The cyclic electron pathways around photosystem I in Chlamydomonas reinhardtii as determined in vivo by photoacoustic measurements of energy storage

The photoacoustic technique was used to measure energy storage by cyclic electron transfer around photosystem I in intact Chlamydomonas reinhardtii cells illuminated with far-red light (>715 nm). The in-vivo cyclic pathway was characterized by investigating the effects of various chemicals on energy storage. Participation of plastoquinone and ferredoxin in the cyclic electron flow was confirmed by the complete suppression of energy storage in the presence of the plastoquinol antagonist 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) and the ferredoxin inhibitors/competitors methylviologen, phenylmercuric acetate and p-benzoquinone. Two alternative electron cycles are demonstrated to operate in vivo. One cycle is sensitive to antimycin A, myxothiazol and 2-(n-heptyl)-4-hydroxyquinoline N-oxide (HQNO) and is catalyzed by ferredoxin which reduces plastoquinone through a route involving cytochrome b ₆ and its protonmotive Q-cycle. The other cycle is unaffected by the above-mentioned inhibitors but is sensitive to N-ethylmaleimide (NEM), an inhibitor of the ferredoxin-NADP reductase, and 2-monophosphoadenosine-5-diphosphoribose (PADR), an analogue of NADP, showing that the electron recycling was mediated by NADPH. Possibly, electrons enter the plastoquinone pool through the action of a NAD(P)H dehydrogenase, which is insensitive to classical inhibitors of the mitochondrial NADH dehydrogenase. Loss of energy storage by photosystem-I-driven cyclic electron transfer in farred light was observed only when antimycin A, myxothiazol or HQNO was used in combination with NEM or PADR. Analysis of the light-intensity dependence and the rate of in-vivo cyclic electron transfer in the presence of various inhibitors indicates that the NADPH-dependent electron-cycle is the preferential cyclic pathway in Chlamydomonas cells illuminated with far-red light.Abbreviations A^max maximal photothermal signal - Cyt cytochrome - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU (diuron) 3-(3,4-dichlorophenyl)-1,1-dimethylurea - ES photochemical energy storage - FNR ferredoxin NADP⁺ reductase - HQNO 2-(n-heptyl)-4-hydroxyquinoline N-oxide - NEM N-ethylmaleimide - P₇₀₀ reaction-center pigment of PSI - PADR 2-monophosphoadenosine-5-diphosphoribose - pBQ p-benzoquinone - PMA phenylmercuric acetate We are very grateful to Dr. M.-H. Montane (Cadarache, Saint-Paul-lez-Durance, France) for her advice in the electroporation experiments.     

NAD(P)H Dehydrogenase-Dependent, Antimycin A-Sensitive Electron Donation to Plastoquinone in Tobacco Chloroplasts

Cyclic electron transport around PSI through the NAD(P)H dehydrogenasecomplex (NDH) in tobacco leaf disks, measured as an increasein the dark level of Chl fluorescence after the onset of darkness,was inhibited by antimycin A, an inhibitor of ferredoxin quinonereductase (FQR), suggesting that antimycin A inhibits not onlythe FQR-mediated cyclic flow but also the NDH-dependent flow.This electron flow was inhibited also by amytal, an inhibitorof mitochondrial NDH and by nigericin. The reduction of plastoquinonewas detected when NADPH and ferredoxin were added to the suspensionof the osmotically ruptured chloroplasts of the wild type andNDH-defective mutant. Because the addition of NADPH alone didnot induce the reduction, membrane-bound ferredoxin NADP⁺reductase(FNR) was supposed to reduce ferredoxin, which may be a moredirect electron donor for the plastoquinone reduction. The presenceof two types of reducing enzymes was suggested from the bi-phasicinhibition of plastoquinone reduction by antimycin A in thewild type. It is proposed that the reducing activity inhibitedby antimycin A at a low concentration is attributed to FQR andthe less sensitive activity to NDH. (Received June 29, 1998; Accepted September 7, 1998)     

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.     

