assayed by chlorophyll fluorescence kinetics

The fluorescence kinetics of both intact needles and isolated chloroplasts of summer active and winter stressed Pinus sylvestris were measured at both room temperature and 77 K. It was confirmed that winter stress inhibited the photochemical capacity of photosystem II but also that winter stress caused the strongest inhibition of the electron transport at the site where the plastoquinone pool is reduced. Parallel analyses of the fluorescence characteristics of photosystem II (F₆₉₃) and photosystem I (F₇₂₉) during photosystem II trap closure furthermore revealed that the yield of spillover of excitation energy from photosystem II to photosystem I decreased upon winter stress. We suggest that this is because of an increased radiationless decay of excitation energy both at the reaction center and antennae levels of photosystem II. There is, however, also a possibility that the decreased yield of spill-over is accentuated by a partial detachment of the light harvesting chlorophyll a/b complex from photosystem II upon winter stress.Paper presented at the FESPP meeting in Strasbourg (1984).     

State 1-State 2 transitions in the cyanobacterium Synechococcus 6301 are controlled by the redox state of electron carriers between Photosystems I and II

The mechanism by which state 1-state 2 transitions in the cyanobacterium Synechococcus 6301 are controlled was investigated by examining the effects of a variety of chemical and illumination treatments which modify the redox state of the plastoquinone pool. The extent to which these treatments modify excitation energy distribution was determined by 77K fluorescence emission spectroscopy. It was found that treatment which lead to the oxidation of the plastoquinone pool induce a shift towards state 1 whereas treatments which lead to the reduction of the plastoquinone pool induce a shift towards state 2. We therefore propose that state transitions in cyanobacteria are triggered by changes in the redox state of plastoquinone or a closely associated electron carrier. Alternative proposals have included control by the extent of cyclic electron transport around PS I and control by localised electrochemical gradients around PS I and PS II. Neither of these proposals is consistent with the results reported here.Abbreviations DBMIB 2,5-dibromo-3methyl-6-isopropyl-p-benzoquinone - Chl chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DQH₂ duroquinol (tetramethyl-p-hydroquinone) - LHC II light-harvesting chlorophyll a/b-binding protein of PS II - Light 1 light predominantly exciting PS I - Light 2 light predominantly exciting PS II - M.V. methyl viologen - PS photosystem     

Changes in the mode of electron flow to photosystem I following chilling-induced photoinhibition in a C3 plant, Cucumis sativus L.

This study provides evidence for enhanced electron flow from the stromal compartment of the photosynthetic membranes to P700⁺ via the cytochrome b₆/f complex (Cyt b₆/f) in leaves of Cucumis sativus L. submitted to chilling-induced photoinhibition. The above is deduced from the P700 oxidation–reduction kinetics studied in the absence of linear electron transport from water to NADP⁺, cyclic electron transfer mediated through the Q-cycle of Cyt b₆/f and charge recombination in photosystem I (PSI). The segregation of these pathways for P700⁺ rereduction were achieved by the use of a 50-ms multiple turnover white flash or a strong pulse of white or far-red illumination together with inhibitors. In cucumber leaves, chilling-induced photoinhibition resulted in ∼20% loss of photo-oxidizible P700. The measurement of P700⁺ was greatly limited by the turnover of cyclic processes in the absence of the linear mode of electron transport as electrons were rapidly transferred to the smaller pool of P700⁺. The above is explained by integrating the recent model of the cyclic electron flow in C₃ plants based on the Cyt b₆/f structural data [Joliot and Joliot (2006) Biochim Biophys Acta 1757:362–368] and a photoprotective function elicited by a low NADP⁺/NAD(P)H ratio [Rajagopal et al. (2003) Biochemistry 42:11839–11845]. Over-reduction of the photosynthetic apparatus results in the accumulation of NAD(P)H in vivo to prevent NADP⁺-induced reversible conformational changes in PSI and its extensive damage. As the ferredoxin:NADP reductase is fully reduced under these conditions, even in the absence of PSII electron transport, the reduced ferredoxin generated during illumination binds at the stromal openings in the Cyt b₆/f complex and activates cyclic electron flow. On the other hand, the excess electrons from the NAD(P)H pool are routed via the Ndh complex in a slow process to maintain moderate reduction of the plastoquinone pool and redox poise required for the operation of ferredoxin:plastoquinone reductase mediated cyclic flow.     

