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Leaves of Arabidopsis thaliana plants grown in short days (8 h light) generate more reactive oxygen species in the light than leaves of plants grown in long days (16 h light). The importance of the two PsaE isoforms of photosystem I, PsaE1 and PsaE2, for O 2 reduction was studied in plants grown under these different growth regimes. In short day conditions a mutant affected in the amount of PsaE1 (psae1-1) reduced more efficiently O 2 than a mutant lacking PsaE2 (psae2-1) as shown by spin trapping EPR spectroscopy on leaves and by following the kinetics of P700 + reduction in isolated photosystem I. In short day conditions higher O 2 reduction protected photosystem II against photoinhibition in psae1-1. In contrast in long day conditions the presence of PsaE1 was clearly beneficial for photosynthetic electron transport and for the stability of the photosynthetic apparatus under photoinhibitory conditions. We conclude that the two PsaE isoforms have distinct functions and we propose that O 2 reduction at photosystem I is beneficial for the plant under certain environmental conditions., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Photosynthetic complex I enables cyclic electron flow around photosystem I, a regulatory mechanism for photosynthetic energy conversion. We report a 3.3-angstrom-resolution cryo-electron microscopy structure of photosynthetic complex I from the cyanobacterium Thermosynechococcus elongatus. The model reveals structural adaptations that facilitate binding and electron transfer from the photosynthetic electron carrier ferredoxin. By mimicking cyclic electron flow with isolated components in vitro, we demonstrate that ferredoxin directly mediates electron transfer between photosystem I and complex I, instead of using intermediates such as NADPH (the reduced form of nicotinamide adenine dinucleotide phosphate). A large rate constant for association of ferredoxin to complex I indicates efficient recognition, with the protein subunit NdhS being the key component in this process., (Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

The plastid terminal oxidase PTOX catalyzes the oxidation of plastoquinol (PQH2) coupled with the reduction of oxygen to water. In vivo PTOX is attached to the thylakoid membrane. PTOX is important for plastid development and carotenoid biosynthesis, and its role in photosynthesis is controversially discussed. To analyze PTOX activity in photosynthetic electron transport recombinant purified PTOX fused to the maltose-binding protein was added to photosystem II-enriched membrane fragments. These membrane fragments contain the plastoquinone (PQ) pool as verified by thermoluminescence. Experimental evidence for PTOX oxidizing PQH2 is demonstrated by following chlorophyll fluorescence induction. Addition of PTOX to photosystem II-enriched membrane fragments led to a slower rise, a lower level of the maximal fluorescence and an acceleration of the fluorescence decay. This effect was only observed at low light intensities indicating that PTOX cannot compete efficiently with the reduction of the PQ pool by photosystem II at higher light intensities. PTOX attached tightly to the membranes since it was only partly removable by membrane washings. Divalent cations enhanced the effect of PTOX on chlorophyll fluorescence compared to NaCl most likely because they increase connectivity between photosystem II centers and the size of the PQ pool. Using single turnover flashes, it was shown that the level of reactive oxygen species, generated by PTOX in a side reaction, increased when the spacing between subsequent double flashes was enlarged. This shows that PTOX generates reactive oxygen species under limited substrate availability., (Copyright © 2014 Elsevier B.V. All rights reserved.)

A multi-enzyme electron-transfer chain involving solubilized photosystem I (PSI) as photocatalytic unit, cytochrome c6 and ferredoxin as electron carriers and ferredoxin/NADPH oxidoreductase (FNR) as electron acceptor was reconstituted in an electrochemical cell and studied by cyclic voltammetry. The working gold electrodes were modified to react selectively with cytochrome c6. Quantitative analysis of the photocatalytic current under continuous illumination allowed the determination of the values kon and koff for the ferredoxin/PSI interaction. An efficient recycling system for NADPH was established, and the dissociation constant of the oxidized ferredoxin/semiquinone FNR complex was extracted by modeling the catalytic efficiency of the chain as a function of ferredoxin concentration. The value determined hereby is consistent with a shift of -50 to -100 mV of the reduction potential of ferredoxin when complexed with FNR.

