Quantification of cyclic electron flow around Photosystem I in spinach leaves during photosynthetic induction

The variation of the rate of cyclic electron transport around Photosystem I (PS I) during photosynthetic induction was investigated by illuminating dark-adapted spinach leaf discs with red + far-red actinic light for a varied duration, followed by abruptly turning off the light. The post-illumination re-reduction kinetics of P700⁺, the oxidized form of the photoactive chlorophyll of the reaction centre of PS I (normalized to the total P700 content), was well described by the sum of three negative exponential terms. The analysis gave a light-induced total electron flux from which the linear electron flux through PS II and PS I could be subtracted, yielding a cyclic electron flux. Our results show that the cyclic electron flux was small in the very early phase of photosynthetic induction, rose to a maximum at about 30 s of illumination, and declined subsequently to <10% of the total electron flux in the steady state. Further, this cyclic electron flow, largely responsible for the fast and intermediate exponential decays, was sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethyl urea, suggesting an important role of redox poising of the cyclic components for optimal function. Significantly, our results demonstrate that analysis of the post-illumination re-reduction kinetics of P700⁺ allows the quantification of the cyclic electron flux in intact leaves by a relatively straightforward method.     

Effect of temperature on electron transport activities of glutaraldehyde-fixed pea thylakoids

Temperature-induced changes in Hill activity of glutaraldehyde-fixed pea ( Pisum sativum L. cv. Alaska) thylakoids have been examined. Using ferricyanide as electron acceptor, a temperature-induced change occurred at ca 12–14°C for both control and fixed thylakoids. In contrast to the controls, fixed thylakoids not only showed a change in slope of the Arrhenius plots but also a discontinuity which has not been observed in previous studies. A drop in activity coincided with the decrease in slope: the extent of the reduction depended on the concentration of glutaraldehyde used for fixation. Using a lipophilic electron acceptor, a temperature-induced change also occurred at 12–14°C, but there was no reduction in activities of fixed thylakoids at temperatures above the change in slope.
The results indicate that a temperature-induced change in fixed thylakoids restricts the access of ferricyanide to its reductant(s) in the membrane but that fixation does not affect the temperature-induced change per se. The results confirm that temperature has a general effect on the functioning of thylakoid membranes. The data demonstrate that calculations of the extent of inhibition by glutaraldehyde of Hill activity with ferricyanide should take into account the temperature at which assays are performed.     

Proton cellular influx as a probable mechanism of variation potential influence on photosynthesis in pea

Electrical signals (action potential and variation potential, VP) caused by environmental stimuli are known to induce various physiological responses in plants, including changes in photosynthesis; however, their functional mechanisms remain unclear. In this study, the influence of VP on photosynthesis in pea (Pisum sativum L.) was investigated and the proton participation in this process analysed. VP, induced by local heating, inactivated photosynthesis and activated respiration, with the initiation of the photosynthetic response connected with inactivation of the photosynthetic dark stage; however, direct VP influence on the light stage was also probable. VP generation was accompanied with pH increases in apoplasts (0.17–0.30 pH unit) and decreases in cytoplasm (0.18–0.60 pH unit), which probably reflected H⁺‐ATPase inactivation and H⁺ influx during this electrical event. Imitation of H⁺ influx using the protonophore carbonyl cyanide m‐chlorophenylhydrazone (CCCP) induced a photosynthetic response that was similar with a VP‐induced response. Experiments on chloroplast suspensions showed that decreased external pH also induced an analogous response and that its magnitude depended on the magnitude of pH change. Thus, the present results showed that proton cellular influx was the probable mechanism of VP's influence on photosynthesis in pea. Potential means of action for this influence are discussed.     

Localization of electron transport inhibition in plastoquinone reactions

Reduction kinetics of P700 following a short flash are measured in spinach chloroplasts after oxidation of the electron carriers between the two photoreactions by far-red light. Three features of the kinetics allow us to localize simultaneously inhibition at different sites between photoreaction II and the reducing site of plastoquinol. These are the initial lag, the halftime, and the area under the transient of the P700 absorbance change, which indicate the electron transfer time from photoreaction II to the reducing site of plastoquinol, the rate of plastoquinol oxidation, and the number of electrons transferred to the special plastoquinone B functioning as secondary electron acceptor of photosystem II, respectively. As an additional diagnostic parameter for inhibition before and after the plastoquinone pool, the area under the transient of the P700 absorbance change is used after long flashes. This area is proportional to the amount of reduced plastoquinone as shown by the absorbance change at 265 nm. The effects of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) are compared with those of 2-bromo-4-nitrothymol, 2,4-dinitrophenyl ether of 2-iodo-4-nitrothymol, and Illoxan as representatives for new classes of inhibitors. While 2-halogeno-4-nitrothymols inhibit the reduction of plastoquinone similarly to DCMU, their diphenyl ether derivatives inhibit selectively the oxidation of plastoquinol.     