Changes in Photosystem II Account for the Circadian Rhythm in Photosynthesis in Gonyaulax polyedra

Cell-free extracts that show activity in photosynthetic electron flow have been prepared from the unicellular dinoflagellate, Gonyaulax polyedra. Electron flow, as O₂ uptake, was measured through both photo-system I and II from water to methyl viologen, through photosystem I alone from reduced 2,6-dichlorophenol indophenol to methyl viologen which does not include the plastoquinone pool or from duroquinol to methyl viologen which includes the plastoquinone pool. Electron flow principally through photosystem II was measured from water to diaminodurene and ferricyanide, as O₂ evolution. Cultures of Gonyaulax were grown on a 12-hour light:12 hour dark cycle to late log phase, then transferred to constant light at the beginning of a light period. After 3 days, measurements of electron flow were made at the maximum and minimum of the photosynthetic rhythm, as determined from measurements of the rhythm of bioluminescence. Photosynthesis was also measured in whole cells, either as ¹⁴C fixation or O₂ evolution. Electron flow through both photosystems and through photosystem II alone were clearly rhythmic, while electron flow through photosystem I, including or excluding the plastoquinone pool, was constant with time in the circadian cycle. Thus, only changes in photosystem II account for the photosynthesis rhythm in Gonyaulax.     

State transitions at the crossroad of thylakoid signalling pathways

In order to maintain optimal photosynthetic activity under a changing light environment, plants and algae need to balance the absorbed light excitation energy between photosystem I and photosystem II through processes called state transitions. Variable light conditions lead to changes in the redox state of the plastoquinone pool which are sensed by a protein kinase closely associated with the cytochrome b ₆ f complex. Preferential excitation of photosystem II leads to the activation of the kinase which phosphorylates the light-harvesting system (LHCII), a process which is subsequently followed by the release of LHCII from photosystem II and its migration to photosystem I. The process is reversible as dephosphorylation of LHCII on preferential excitation of photosystem I is followed by the return of LHCII to photosystem II. State transitions involve a considerable remodelling of the thylakoid membranes, and in the case of Chlamydomonas, they allow the cells to switch between linear and cyclic electron flow. In this alga, a major function of state transitions is to adjust the ATP level to cellular demands. Recent studies have identified the thylakoid protein kinase Stt7/STN7 as a key component of the signalling pathways of state transitions and long-term acclimation of the photosynthetic apparatus. In this article, we present a review on recent developments in the area of state transitions.     

Cytochrome b6-f complex : The carburettor of exciton distribution in oxygenic photosynthesis

Efficient oxygenic photosynthesis not only requires synchronous turover and operation of photosystem I (PS I) and photosystem II (PS II) but also the preferential turnover of PS I for cyclic photophosphorylation to maintain required ATP and NADPH ratio during carbon dioxide reduction. Ohe initial higher rate of turnover of PS IIin viva is accounted by the fact that (i) PS I contains only about one-third of total chlorophylls, (ii) about 90% of light harvesting a/b protein (LAC) which accounts for about 50% of the total chlorophylls, remains associated with PS II as PS II-LHC II complexes (PS II_α and (iii) the ratio of PS II/PS I is always greater than unity, in the range of 1–2 : 1 under different environmental regimes. Ohe initial preferential feeding of PS II, due to its larger antenna, is bound to result in faster rate of turn over of PS II than PS I, leading to higher rate of reduction of an intersystem carrier than the rate of its oxidation by PS I. Ohe light dependent phosphorylation of a ‘mobile’ and small pool (−20%) of LHC II of PS II_α (possibly located at the edge of appressed regions of the membranes) increases the repulsive forces of LHC II resulting in its migration to non-appressed region associating itself with PS 1. Ohe phosphorylation itself is controlled by the redox state of an intermediate of electron transport. Several experimental approaches have provided evidence which suggest that (i) phosphorylation of LAC II involves interaction of cyt b5-f complex with LAC II kinase and the interaction of QA with cyt b₅-f complex and (ii) different kinases may be involved in phosphorylation of LHC IIversus PS II polypeptides. Ohe major purpose of light dependent LAC II phosphorylation and its consequent migration close to PS I appears to balance the rate of cyclicversus non-cyclic photophosphorylation. Ohe mechanism by which cyt b₅-f complex controls the activation of LAC II is not known. Ohe role of membrane bound ealmodulin, electron transfer through cyt b₆-f complex in activation of LAC II kinase should be explored.     