Effects of 2,5-Dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) on Light Induced and Glycolytic Phosphate Uptake in Scenedesmus

The effects of DBMIB on photophosphorylation and glycolysis in Scenedesmus obtusiusculus Chod. were investigated by measuring the uptake of inorganic phosphate. To analyze the effects of DBMIB on the different energy coupling possibilities in open chain and cyclic photophosphorylation, DBMIB was given to the algae in narrow concentration intervals between 10^?6M to 10^?4M, either alone, or in combination with DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) or desaspidin. DBMIB inhibits non-cyclic as well as cyclic photophosphorylation in Scenedesmus. However, the DCMU resistant photophosphorylation reactions are less sensitive to DBMIB than the open chain photophosphorylating system in non-DCMU treated cells. Low concentrations of DBMIB even released a part of the DCMU inhibition. Experiments with combinations of DBMIB and desaspidin also indicated that cyclic photophosphorylation is less sensitive to DBMIB than non-cyclic. The inhibition of DCMU resistant cyclic phosphorylation by DBMIB, which is a competitive inhibitor of quinones, indicated a participation of plastoquinones in this type of energy coupling as well as in the non-cyclic and DCMU-sensitive processes. The cyclic and the non-cyclic photophosphorylation pathways probably use different parts of the plastoquinone pool. For the purpose of the experiments, it was necessary to produce data for the effect of DBMIB (10^?6–10^?4M) on glycolysis. The highest concentration gave 50% inhibition.     

Three Substrate Binding Sites on Spinach Ferredoxin:NADP+ Oxidoreductase. Studies with Selectively Acting Inhibitors

The effects of phenylmercuric acetate (PMA) and apoferredoxin (apoFd) on the diaphorase activity of spinach ferredoxin:NADP⁺ oxidoreductase (FNR) in the presence of dibromothymoquinone (DBMIB) or cytochrome c (Cyt c) were studied. PMA inhibited effectively (I₅₀ = < 5 M) ferredoxin-dependent Cyt c reduction but did not affect evidently the enzyme activity in the presence of DBMIB as an electron acceptor. ApoFd caused also inhibition of Cyt c reduction but slightly stimulated, like ferredoxin, DBMIB reduction. We confirm a hypothesis according to which three binding sites for substrates [NADP(H), Fd-Cyt c, quinone/dichlorophenol indophenol] occur within the molecule of isolated FNR.     

Sulfide-quinone and sulfide-cytochrome reduction in Rhodobacter capsulatus

The reduction by sulfide of exogenous ubiquinone is compared to the reduction of cytochromes in chromatophores of Rhodobacter capsulatus. From titrations with sulfide values for V_max of 300 and 10 moles reduced/mg bacteriochlorophyll a·h, and for K_m of 5 and 3 M were estimated, for decyl-ubiquinone-and cytochrome c-reduction, respectively. Both reactions are sensitive to KCN, as has been found for sulfide-quinone reductase (SQR) in Oscillatoria limnetica, which is a flavoprotein. Effects of inhibitors interfering with quinone binding sites suggest that at least part of the electron transport from sulfide in R. capsulatus employs the cytochrome bc ₁-complex via the ubiquinone pool.Abbreviations BChl a bacteriochlorophyll a - DAD diaminodurene - decyl-UQ decyl-ubiquinone - LED light emitting diode - NQNO 2-n-nonyl-4-hydroxyquinoline-N-oxide - PQ-1 plastoquinone 1 - SQR sulfide-quinone reductase (E.C. 1.8.5.'.) - UQ ubiquinone 10 - Q_c the quinone reduction site on the cytochrome b ₆ f/bc ₁, complex (also termed Q_i or Q_r or Q_n) - Q_s the quinone reduction site on SQR - Q_z quinol oxidation site on the b ₆ f/bc ₁, complex (also termed Q_o or Q_p)     

Plastoquinone as a common link between photosynthesis and respiration in a blue-green alga