The role of water-water cycle in regulating the redox state of photosystem I under fluctuating light

The regulation of photosystem I (PSI) redox state under fluctuating light was investigated for four species using P700 measurement and electrochromic shift analysis. Species included the angiosperms Camellia japonica, Bletilla striata and Arabidopsis thaliana and the fern Cyrtomium fortunei. For the first seconds after transition from low to high light, all species showed relatively low levels of the proton gradient (ΔpH) across the thylakoid membranes. At this moment, PSI was highly oxidized in C. japonica and C. fortunei but was over-reduced in B. striata and A. thaliana. In B. striata and A. thaliana, the redox state of PSI was largely dependent on ΔpH. In contrast, the rapid oxidation of P700 in C. japonica was relatively independent of ΔpH, but was mainly dependent on the outflow of electrons to O₂ via the water-water cycle. In the fern C. fortunei, PSI redox state was rapidly regulated by the fast photo-reduction of O₂ rather than the ΔpH. These results indicate that mechanisms regulating PSI redox state under fluctuating light differ greatly between species. We propose that the water-water cycle is an important mechanism regulating the PSI redox state in angiosperms.     

Interference of electron donors in the measurement of the proton gradient and membrane potential by flow dialysis

The three most commonly used electron donors for flow dialysis measurements of membrane potential lead to the development of an apparent but artifactual membrane potential with the interior negative in the presence or absence of membrane vesicles. The same three electron donors used in flow dialysis determinations of delta pH in the presence or absence of membrane vesicles lead to the development of an apparent but artifactual delta pH with the interior acidic. These artifacts have been evaluated using two probes for membrane potential, namely, TPP+ and rubidium in the presence of valinomycin and for two probes of delta pH, namely, acetate and DMO. Measurements were made over a range of ionic strengths.     

Analysis of donors of electrons to photosystem I and cyclic electron flow by redox kinetics of P700 in chloroplasts of isolated bundle sheath strands of maize

Bundle sheath chloroplasts of NADP-malic enzyme (NADP-ME) type C₄ species have a high demand for ATP, while being deficient in linear electron flow and oxidation of water by photosystem II (PSII). To evaluate electron donors to photosystem I (PSI) and possible pathways of cyclic electron flow (CEF1) in isolated bundle sheath strands of maize (Zea mays L.), an NADP-ME species, light-induced redox kinetics of the reaction center chlorophyll of PSI (P700) were followed under aerobic conditions. Donors of electrons to CEF1 are needed to compensate for electrons lost from the cycle. When stromal electron donors to CEF1 are generated during pre-illumination with actinic light (AL), they retard the subsequent rate of oxidation of P700 by far-red light. Ascorbate was more effective than malate in generating stromal electron donors by AL. The generation of stromal donors by ascorbate was inhibited by DCMU, showing ascorbate donates electrons to the oxidizing side of PSII. The inhibitors of NADPH dehydrogenase (NDH), amytal and rotenone, accelerated the oxidation rate of P700 by far-red light after AL, indicating donation of electrons to the intersystem from stromal donors via NDH. These inhibitors, however, did not affect the steady-state level of P700⁺ under AL, which represents a balance of input and output of electrons in P700. In contrast, antimycin A, the inhibitor of the ferredoxin-plastoquinone reductase-dependent CEF1, substantially lowered the level of P700⁺ under AL. Thus, the primary pathway of ATP generation by CEF1 may be through ferredoxin-plastoquinone, while function of CEF1 via NDH may be restricted by low levels of ferredoxin-NADP reductase. NDH may contribute to redox poising of CEF1, or function to generate ATP in linear electron flow to O₂ via PSI, utilizing NADPH generated from malate by chloroplastic NADP-ME.     