Fast turnover of ferredoxin/Fd reduction by photosystem-I/PSI requires that it dissociates rapidly after it has been reduced by PSI:Fd intracomplex electron transfer. The rate constants of Fd dissociation from PSI have been determined by flash-absorption spectroscopy with different combinations of cyanobacterial PSIs and Fds, and different redox states of Fd and of the terminal PSI acceptor (F A F B ). Newly obtained values were derived firstly from the fact that the dissociation constant between PSI and redox-inactive gallium-substituted Fd increases upon (F A F B ) reduction and secondly from the characterization and elucidation of a kinetic phase following intracomplex Fd reduction to binding of oxidized Fd to PSI, a process which is rate-limited by the foregoing dissociation of reduced Fd from PSI. By reference to the complex with oxidized partners, dissociation rate constants were found to increase moderately with (F A F B ) single reduction and by about one order of magnitude after electron transfer from (F A F B ) − to Fd, therefore favoring turnover of Fd reduction by PSI. With Thermosynechococcus elongatus partners, values of 270, 730 and > 10000 s −1 were thus determined for (F A F B )Fd oxidized , (F A F B ) − Fd oxidized and (F A F B )Fd reduced , respectively. Moreover, assuming a conservative upper limit for the association rate constant between reduced Fd and PSI, a significant negative shift of the Fd midpoint potential upon binding to PSI has been calculated (< −60 mV for Thermosynechococcus elongatus ). From the present state of knowledge, the question is still open whether this redox shift is compatible with a large (> 10) equilibrium constant for intracomplex reduction of Fd from (F A F B ) − . [ABSTRACT FROM AUTHOR]

The inhibitor methyl viologen (MV) has been widely used in photosynthesis to study oxidative stress. Its effects on electron transfer kinetics in Synechocystis sp. PCC6803 cells were studied to characterize its electron-accepting properties. For the first hundreds of flashes following MV addition at submillimolar concentrations, the kinetics of NADPH formation were hardly modified (less than 15% decrease in signal amplitude) with a significant signal decrease only observed after more flashes or continuous illumination. The dependence of the P700 photooxidation kinetics on the MV concentration exhibited a saturation effect at 0.3 mM MV, a concentration which inhibits the recombination reactions in photosystem I. The kinetics of NADPH formation and decay under continuous light with MV at 0.3 mM showed that MV induces the oxidation of the NADP pool in darkness and that the yield of linear electron transfer decreased by only 50% after 1.5–2 photosystem-I turnovers. The unexpectedly poor efficiency of MV in inhibiting NADPH formation was corroborated by in vitro flash-induced absorption experiments with purified photosystem-I, ferredoxin and ferredoxin-NADP + -oxidoreductase. These experiments showed that the second-order rate constants of MV reduction are 20 to 40-fold smaller than the competing rate constants involved in reduction of ferredoxin and ferredoxin-NADP + -oxidoreductase. The present study shows that MV, which accepts electrons in vivo both at the level of photosystem-I and ferredoxin, can be used at submillimolar concentrations to inhibit recombination reactions in photosystem-I with only a moderate decrease in the efficiency of fast reactions involved in linear electron transfer and possibly cyclic electron transfer. [ABSTRACT FROM AUTHOR]

Illumination of photosystem I (PSI) from the cyanobacterium Synechocystis sp. PCC 6803 at 200 K in the presence of ascorbate leads to the formation of two ascorbyl radicals per PSI, which are formed by P700+ reduction by ascorbate. During photoaccumulation, one half of the ascorbyl radicals is formed with a halftime of 1 min and the other half with a halftime of 7 min. Pulsed electron paramagnetic resonance (EPR) experiments with protonated/deuterated PSI show that a PSI proton/deuteron is strongly coupled to the ascorbyl radical. Our data indicate that reactive ascorbate molecules bind to PSI at two specific locations, which might be symmetrically located with respect to the pseudo-C2 axis of symmetry of the heterodimeric core of PSI. Reduction of P700+ by ascorbate leads to multiple turnover of PSI photochemistry, resulting in partial photoaccumulation of the doubly reduced species (FA-, FB-). A modified form of FB-—in accordance with Chamorovsky and Cammack [Biochim. Biophys. Acta 679 (1982) 146–155], but not of FA-, is observed by EPR after illumination at 200 K, which indicates that reduction of FB at 200 K is followed by some relaxation process, in line with this cluster being the most exposed to the solvent. [Copyright &y& Elsevier]

The reaction center of photosystem I (PSI) reduces soluble ferredoxin on the stromal side of the photosynthetic membranes of cyanobacteria and chloroplasts. The X-ray structure of PSI from the cyanobacterium Synechococcus elongatus has been recently established at a 2.5 A˚ resolution [Nature 411 (2001) 909]. The kinetics of ferredoxin photoreduction has been studied in recent years in many mutants of the stromal subunits PsaC, PsaD and PsaE of PSI. We discuss the ferredoxin docking site of PSI using the X-ray structure and the effects brought by the PSI mutations to the ferredoxin affinity. [Copyright &y& Elsevier]