The role of plastoquinone in a thermophilic blue-green alga, Shynechococcus sp., was studied by measuring reduction kinetics of cytochrome 553 which was oxidized with red flash preferentially exciting photosystem I. Sensitivity of the cytochrome reduction to DBMIB indicates that cytochrome 553 accepts electrons from reduced plastoquinone. Plastoquinone is in turn reduced in cells without electrons from photosystem II, since DCMU, which inhibited methyl viologen photoreduction more strongly than DBMIB, failed to affect the cytochrome reduction. Participation of cyclic electron transport around photosystem I in cytochrome reduction in the presence of DCMU was excluded, because methyl viologen and antimycin A had no effect on the cytochrome kinetics. On the other hand, electron donation from endogenous substrates to plastoquinone was suggested from decreases in rate of the cytochrome reduction by dark starvation of cells and also from restoration of fast reduction kinetics by the addition of exogenous substrates to or by reillumination of starved cells.KCN, which completely suppressed respiratory O₂-uptake, induced a marked acceleration of the cytochrome reduction in starved cells. The poison was less or not effective in stimulating the cytochrome reduction in more extensively starved or reilluminated cells.Results indicate that plastoquinone is functioning not only in the photosynthetic but also in the respiratory electron transport chain, thereby forming a common link between the two energy conservation systems of the blue-green alga.

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Regulation of photosynthetic cyclic electron flow pathways by adenylate status in higher plant chloroplasts

Cylic electron flow (CEF) around Photosystem I in photosynthetic eukaryotes is likely to be necessary to augment ATP production, rapidly- and precisely balancing the plastid ATP/NADPH energy budget to meet the demands of downstream metabolism. Many regulatory aspects of this process are unclear. Here we demonstrate that the higher plant plastid NADH/Fd:plastoquinone reductase (NDH) and proposed PGR5/PGRL1 ferredoxin:plastoquinone reductase (FQR) pathways of CEF are strongly, rapidly and reversibly inhibited in vitro by ATP with K_i values of 670 μM and 240 μM respectively, within the range of physiological changes in ATP concentrations. Control experiments ruled out effects on secondary reactions, e.g. FNR- and cytochrome b₆f activity, nonphotochemical quenching of chlorophyll fluorescence etc., supporting the view that ATP is an inhibitor of CEF and its associated pmf generation and subsequent ATP production. The effects are specific to ATP, with the ATP analog AMP-PNP showing little inhibitory effect, and ADP inhibiting only at higher concentrations. For the FQR pathway, inhibition was found to be classically competitive with Fd, and the NDH pathway showing partial competition with Fd. We propose a straightforward model for regulation of CEF in plants in which CEF is activated under conditions when stromal ATP low, but is downregulated as ATP levels build up, allowing for effective ATP homeostasis. The differences in K_i values suggest a two-tiered regulatory system, where the highly efficient proton pumping NDH is activated with moderate decreases in ATP, with the less energetically-efficient FQR pathway being activated under more severe ATP depletion.     

Two molecular forms of pea ferredoxin in the electron transport chain of chloroplasts

The effects of two molecular forms of water-soluble ferredoxin (Fd I and Fd II) on the kinetics of electron transport in bean chloroplasts (class B) were studied. The light-induced redox transitions of the photosystem I reaction center P700 were measured by the intensity of the EPR signal I produced by P700+. Both forms of ferredoxin, Fd I and Fd II, when added to the chloroplasts in catalytic amounts, stimulate the light-induced electron transfer from P700 to NADP+. Nevertheless, Fd I is a better mediator of the back reactions from NADPH to P700+. This electron transfer pathway is sensitive to the cyclic electron transport inhibitor, antimycin A, and to DCMU inhibitor of electron transport between photosystem II and plastoquinone. It may be concluded that the two molecular forms of ferredoxin, Fd I and Fd II, differ in their ability to catalyze cyclic electron transport in photosystem I. The role of Fd I and Fd II in regulation of electron transport at the acceptor site of photosystem I is discussed.     