Divergent pathways of photosynthetic electron transfer: The autonomous oxygenic and anoxygenic photosystems

The aim of this article is to assemble and integrate, from a personal perspective of a research participant, seldom examined evidence that is incompatible with some basic tenets of photosynthetic electron transport, the cornerstone of which is the Z scheme. The nonconforming evidence pertaining to the mode of ferredoxin reduction and the role of the copper redox protein, plastocyanin, indicates that contrary to the Z scheme ferredoxin is reduced in two experimentally distinguishable ways: oxygenically by PS II (renamed the oxygenic photosystem), without the participation of PS I, and anoxygenically by PS I (renamed the anoxygenic photosystem). It also indicates that plastocyanin is not only, as the Z scheme asserts, the electron donor to the reaction center chlorophyll of PS I (P700) but also to the reaction center chlorophyll of PS II (P680). Other unconventional findings include evidence that the fully functional oxygenic photosystem, when operating separately from the anoxygenic photosystem, reduces plastoquinone to plastoquinol and subsequently oxidizes plastoquinol by two pathways acting in concert: one being the universally recognized DBMIB-sensitive pathway via the Rieske iron-sulfur center of the cytochrome bf complex and the other, a hitherto unrecognized, DBMIB-insensitive electron transport pathway around P680 that centers on cytochrome b-559. These nonconforming findings form the basis of an alternate hypothesis of photosynthetic electron transport that modifies and complements the Z scheme.Abbreviations PS photosystem - PQ oxidized plastoquinone - PQH₂ reduced plastoquinone (plastoquinol) - Q_A and Q_B specialized membrane-bound forms of PQ - PC plastocyanin - Fd ferredoxin - BISC F_AF_B, membrane-bound iron-sulfur centers of PS I - DBM1B 2,5-dibromo-3-methyl-6-isopropyl-n-benzoquinone (dibromothymoquinone) - DNP-INT dinitrophenol ether of iodonitrothymol - NADP⁺ NADPH, oxidized and reduced forms of nicotinamide adenine dinucleotide phosphate - FCCP carbonylcyanide-p-trifluoromethoxyphenyl-hydrazone - CCCP carbonyl cyanide-3-chlorophenylhydrazone - SF 6847 2,6,-di-(t-butyl)-4-(2,2-dicyanovinyl) phenol - diuron (DCMU) 3-(3,4-dichlorophenyl)-1,1-dimethylurea - EPR electron paramagnetic resonance - DCIP 2,6-dichloro-phenolindophenol - UHDBT 5-(n-undecyl)-6-hydroxy-4-7-dioxobenzothiazole; cytochrome b-559_HP-cytochrome b-559_LP, high- and low potential states of cytochrome b-559 - oxygenic reductions reductions in which water is the electron donor - BBY PS II preparation made according to Berthold et al. (1981) Dedicated to Professor Achim Trebst on his 65th birthday.Based in part on lecture in Advanced Course on Trends in Photosynthesis Research, Palma de Mallorca, Spain, September 18, 1990.     