Photosystem I (PSI) is an essential protein complex for oxygenic photosynthesis and is also known to be an important source of reactive oxygen species (ROS) in the light. When ROS are generated within PSI, the photosystem can be damaged. The so-called PSI photoinhibition is a lethal event for oxygenic phototrophs, and it is prevented by keeping the reaction center chlorophyll (P700) oxidized in excess light conditions. Whereas regulatory mechanisms for controlling P700 oxidation have been discovered already, the molecular mechanism of PSI photoinhibition is still unclear. Here, we characterized the damage mechanism of PSI photoinhibition by in vitro transient absorption and electron paramagnetic resonance (EPR) spectroscopy in isolated PSI from cucumber leaves that had been subjected to photoinhibition treatment. Photodamage to PSI was induced by two different light treatments: 1. continuous illumination with high light at low (chilling) temperature (C/LT) and 2. repetitive flashes at room temperature (F/RT). These samples were compared to samples that had been illuminated with high light at room temperature (C/RT). The [Fe S] clusters F X and (F A F B) were destructed in C/LT but not in F/RT. Transient absorption spectroscopy indicated that half of the charge separation was impaired in F/RT, however, low-temperature EPR revealed the light-induced F X signal at a similar size as in the case of C/RT. This indicates that the two branches of electron transfer in PSI were affected differently. Electron transfer at the A-branch was inhibited in F/RT and also partially in C/LT, while the B-branch remained active. • Photosystem I photoinhibition differs between different illumination protocols. • Fe-S clusters are destroyed by illumination at chilling temperature. • The A- and B-branches in photosystem I are affected differently by photoinhibition. [ABSTRACT FROM AUTHOR]

Photosynthetic ferredoxin and its main partner ferredoxin–NADP + -reductase (FNR) are key proteins during the photoproduction of reductive power involved in photosynthetic growth. In this work, we used covalent attachment of ruthenium derivatives to different cysteine mutants of ferredoxin to trigger by laser-flash excitation both ferredoxin reduction and subsequent electron transfer from reduced ferredoxin to FNR. Rates and yields of reduction of the ferredoxin [2Fe–2S] cluster by reductively quenched Ru* could be measured for the first time for such a low redox potential protein whereas ferredoxin–FNR electron transfer was characterized in detail for one particular Ru–ferredoxin covalent adduct. For this adduct, the efficiency of FNR single reduction by reduced ferredoxin was close to 100% under both first-order and diffusion-limited second-order conditions. Interprotein intracomplex electron transfer was measured unambiguously for the first time with a fast rate of c. 6500 s − 1 . Our measurements point out that Ru photosensitizing is a powerful approach to study the functional interactions of ferredoxin with its numerous partners besides FNR. [ABSTRACT FROM AUTHOR]

Biological desert sand crusts are the foundation of desert ecosystems, stabilizing the sands and allowing colonization by higher order organisms. The first colonizers of the desert sands are cyanobacteria. Facing the harsh conditions of the desert, these organisms must withstand frequent desiccation–hydration cycles, combined with high light intensities. Here, we characterize structural and functional modifications to the photosynthetic apparatus that enable a cyanobacterium, Leptolyngbya sp., to thrive under these conditions. Using multiple in vivo spectroscopic and imaging techniques, we identified two complementary mechanisms for dissipating absorbed energy in the desiccated state. The first mechanism involves the reorganization of the phycobilisome antenna system, increasing excitonic coupling between antenna components. This provides better energy dissipation in the antenna rather than directed exciton transfer to the reaction center. The second mechanism is driven by constriction of the thylakoid lumen which limits diffusion of plastocyanin to P 700 . The accumulation of P 700 + not only prevents light-induced charge separation but also efficiently quenches excitation energy. These protection mechanisms employ existing components of the photosynthetic apparatus, forming two distinct functional modes. Small changes in the structure of the thylakoid membranes are sufficient for quenching of all absorbed energy in the desiccated state, protecting the photosynthetic apparatus from photoinhibitory damage. These changes can be easily reversed upon rehydration, returning the system to its high photosynthetic quantum efficiency. [ABSTRACT FROM AUTHOR]