Kinetic and functional characterization of a membrane-bound NAD(P)H dehydrogenase located in the chloroplasts of Pleurochloris meiringensis (Xanthophyceae)

Using isolated chloroplasts or purified thylakoids from photoautotrophically grown cells of the chromophytic alga Pleurochloris meiringensis (Xanthophyceae) we were able to demonstrate a membrane bound NAD(P)H dehydrogenase activity. NAD(P)H oxidation was detectable with menadione, coenzyme Q₀, decylplastoquinone and decylubiquinone as acceptors in an in vitro assay. K _m-values for both pyridine nucleotides were in the molar range (K _m[NADH]=9.8 M, K _m[NADPH]=3.2 M calculated according to Lineweaver-Burk). NADH oxidation was optimal at pH 9 while pH dependence of NADPH oxidation showed a main peak at 9.8 and a smaller optimum at pH 7.5–8. NADH oxidation could be completely inhibited with rotenone, an inhibitor of mitochondrial complex I dehydrogenase, while NADPH oxidation revealed the typical inhibition pattern upon addition of oxidized pyridine nucleotides reported for ferredoxin: NADP⁺ reductase. Partly-denaturing gel electrophoresis followed by NAD(P)H dehydrogenase activity staining showed that NADPH and NADH oxidizing proteins had different electrophoretic mobilities. As revealed by denaturing electrophoresis, the NADH oxidizing enzyme had one main subunit of 22 kDa and two further polypeptides of 29 and 44 kDa, whereas separation of the NADPH depending protein yielded five bands of different molecular weight. Measurement of oxygen consumption due to PS I mediated methylviologen reduction upon complete inhibition of PS II showed that the NAD(P)H dehydrogenase is able to catalyze an input of electrons from NADH to the photosynthetic electron transport chain in case of an oxidized plastoquinone-pool. We suggest ferredoxin: NADP⁺ reductase to be the main NADPH oxidizing activity while a thylakoidal NAD(P)H: plastoquinone oxidoreductase involved in the chlororespiratory pathway in the dark acts mainly as an NADH oxidizing enzyme.Abbreviations Coenzyme Q₀-2,3-dimethoxy-5-methyl-1,4-benzoquinone - FNR ferredoxin: NADP⁺ reductase - MD menadione - MV methylviologen - NDH NAD(P)H dehydrogenase - PQ plastoquinone - PQ₁₀ decylplastoquinone - SDH succinate dehydrogenase - UQ₁₀ decylubiquinone (2,3-dimethoxy-5-methyl-6-decyl-1,4-benzoquinone)     

Sulfate reduction in Catharanthus roseus (L.)

Cell homogenates from Catharanthus roseus (L.) G. Don. grown S-autotrophically on sulfate in the dark are capable of reducing adenylysulfate (APS) to cysteine. This reduction required a particulate protein fraction from the cell extract and reduced ferredoxin as the electron donor. The protein fraction (MW 700,000±50,000) was found to contain Fd:NADP⁺ reductase, glutathione reductase and an unspecific dithiol reductase, and APS-sulfotransferase and thiosulfonate reductase activity. Resolution into these individual enzyme activities led to a non-restorable loss of the APS reducing activity. It was observed that a slow gradual decay of the APS reducing activity was accompanied by a likewise slow generation of a ferredoxin-dependent sulfite reductase.Enzymes and abbreviations APS Adenosine 5-phosphosulfate - APS-kinase E.C.2.7.1.25 - ATP-sulfurylase E.C.2.7.7.4 - Fd ferredoxin - Fd-NADP⁺-reductase E.C.1.6.7.1. - Glutathione reductase E.C.1.6.4.2. - G6P Glucose 6-phosphate - G6PDH glucose 6-phosphate dehydrogenase, E.C.1.1.49 - GSSG oxidized glutathione - GSSO₃H S-sulfoglutathione - MVH reduced methylviologen - OASS O-acetylserine sulfhydrylase-E.C. 4.2.99.8 - Sulfite reductase E.C.1.8.1.2     

Spectrophotometric measurement of plastoquinone photoreduction in the blue-green alga,Phormidium uncinatum

The redox state of plastoquinone was measured in vivo in the blue-green alga, Phormidium uncinatum by means of a double beam UV-spectrophotometer. The difference in absorbance of the oxidized and the reduced forms of plastoquinone was amplified, and stored and averaged in a computer. The redox state was changed by two alternating actinic light beams. When one actinic wavelength was kept constant at 700 nm (PSI) variation of the other yielded an action spectrum representing photosystem II. The inhibitors of the photosynthetic electron transport chain, DCMU and DBMIB, reduced the difference in absorbance between the oxidized and reduced forms of plastoquinone.Abbreviations DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU 3-(3,4-dichlorophenyl)-1, 1-dimethylurea     