The effect of inhibitors on the electron flow triggering photo-phobic reactions in cyanophyceae

1. Since photo-phobic reactions in the blue green alga Phormidium uncinatum seem to be triggered by changes of electron flow rates into or out of an electron pool situated in the electron transport chain between photosystem II and I, the effect of inhibitors affecting the electron transport chain has been studied.
2. Dose response curves of the phobic reaction have been measured by varying the trap energy in double beam light trap experiments with constant pairs of monochromatic light. From these dose response curves the effects of the inhibitors on both types of phobic reactions, i.e. exit reactions and entrance reactions, have been calculated.
3. Dibromothymoquinone (DBMIB) inhibits the electron transport between the electron pool and photosystem I by preventing the reoxidation of plastoquinone. The phobic entrance reaction, which results in an emptying of the light trap, is triggered by changes in the electron flow out of the pool; thus it is more effected by DBMIB than the exit reaction, which is mediated by the electron transport into the pool.
4. The phobic exit reaction, which results in accumulations in the light trap, is triggered by changes in the electron flow into the electron pool via photosystem II. 3-[3,4-dichlorophenyl]-1,1-dimethylurea (DCMU) inhibits the electron transport near photosystem II; thus it affects the exit reaction more than the entrance reaction.

     

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.     

Enhanced leaf photosynthesis as a target to increase grain yield: insights from transgenic rice lines with variable Rieske FeS protein content in the cytochrome b6/f complex

Although photosynthesis is the most important source for biomass and grain yield, a lack of correlation between photosynthesis and plant yield among different genotypes of various crop species has been frequently observed. Such observations contribute to the ongoing debate whether enhancing leaf photosynthesis can improve yield potential. Here, transgenic rice plants that contain variable amounts of the Rieske FeS protein _in the cytochrome (cyt) b₆/f complex between 10 and 100% of wild‐type levels have been used to investigate the effect of reductions of these proteins on photosynthesis, plant growth and yield. Reductions of the cyt b₆/f complex did not affect the electron transport rates through photosystem I but decreased electron transport rates through photosystem II, leading to concomitant decreases in CO₂ assimilation rates. There was a strong control of plant growth and grain yield by the rate of leaf photosynthesis, leading to the conclusion that enhancing photosynthesis at the single‐leaf level would be a useful target for improving crop productivity and yield both via conventional breeding and biotechnology. The data here also suggest that changing photosynthetic electron transport rates via manipulation of the cyt b₆/f complex could be a potential target for enhancing photosynthetic capacity in higher plants.     

Cytochrome b6f complex is required for phosphorylation of light-harvesting chlorophyll a/b complex II in chloroplast photosynthetic membranes

The light-harvesting chlorophyll a/b complex (LHC II) and four photosystem II (PS II) core proteins (8.3, 32, 34 and 44 kDa) become phosphorylated in response to reduction of the intersystem electron transport chain of green plant chloroplasts. Previous studies indicated that reduction of the plastoquinone (PQ) pool is the key event in kinase activation. However, we show here that, unlike PS II proteins, LHC II is phosphorylated only when the cytochrome b6f complex is active. Two lines of evidence support this conclusion. (1) 2,5-Dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) and the 2,4-dinitrophenyl ether of iodonitrothymol (DNP-INT), which are known to block electron flow into the cytochrome complex, selectively inhibit LHC II phosphorylation in spinach thylakoids. (2) The hcf6 mutant of maize, which contains PQ but lacks the cytochrome b6f complex, phosphorylates the four PS II proteins but fails to phosphorylate LHC II in vivo or in vitro.     

Inhibition of chlororespiration by myxothiazol and antimycin A in Chlamydomonas reinhardtii

Myxothiazol and antimycin A are shown to suppress the oxygen transient previously attributed to the flash-induced inhibition of chlororespiration in Chlamydomonas reinhardtii (Peltier et al. 1987, Biochim Biophys Acta 893: 83–90). However, these two compounds do not affect the photosynthetic electron transport chain as inferred by the insensitivity of the CO₂-dependent photosynthetic O₂ evolution and of the flash-induced electrochromic effect. Chlorophyll fluorescence induction measurements carried out in dark-adapted cells of a mutant of Chlamydomonas lacking photosystem 1, show that myxothiazol and antimycin A significantly increase the redox state of the photosystem 2 acceptors. We conclude from these results that chlororespiration is inhibited by myxothiazol and antimycin A and that the site of inhibition is located on the dark oxidation pathway of the plastoquinone pool. This inhibition is interpreted through the involvement of a myxothiazol and antimycin A sensitive cytochrome in the chlororespiratory chain.Abbreviations cyt cytochrome - PQ plastoquinone - PS photosystem     

