A complex and dynamic redox network regulates oxygen reduction at photosystem I in Arabidopsis.

Thiol-dependent redox regulation of enzyme activities plays a central role in regulating photosynthesis. Besides the regulation of metabolic pathways, alternative electron transport is subjected to thiol-dependent regulation. We investigated the regulation of O2 reduction at photosystem I.

Photo-oxidative damage of photosystem I by repetitive flashes and chilling stress in cucumber leaves.

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.

First analysis of the regulatory protein CP12 of the model cyanobacterium PCC 6803: Biotechnological implications.

We report the first analysis of a canonical CP12 regulatory protein, namely the unique CP12 of the model cyanobacterium PCC 6803, which has the advantage of being able to grow photoautotrophically, photomixotrophically, and photoheterotrophically. The data showed that CP12 is dispensable to cell growth under standard (continuous) light and light/dark cycle, whereas it is essential for the catabolism of exogenously added glucose that normally sustains cell growth in absence of photosynthesis. Furthermore, to be active in glucose catabolism, CP12 requires its three conserved features: its AWD_VEEL motif and its two pairs of cysteine residues.

The success of the bloom-forming cyanobacteria Planktothrix: Genotypes variability supports variable responses to light and temperature stress.

Cyanobacterial blooms can modify the dynamic of aquatic ecosystems and have harmful consequences for human activities. Moreover, cyanobacteria can produce a variety of cyanotoxins, including microcystins, but little is known about the role of environmental factors on the prevalence of microcystin producers in the cyanobacterial bloom dynamics. This study aimed to better understand the success of Planktothrix in various environments by unveiling the variety of strategies governing cell responses to sudden changes in light intensity and temperature.

Identification of the electron donor to flavodiiron proteins in Synechocystis sp. PCC 6803 by in vivo spectroscopy.

Flavodiiron proteins (FDPs) of photosynthetic organisms play a photoprotective role by reducing oxygen to water and thus avoiding the accumulation of excess electrons on the photosystem I (PSI) acceptor side under stress conditions. In Synechocystis sp. PCC 6803 grown under high CO, both FDPs Flv1 and Flv3 are indispensable for oxygen reduction.

Near-infrared in vivo measurements of photosystem I and its lumenal electron donors with a recently developed spectrophotometer.

In photosynthesis research, non-invasive in vivo spectroscopic analyses have been used as a practical tool for studying photosynthetic electron transport. Klas-NIR spectrophotometer has been recently developed by Klughammer and Schreiber (Photosynth Res 128:195-214, 2016) for in vivo measurements of redox changes of P700, plastocyanin (Pcy) and ferredoxin (Fd). Here we show examples using the Klas-NIR spectrophotometer for the evaluation of the redox states and quantities of these components in plant leaves and cyanobacterial suspensions.

Role of the two PsaE isoforms on O reduction at photosystem I in Arabidopsis thaliana.

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 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 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.

An alternative plant-like cyanobacterial ferredoxin with unprecedented structural and functional properties.

Photosynthetic [2Fe-2S] plant-type ferredoxins have a central role in electron transfer between the photosynthetic chain and various metabolic pathways. Several genes are coding for [2Fe2S] ferredoxins in cyanobacteria, with four in the thermophilic cyanobacterium Thermosynechococcus elongatus. The structure and functional properties of the major ferredoxin Fd1 are well known but data on the other ferredoxins are scarce.

Near-infrared in vitro measurements of photosystem I cofactors and electron-transfer partners with a recently developed spectrophotometer.

A kinetic-LED-array-spectrophotometer (Klas) was recently developed for measuring in vivo redox changes of P700, plastocyanin (PCy), and ferredoxin (Fd) in the near-infrared (NIR). This spectrophotometer is used in the present work for in vitro light-induced measurements with various combinations of photosystem I (PSI) from tobacco and two different cyanobacteria, spinach plastocyanin, cyanobacterial cytochrome c (cyt. c), and Fd.

Light stress in green and red Planktothrix strains: The orange carotenoid protein and its related photoprotective mechanism.

Photosynthetic organisms need to sense and respond to fluctuating environmental conditions, to perform efficient photosynthesis and avoid the formation of harmful reactive oxygen species. Cyanobacteria have developed a photoprotective mechanism that decreases the energy arriving at the reaction centers by increasing thermal energy dissipation at the level of the phycobilisome, the extramembranal light-harvesting antenna. This mechanism is triggered by the photoactive orange carotenoid protein (OCP).

Different roles for ApcD and ApcF in Synechococcus elongatus and Synechocystis sp. PCC 6803 phycobilisomes.

The phycobilisome, the cyanobacterial light harvesting complex, is a huge phycobiliprotein containing extramembrane complex, formed by a core from which rods radiate. The phycobilisome has evolved to efficiently absorb sun energy and transfer it to the photosystems via the last energy acceptors of the phycobilisome, ApcD and ApcE. ApcF also affects energy transfer by interacting with ApcE.