Photosynthetic electron transport in thylakoids from cotton cultivars (Gossypium sp.) differing in tolerance to prometryn

A method to determine photosynthetic electron transport in thylakoid membranes is described for Gossypium barbadense (cv. Pima S-7) and G. hirsutum (cv. DP 5415). These cultivars differed markedly in tolerance to prometryn, a PS II inhibitor. The rates of photosynthetic electron transport obtained were 245 mole oxygen mg^–1 chl h¹. Plant age and leaf size influenced the activity of the thylakoid preparations. Thylakoids from leaves of plants 24 to 37 d and 50–70 mm in diameter had the highest activities; thylakoids from cotyledons, fully expanded leaves and young leaves had low activity. Thylakoids from both species had similar photosynthetic activities and I₅₀'s for prometryn, atrazine and diuron. Thus, tolerance to prometryn was not due to differential binding at D1 protein.Abbreviations PSII photosystem II - DAP day after planting - DQ duroquinone - DBMIB dibromothymoquinone - DMBQ 2,5-dimethyl-p-benzoquinone - I₅₀ concentration to inhibit reaction by 50% - Q_A quinone A - Q_B quinone B     

Coordinated, coenzyme Q reversible, 2,5-dibromothymoquionine inhibition of electron transport and ATPase in Escherichia coli.

2,6-dibromothymoquinone (DBMIB) and other coenzyme Q analogs partially inhibit electron transport and the membrane-bound Mg⁺⁺ stimulated ATPase of $\text{E. coli}$ membranes. The inhibitions by DBMIB are fully reversed by coenzyme Q₆, and other analogs show partial reversal by coenzyme Q₆. Electron transport reactions inhibited are NADH and lactate oxidase, NADH menadione reductase, lactate phenazinemethosulfate reductase and duroquinol oxidase. The concentrations of DBMIB required are similar for electron transport and ATPase inhibition and inhibitions are all increased by uncouplers. Electron transport and ATPase are not inhibited in a DBMIB insensitive mutant. Soluble ATPase extracted from the membranes does not show DBMIB inhibition under either high or low Mg⁺⁺ conditions. Lipophilic chelators show additional inhibition over DBMIB. It appears that coenzyme Q functions at three sites in $\text{E. coli}$ electron transport where ATPase activity is controlled. Coenzyme Q deficient mutants also show decreased electron transport and ATPase activity which is restored by coenzyme Q.     

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     

The non-photochemical reduction of plastoquinone in leaves

Although it is generally assumed that the plastoquinone pool of thylakoid membranes in leaves of higher plants is rapidly oxidized upon darkening, this is often not the case. A multiflash kinetic fluorimeter was used to monitor the redox state of the plastoquinone pool in leaves. It was found that in many species of plants, particularly those using the NAD-malic enzyme C₄ system of photosynthesis, the pool actually became more reduced following a light to dark transition. In some Amaranthus species, plastoquinone remained reduced in the dark for several hours. Far red light, which preferentially drives Photosystem I turnover, could effectively oxidize the plastoquinone pool. Plastoquinone was re-reduced in the dark within a few seconds when far red illumination was removed. The underlying mechanism of the dark reduction of the plastoquinone pool is still uncertain but may involve chlororespiratory activity.Abbreviations apparent F_o observed fluorescence yield after dark adaptation - F_m maximum fluorescence when all Q_A is fully reduced - F_o minimum fluorescence yield when Q_A is fully oxidized and non-photochemical quenching is fully relaxed - F_s steady state fluorescence yield - PPFD photosynthetic photon flux density - PQ plastoquinone - Q_A primary quinone acceptor of the Photosystem II reaction center - Q_B secondary quinone acceptor to the Photosystem II reaction center - F F_m minus F_s     

Ferredoxin-quinone reductase benefits cyclic electron flow around photosystem 1 in tobacco leaves upon exposure to chilling stress under low irradiance