Effect of exogenous glucose on electron flow to photosystem I and respiration in cyanobacterial cells

The effects of exogenous glucose on the rates of alternative pathways of photosystem II (PSII)-independent electron flow to PSI and of dark respiration in Synechocystis sp. 6803 cells were studied. The presence of glucose was shown to accelerate the electron flow to P700⁺, the PSI primary electron donor oxidized with Far-red light (FRL), which excites specifically only PSI. An increase in the glucose concentration was accompanied by a further activation of electron flow to PSI, which was supported by the dark donation of reducing equivalents to the electron transport chain. An increase in the external glucose concentration resulted also in the disappearance of lag-phase in the kinetics of P700⁺ reduction, which was observed in the cells incubated without glucose after FRL switching off. A similarity of nonphotochemical processes of electron transfer to PSI in cyanobacteria and higher plants was supposed, basing on the earlier observed fact of the occurrence of such lagphase in higher plants and its dependence on the exhausting of stromal reductants in the light. Acceleration of dark electron flow to PSI in the presence of glucose, a major respiratory substrate, may indicate the coupling between nonphotochemical processes in the photosynthetic and respiratory chains of electron transport in cyanobacterial cells. A close correlation between photosynthesis and respiration in cyanobacterial cells is also confirmed by a sharp acceleration of respiration with an increase in the glucose concentration in medium.     

Benzothiadiazole and l-2-oxothiazolidine-4-carboxylic acid reduce the severity of Sharka symptoms in pea leaves: effect on antioxidative metabolism at the subcellular level

The effect of treatment with benzothiadiazole (BTH) or l -2-oxothiazolidine-4-carboxylic acid (OTC), and their interaction with Plum pox virus (PPV) infection, on antioxidative metabolism of pea plants was studied at the subcellular level. PPV infection produced a 20% reduction in plant growth. Pre-treatment of pea plants with OTC or BTH afforded partial protection against PPV infection, measured as the percentage of leaves showing symptoms, but neither BTH nor OTC significantly reduced the virus content. PPV infection caused oxidative stress, as monitored by increases in lipid peroxidation and protein oxidation in soluble and chloroplastic fractions. In leaves of non-infected plants, OTC increased the content of reduced glutathione (GSH) and total glutathione; accordingly, an increase in the redox state of glutathione was observed. An increase in oxidized glutathione (GSSG) was found in symptomatic leaves from infected plants. A similar increase in GSSG was also observed in asymptomatic leaves from infected, untreated plants. However, no changes in GSSG occurred in asymptomatic leaves from infected plants treated with BTH and OTC and, accordingly, a higher redox state of GSH was recorded in those leaves, which could have had a role in the reduction of symptoms, as observed in asymptomatic leaves from infected plants treated with BTH or OTC. Treatment with BTH or OTC had some effect on antioxidant enzymes in soluble and chloroplastic fractions from infected pea leaves. An increase in antioxidative mechanisms, such as GSH-related enzymes (DHAR, GR and G6PDH), as well as APX and POX, at the subcellular level was observed, which could play a role in reducing the severity of cellular damage induced by Sharka in pea leaves.     

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.     

Characterization of photosynthetic electron transport in bundle sheath cells of maize. I. Ascorbate effectively stimulates cyclic electron flow around PSI

Redox changes of the reaction-center chlorophyll of photosystem I (P700) and chlorophyll fluorescence yield were measured in bundle sheath strands (BSS) isolated from maize (Zea mays L.) leaves. Oxidation of P700 in BSS by actinic light was suppressed by nigericin, indicating the generation of a proton gradient across the thylakoid membranes of BSS chloroplasts. Methyl viologen, which transfers electrons from photosystem I (PSI) to O₂, caused a considerable decrease in the reduction rate of P700⁺ in BSS after turning off actinic light, showing that electron flow from the acceptor side of PSI to stromal components is critical for this reduction. Ascorbate (Asc), and to a lesser extent malate (Mal), caused a lower level of P700⁺ in BSS under aerobic conditions in far-red light, implying electron donation from these substances to the intersystem carriers. When Asc or Mal was added to BSS during pre-illumination under anaerobic conditions in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU), the far-red-induced level of P700⁺ was lowered. The results suggest Asc and Mal can cause reduction of stromal donors, which in turn establishes conditions for rapid PSI-driven P700⁺ reduction. Addition of these metabolites also strongly stimulated the development of a proton gradient in thylakoids under aerobic conditions in the absence of DCMU, i.e. under conditions analogous to those in vivo. Ascorbate was a much more effective electron donor than Mal, suggesting it has a physiological role in activation of cyclic electron flow around PSI.     