Autotrophic synthesis of activated acetic acid from two CO2 inMethanobacterium thermoautotrophicum

A cell-free preparation of heterocysts from Anabaena variabilis showed high nitrogenase activities with several physiological electron donors, dependent on addition of an ATP-generating system. Light-induced acetylene reduction with the artificial electron donor to photosystem I, diaminodurol, exhibited the same light saturation as with hydrogen as donor. Inhibitors of electron flow through plastoquinone affected light-induced, hydrogen- or NADH-dependent nitrogenase activity in a similar way. Several uncoupling agents were without effect, indicating that energized membranes are not a prerequisite for nitrogen fixation. We conclude that NADH or hydrogen deliver electrons to nitrogenase via photosystem I and ferredoxin, feeding in at the plastoquinone site.In the light, addition of NADP induced a lag in H₂- or NADH-supported acetylene reduction apparently by competing with nitrogenase for electrons at the reducing side of photosystem I. Time reversal of this inibition reflects a regulation of photosystem I-dependent nitrogenase activity by the NADPH/NADP ratio in the cell. This was directly demonstrated by differently adjusted NADPH/NADP ratios.NADPH donates electrons to nitrogenase in the dark and in the light, the light reaction being DBMIB-sensitive. NADPH-supported acetylene reduction was inhibited by NADP. This inhibition was not reversed with time, pointing to an involvement of ferredoxin: NADP oxidoreductase (EC 1.18.1.2) in this pathway. Apparently, in the dark, this enzyme is able to directly reduce ferredoxin, whereas in the light electrons from NADPH first have to pass through photosystem I before reducing ferredoxin, hence nitrogenase.Intermediates of glycolysis, like glucose-6-phosphate, fructose-1,6-bisphosphate, and dihydroxyacetone phosphate supported nitrogenase activity in the dark, each with catalytic amounts of both NAD and NADP as equally effective cofactors.We conclude that in heterocysts electrons for nitrogen fixation are essentially supplied by dark reactions, mainly by glycolysis. NADH (and hydrogen) contribute electrons via photosystem I in the light, whereas the NADPH/NADP ratio regulates linear and cyclic electron flow at the reducing side of photosystem I to provide a ratio of ATP/electrons most effective for nitrogenase.Abbvreviations ATCC American Type Culture Collection - Diaminodurol (DAD) 2,3,5,6-tetramethyl-p-phenylenediamine dihydrochloride - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DNP-INT 2,4-dinitrophenyl ether of 2-iodo-4-nitrothymol - E Einstein (mol photons) - FNR ferredoxin - NADP oxidoreductase (EC 1.18.1.2) - HEPES N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid - Metronidazole 1-(2-hydroxyethyl)-2-methyl-5-nitroimidazole     

On an Inhibitor of Ferredoxin Catalyzed Cyclic Photophosphorylation in Thylakoid Membranes*

Dibromo- and diiodo-naphthoquinones are shown to be inhibitors of the cytochrome b₆/f complex in isolated thylakoid membranes from spinach chloroplasts. Dibromo-naphthoquinone inhibits ferredoxin catalyzed cyclic photophosphorylation at 0.1 μM concentrations, but non cyclic e-flow only at 10 μM. It does not inhibit cyclic systems with artifical cofactors, nor non-cyclic electron flow from duroquinol through photosystem I via the cytochrome b₆/f complex. Dibromo-naphthoquinone does however, lower the stoichiometry for ATP formation in the duroquinol donor system. This inhibitory pattern is quite different from that of DBMIB, but very similar to that of antimycin. This antimycin-like behaviour of these inhibitors is interpreted to indicate a) the existence of a Q_c site in the cytochrome b₆/f complex and its obligate function in ferredoxin catalyzed cyclic electron flow and b) a non-essential role of the Q_c site in non-cyclic electron flow, but which — when operative — pumps an extra proton across the thylakoid membrane increasing the ATP yield.     