The function of chloroplast ferredoxin quinone reductase (FQR)-dependent flow was examined by comparing a wild type tobacco and a tobacco transformant (ΔndhB) in which the ndhB gene had been disrupted with their antimycin A (AA)-fed leaves upon exposure to chilling temperature (4 °C) under low irradiance (100 μmol m⁻² s⁻¹ photon flux density). During the chilling stress, the maximum photochemical efficiency of photosystem (PS) 2 (F_v/F_m) decreased markedly in both the controls and AA-fed leaves, and P700⁺ was also lower in AA-fed leaves than in the controls, implying that FQR-dependent cyclic electron flow around PS1 functioned to protect the photosynthetic apparatus from chilling stress under low irradiance. Under such stress, non-photochemical quenching (NPQ), particularly the fast relaxing NPQ component (q_f) and the de-epoxidized ratio of the xanthophyll cycle pigments, (A+Z)/(V+A+Z), formed the difference between AA-fed leaves and controls. The lower NPQ in AA-fed leaves might be related to an inefficient proton gradient across thylakoid membranes (ΔpH) because of inhibiting an FQR-dependent cyclic electron flow around PS1 at chilling temperature under low irradiance.     

Light adaptation of cyclic electron transport through Photosystem I in the cyanobacterium Synechococcus sp. PCC 7942

Photosystem I-driven cyclic electron transport was measured in intact cells of Synechococcus sp PCC 7942 grown under different light intensities using photoacoustic and spectroscopic methods. The light-saturated capacity for PS I cyclic electron transport increased relative to chlorophyll concentration, PS I concentration, and linear electron transport capacity as growth light intensity was raised. In cells grown under moderate to high light intensity, PS I cyclic electron transport was nearly insensitive to methyl viologen, indicating that the cyclic electron supply to PS I derived almost exclusively from a thylakoid dehydrogenase. In cells grown under low light intensity, PS I cyclic electron transport was partially inhibited by methyl viologen, indicating that part of the cyclic electron supply to PS I derived directly from ferredoxin. It is proposed that the increased PSI cyclic electron transport observed in cells grown under high light intensity is a response to chronic photoinhibition.Abbreviations DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - ES energy storage - MV methyl viologen - PA_m photoacoustic thermal signal with strong non-modulated background light added - PA_s photoacoustic thermal signal without background light added CIW/DPB Publication No. 1205.     

Shikonin isovalerate: An inhibitor of photosystem II in Chlorogloeopsis fritschi

Shikonin isovalerate, extracted from the roots of the desert plant Arnebia decumbens, was tested for its effect on photosynthetic electron transport system of Chlorogloeopsis fritschii. The ferricyanide-Hill reaction with water and DPC as electron donors was inhibited completely with 10^-5 M shikonin isovalerate. The photoreduction of DCPIP through photosystem II was only slightly inhibited. Photosystem I from durohydroquinone to methyl viologen was not affected using 10^-6 M shikonin isovalerate. The same concentration caused 49% inhibition of cyclic photophosphorylation. These results suggest that shikonin isovalerate inhibits photosynthetic electron flow at the plastoquinone pool.Abbreviations DCMU 3-(3,4-dichlorophenyl)-N,N-dimethyl urea - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-P-benzoquinone - DCPIP 2–6-dichlorophenolindophenol - DPC Diphenylcarbazide - Tricine N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine     

Open reading frame ssr2016 is required for antimycin A-sensitive photosystem I-driven cyclic electron flow in the cyanobacterium Synechocystis sp. PCC 6803

Open reading frame ssr2016 encodes a protein with substantial sequence similarities to PGR5 identified as a component of the antimycin A-sensitive ferredoxin:plastoquinone reductase (FQR) in PSI cyclic photophosphorylation in Arabidopsis thaliana. We studied cyclic electron flow in Synechocystis sp. PCC 6803 in vivo in ssr2016 deletion mutants generated either in a wild-type background or in a ndhB deletion mutant. Our results indicate that ssr2016 is required for FQR and that it operates in a parallel pathway to the NDH1 complex. The ssr2016 deletion mutants are high light sensitive, suggesting that FQR might be important in controlling redox poise under adverse conditions.