Photosynthetic regulation under fluctuating light in field-grown Cerasus cerasoides: A comparison of young and mature leaves

Photosystem I (PSI) is a potential target of photoinhibition under fluctuating light. However, photosynthetic regulation under fluctuating light in field-grown plants is little known. Furthermore, it is unclear how young leaves protect PSI against fluctuating light under natural field conditions. In the present study, we examined chlorophyll fluorescence, P700 redox state and the electrochromic shift signal in the young and mature leaves of field-grown Cerasus cerasoides (Rosaceae). Within the first seconds after any increase in light intensity, young leaves showed higher proton gradient (ΔpH) across the thylakoid membranes than the mature leaves, preventing over-reduction of PSI in the young leaves. As a result, PSI was more tolerant to fluctuating light in the young leaves than in the mature leaves. Interestingly, after transition from low to high light, the activity of cyclic electron flow (CEF) in young leaves increased first to a high level and then decreased to a stable value, while this rapid stimulation of CEF was not observed in the mature leaves. Furthermore, the over-reduction of PSI significantly stimulated CEF in the young leaves but not in the mature leaves. Taken together, within the first seconds after any increase in illumination, the stimulation of CEF favors the rapid lumen acidification and optimizes the PSI redox state in the young leaves, protecting PSI against photoinhibition under fluctuating light in field-grown plants.     

Photosystem I-dependent cyclic electron transport is important in controlling Photosystem II activity in leaves under conditions of water stress

Leaves of the C₃ plant Brassica oleracea were illuminated with red and/or far-red light of different photon flux densities, with or without additional short pulses of high intensity red light, in air or in an atmosphere containing reduced levels of CO₂ and/or oxygen. In the absence of CO₂, far-red light increased light scattering, an indicator of the transthylakoid proton gradient, more than red light, although the red and far-red beams were balanced so as to excite Photosystem II to a comparable extent. On red background light, far-red supported a transthylakoid electrical field as indicated by the electrochromic P₅₁₅ signal. Reducing the oxygen content of the gas phase increased far-red induced light scattering and caused a secondary decrease in the small light scattering signal induced by red light. CO₂ inhibited the light-induced scattering responses irrespective of the mode of excitation. Short pulses of high intensity red light given to a background to red and/or far-red light induced appreciable additional light scattering after the flashes only, when CO₂ levels were decreased to or below the CO₂ compensation point, and when far-red background light was present. While pulse-induced light scattering increased, non-photochemical fluorescence quenching increased and F₀ fluorescence decreased indicating increased radiationless dissipation of excitation energy even when the quinone acceptor Q_A in the reaction center of Photosystem II was largely oxidized. The observations indicate that in the presence of proper redox poising of the chloroplast electron transport chain cyclic electron transport supports a transthylakoid proton gradient which is capable of controlling Photosystem II activity. The data are discussed in relation to protection of the photosynthetic apparatus against photoinactivation.Abbreviations F, F_M, F'_M, F"_M, F₀, F'₀ chlorophyll fluorescence levels - _exc quantum efficiency of excitation energy capture by open Photosystem II - _{PS II} quantum efficiency of electron flow through Photosystem II - P₅₁₅ field indicating rapid absorbance change peaking at 522 nm - P₇₀₀ primary donor of Photosystem I - Q_A primary quinone acceptor in Photosystem II - Q_N non-photochemical fluorescence quenching - Q_q photochemical quenching of chlorophyll fluorescence     

Properties of ATP-driven reverse electron flow in chloroplasts

1. The reverse reactions induced by coupled ATP hydrolysis were studied in spinach chloroplasts by measurements of the ATP-induced increase in chlorophyll fluorescence reflecting reverse electron flow, and of the ATP-induced decrease in 9-aminoacridine fluorescence, representing formation of the transthylakoidal proton gradient (ΔpH). ATP-driven reverse electron flow was kinetically analysed into three phases, of which only the second and third one were paralleled by corresponding phases in ΔpH formation. The rapid first phase and formation of a ΔpH occur also in the absence of the electron transfer mediator phenazine methosulfate.2. The rate and extent of the reverse reactions were measured at temperatures in the range from 0 to 30°C. The rate of formation of ΔpH and of reverse electron flow were faster at high temperatures, but the maximal extent of ΔpH and chlorophyll fluorescence increase were observed at the lowest temperature. Considering rate and extent of the ATP-stimulated reactions, a temperature optimum around 15°C was found. Light activation of the ATPase occurred throughout the range studied. At 0°C and in the presence of inorganic phosphate the activated state for ATPase was maintained for more then 10 min.3. The ATP-induced rise in chlorophyll fluorescence yield was found to be of similar magnitude as the rise induced by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), when both were measured with an extremely weak measuring beam. It is concluded, that both effects, although derived via distinctly different pathways, are limited by the same electron donating or electron accepting pool.     