Non-photochemical reduction of thylakoid photosynthetic redox carriers in vitro: Relevance to cyclic electron flow around photosystem I?

Non-photochemical (dark) increases in chlorophyll a fluorescence yield associated with non-photochemical reduction of redox carriers (F_npr) have been attributed to the reduction of plastoquinone (PQ) related to cyclic electron flow (CEF) around photosystem I. In vivo, this rise in fluorescence is associated with activity of the chloroplast plastoquinone reductase (plastid NAD(P)H:plastoquinone oxidoreductase) complex. In contrast, this signal measured in isolated thylakoids has been attributed to the activity of the protein gradient regulation-5 (PGR5)/PGR5-like (PGRL1)-associated CEF pathway. Here, we report a systematic experimentation on the origin of F_npr in isolated thylakoids. Addition of NADPH and ferredoxin to isolated spinach thylakoids resulted in the reduction of the PQ pool, but neither its kinetics nor its inhibitor sensitivities matched those of F_npr. Notably, F_npr was more rapid than PQ reduction, and completely insensitive to inhibitors of the PSII Q_B site and oxygen evolving complex as well as inhibitors of the cytochrome b₆f complex. We thus conclude that F_npr in isolated thylakoids is not a result of redox equilibrium with bulk PQ. Redox titrations and fluorescence emission spectra imply that F_npr is dependent on the reduction of a low potential redox component (E_m about − 340 mV) within photosystem II (PSII), and is likely related to earlier observations of low potential variants of Q_A within a subpopulation of PSII that is directly reducible by ferredoxin. The implications of these results for our understanding of CEF and other photosynthetic processes are discussed.     

Inhibition of electron transport at the cytochrome b(6)f complex protects photosystem II from photoinhibition

Photoinhibition of photosystem II (PS II) activity was studied in thylakoid membranes illuminated in the presence of the inhibitor of the cytochrome b(6)f complex 2'iodo-6-isopropyl-3-methyl-2',4, 4'-trinitrodiphenylether (DNP-INT). DNP-INT was found to decrease photoinhibition. In the absence of DNP-INT, anaerobosis, superoxide dismutase and catalase protected against photoinhibition. No effect of these treatments was observed in the presence of DNP-INT. These data demonstrate that photoinhibition under these conditions is caused by reactive oxygen species which are formed most probably by the reduction of oxygen at photosystem I. The results are discussed in terms of the importance of photosynthetic control in protection against photoinhibition in vivo.     

Competition between the photosynthetic and the (chloro)respiratory electron transport chains in cyanobacteria, green algae and higher plants. Effect of heat stress

By measuring the effect of cyanide on the flash-induced redox reactions of the cytochrome (cyt) b ₆/f complex we carried out a comparative study in order to characterize the interaction between the photosynthetic and the respiratory electron transport systems in cyanobacterial (Synechococcus sp. PCC 6301) and green algal (Chlamydomonas reinhardtii) cells, and in tobacco (Nicotiana tabacum L. cv. Petit Havana SR1) protoplasts. We found that the addition of 1 mM KCN resulted in a significant acceleration of the rereduction-rate of cyt f ⁺. This enhancement of the activity of the cyt b ₆/f complex apparently occurred with the same mechanism in prokaryotes and eukaryotes, and its dependence on the concentration of KCN in eukaryotes ruled out an origin in mitorespiration, superoxide dismutase and plastocyanin, strongly suggesting that a cyanide-sensitive terminal oxidase, a putative component of chlororespiration, competes with photosystem 1 (PS1) for electrons from the plastoquionone (PQ) pool. Concerning the physiological role of the competition between the (chloro)respiratory and the photosynthetic electron transport systems, our data obtained with cyanobacterial and algal cells incubated at elevated temperatures (30–50 °C) showed that the respiratory control over photosynthesis became significant in cells exposed to heat-stress. This revised version was published online in August 2006 with corrections to the Cover Date.