Contribution of NDH-dependent cyclic electron transport around photosystem I to the generation of proton motive force in the weak mutant allele of pgr5

In angiosperms, cyclic electron transport (CET) around photosystem I (PSI) consists of two pathways, depending on PGR5/PGRL1 proteins and the chloroplast NDH complex. In single mutants defective in chloroplast NDH, photosynthetic electron transport is only slightly affected at low light intensity, but in double mutants impaired in both CET pathways photosynthesis and plant growth are severely affected. The question is whether this strong mutant phenotype observed in double mutants can be simply explained by the additive effect of defects in both CET pathways. In this study, we used the weak mutant allele of pgr5-2 for the background of double mutants to avoid possible problems caused by the secondary effects due to the strong mutant phenotype. In two double mutants, crr2-2 pgr5-2 and ndhs-1 pgr5-2, the plant growth was unaffected and linear electron transport was only slightly affected. However, NPQ induction was more severely impaired in the double mutants than in the pgr5-2 single mutant. A similar trend was observed in the size of the proton motive force. Despite the slight reduction in photosystem II parameters, PSI parameters were severely affected in the pgr5-2 single mutant, the phenotype that was further enhanced by adding the NDH defects. Despite the lack of ?pH-dependent regulation at the cytochrome b₆f complex (donor-side regulation of PSI), the plastoquinone pool was more reduced in the double mutants than in the pgr5-2 single mutants. This phenotype suggests that both PGR5/PGRL1- and NDH-dependent CET contribute to supply sufficient acceptors from PSI by balancing the ATP/NADPH production ratio.     

Induction of cyclic electron flow around photosystem 1 and state transition are correlated with salt tolerance in soybean

We investigated the role of cyclic electron flow around photosystem 1 (CEF1) and state transition (ST) in two soybean cultivars that differed in salt tolerance. The CEF1 and maximum photochemical efficiency (F_v/F_m) were determined under control and NaCl (50 mM) stress and the NaCl-induced light-harvesting complex 2 (LHC2) phosphorylation in vitro was analysed in light and dark. NaCl induced the increase of CEF1 more greatly in wild soybean Glycine cyrtoloba (cv. ACC547) than in cultivated soybean Glycine max (cv. Melrose). The F_v/F_m was reduced less in G. cyrtoloba than in G. max after 10-d NaCl stress. In G. cyrtoloba, the increase of CEF1 was associated with enhancement of LHC2 phosphorylation in thylakoid membrane under both dark and light. However, in G. max the NaCl treatment decreased the LHC2 phosphorylation. Treatment with photosynthetic electron flow inhibitors (DCMU, DBMIB) inhibited LHC2 phosphorylation more in G. max than in G. cyrtoloba. Thus the NaCl-induced up-regulation in CEF1 and ST might contribute to salt resistance of G. cyrtoloba.     

Regulation of pepc gene expression in Anabaena sp. PCC 7120 and its effects on cyclic electron flow around photosystem I and tolerances to environmental stresses

Since pepc gene encoding phosphoenolpyruvate carboxylase(PEPCase) has been cloned from Anabaena sp. PCC7120 and other cyanobacteria, the effects of pepc gene expression on photosynthesis have not been reported yet. In this study, we constructed mutants containing either upregulated(forward) or downregulated(reverse) pepc gene in Anabaena sp. PCC 7120. Results from real-time quantitative polymerase chain reaction(RT-q PCR), Western blot and enzymatic analysis showed that PEPCase activity was significantly reduced in the reverse mutant compared with the wild type, and that of the forward mutant was obviously increased.Interestingly, the net photosynthesis in both the reverse mutant and the forward mutant were higher than that of the wild type, but dark respiration was decreased only in the reverse mutant. The absorbance changes of P700 upon saturation pulse showed the photosystem I(PSI) activity was inhibited, as reflected by Y(I), and Y(NA) was elevated, and Rdark reduction of P700 t was stimulated, indicating enhanced cyclic electron flow(CEF) around PSI in the reverse mutant.Additionally, the reverse mutant photosynthesis was higher than that of the wild type in low temperature, low and high pH,and high salinity, and this implies increased tolerance in the reverse mutant through downregulated pepc gene.     

Photo-oxidation of P740, the primary electron donor in photosystem I from Acaryochloris marina

Fourier transform infrared spectroscopy (FTIR) difference spectroscopy in combination with deuterium exchange experiments has been used to study the photo-oxidation of P740, the primary electron donor in photosystem I from Acaryochloris marina. Comparison of (P740(+)-P740) and (P700(+)-P700) FTIR difference spectra show that P700 and P740 share many structural similarities. However, there are several distinct differences also: 1), The (P740(+)-P740) FTIR difference spectrum is significantly altered upon proton exchange, considerably more so than the (P700(+)-P700) FTIR difference spectrum. The P740 binding pocket is therefore more accessible than the P700 binding pocket. 2), Broad, "dimer" absorption bands are observed for both P700(+) and P740(+). These bands differ significantly in substructure, however, suggesting differences in the electronic organization of P700(+) and P740(+). 3), Bands are observed at 2727(-) and 2715(-) cm(-1) in the (P740(+)-P740) FTIR difference spectrum, but are absent in the (P700(+)-P700) FTIR difference spectrum. These bands are due to formyl CH modes of chlorophyll d. Therefore, P740 consists of two chlorophyll d molecules. Deuterium-induced modification of the (P740(+)-P740) FTIR difference spectrum indicates that only the highest frequency 13(3) ester carbonyl mode of P740 downshifts, indicating that this ester mode is weakly H-bonded. In contrast, the highest frequency ester carbonyl mode of P700 is free from H-bonding. Deuterium-induced changes in (P740(+)-P740) FTIR difference spectrum could also indicate that one of the chlorophyll d 3(1) carbonyls of P740 is hydrogen bonded.     

Photoinhibition in Chlamydomonas reinhardtii: Effect on state transition,intersystem energy distribution and Photosystem I cyclic electron flow

The energy distribution, state transitions and photosynthetic electron flow during photoinhibition of Chlamydomonas reinhardtii cells have been studied in vivo using photoacoustics and modulated fluorescence techniques. In cells exposed to 2500 W/m² light at 21 °C for 90 min, 90% of the oxygen evolution activity was lost while photochemical energy storage as expressed by the parameter photochemical loss (P.L.) at 710–720 nm was not impaired. The energy storage vs. modulation frequency profile indicated an endothermic step with a rate constant of 2.1 ms. The extent of the P.L. was not affected by DCMU but was greatly reduced by DBMIB. The regulatory mechanism of the state 1 to state 2 transition process was inactivated and the apparent light absorption cross section of photosystem II increased during the first 20 min of photoinhibition followed by a significant decrease relative to that of photosystem I. These results are consistent with the inactivation of the LHC II kinase and the presence of an active cyclic electron flow around photosystem I in photoinhibited cells.Abbreviations PS I, PS II Photosystem I and Photosystem II respectively - P.L. photochemical loss - DCMU 3-(3,4-dichlorophenyl-1,1-dimethyl urea - LHC II light harvesting chlorophyll a,b-protein complex of PS II - DBMIB 2,5 dibromo-3-methyl-6-isopropyl-p-benzoquinone     

The secondary electron acceptor of photosystem I in Gloeobacter violaceus PCC 7421 is menaquinone-4 that is synthesized by a unique but unknown pathway

The secondary electron acceptor of photosystem (PS) I in the cyanobacterium Gloeobacter violaceus PCC 7421 was identified as menaquinone-4 (MQ-4) by comparing high performance liquid chromatograms and absorption spectra with an authentic compound. The MQ-4 content was estimated to be two molecules per one molecule of chlorophyll (Chl) a', a constituent of P700. Comparative genomic analyses showed that six of eight men genes, encoding phylloquinone/MQ biosynthetic enzymes, are missing from the G. violaceus genome. Since G. violaceus clearly synthesizes MQ-4, the combined results indicate that this cyanobacterium must have a novel pathway for the synthesis of 1,4-dihydroxy-2-naphthoic acid.