Electron transport pathways in isolated chromoplasts from Narcissus pseudonarcissus L.

During daffodil flower development, chloroplasts differentiate into photosynthetically inactive chromoplasts having lost functional photosynthetic reaction centers. Chromoplasts exhibit a respiratory activity reducing oxygen to water and generating ATP. Immunoblots revealed the presence of the plastid terminal oxidase (PTOX), the NAD(P)H dehydrogenase (NDH) complex, the cytochrome b f complex, ATP synthase and several isoforms of ferredoxin-NADP oxidoreductase (FNR), and ferredoxin (Fd).

The Cytochrome Complex Is Not Involved in Cyanobacterial State Transitions.

Photosynthetic organisms must sense and respond to fluctuating environmental conditions in order to perform efficient photosynthesis and to avoid the formation of dangerous reactive oxygen species. The excitation energy arriving at each photosystem permanently changes due to variations in the intensity and spectral properties of the absorbed light. Cyanobacteria, like plants and algae, have developed a mechanism, named "state transitions," that balances photosystem activities.

Structural adaptations of photosynthetic complex I enable ferredoxin-dependent electron transfer.

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 The model reveals structural adaptations that facilitate binding and electron transfer from the photosynthetic electron carrier ferredoxin.

X-ray structure of an asymmetrical trimeric ferredoxin-photosystem I complex.

Photosystem I (PSI), a large protein complex located in the thylakoid membrane, mediates the final step in light-driven electron transfer to the stromal electron carrier protein ferredoxin (Fd). Here, we report the first structural description of the PSI-Fd complex from Thermosynechococcus elongatus. The trimeric PSI complex binds three Fds in a non-equivalent manner.

Dynamics and energetics of cyanobacterial photosystem I:ferredoxin complexes in different redox states.

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 (FF). Newly obtained values were derived firstly from the fact that the dissociation constant between PSI and redox-inactive gallium-substituted Fd increases upon (FF) 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.

Gallium ferredoxin as a tool to study the effects of ferredoxin binding to photosystem I without ferredoxin reduction.

Reduction of ferredoxin by photosystem I (PSI) involves the [4Fe-4S] clusters F and F harbored by PsaC, with F being the direct electron transfer partner of ferredoxin (Fd). Binding of the redox-inactive gallium ferredoxin to PSI was investigated by flash-absorption spectroscopy, studying both the P700 decay and the reduction of the native iron Fd in the presence of Fd. Fd binding resulted in a faster recombination between P700 and (F, F), a slower electron escape from (F, F) to exogenous acceptors, and a decreased amount of intracomplex Fd reduction, in accordance with competitive binding between Fd and Fd.

Photoreduction of the ferredoxin/ferredoxin-NADP(+)-reductase complex by a linked ruthenium polypyridyl chromophore.

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.

Pre-steady-state kinetic studies of redox reactions catalysed by Bacillus subtilis ferredoxin-NADP(+) oxidoreductase with NADP(+)/NADPH and ferredoxin.

Ferredoxin-NADP(+) oxidoreductase ([EC1.18.1.

Electron-transfer kinetics in cyanobacterial cells: methyl viologen is a poor inhibitor of linear electron flow.

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.

In vitro analysis of the plastid terminal oxidase in photosynthetic electron transport.

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.

Kinetic studies of a ferredoxin-dependent cyanobacterial nitrate reductase.

A flash photolysis study of electron transfer (ET) kinetics from reduced ferredoxin (photoreduced by Photosystem I) to the ferredoxin-dependent nitrate reductase from the cyanobacterium Synechococcus sp. PCC 7942 has been carried out. In the presence of nitrate, under conditions where only a single electron is transferred to nitrate reductase, the rate of enzyme reduction shows a biphasic concentration dependence: At low enzyme concentrations the dependence is approximately linear, with an estimated second-order rate constant of 7.

NADPH fluorescence in the cyanobacterium Synechocystis sp. PCC 6803: a versatile probe for in vivo measurements of rates, yields and pools.

We measured the kinetics of light-induced NADPH formation and subsequent dark consumption by monitoring in vivo its fluorescence in the cyanobacterium Synechocystis PCC 6803. Spectral data allowed the signal changes to be attributed to NAD(P)H and signal linearity vs the chlorophyll concentration was shown to be recoverable after appropriate correction. Parameters associated to reduction of NADP(+) to NADPH by ferredoxin-NADP(+)-oxidoreductase were determined: After single excitation of photosystem I, half of the signal rise is observed in 8ms; Evidence for a kinetic limitation which is attributed to an enzyme bottleneck is provided; After two closely separated saturating flashes eliciting two photosystem I turnovers in less than 2ms, more than 50% of the cytoplasmic photoreductants (reduced ferredoxin and photosystem I acceptors) are diverted from NADPH formation by competing processes.

Detection of the photosystem I:ferredoxin complex by backscattering interferometry.

The dissociation constant K(d) of the photosystem I (PSI):ferredoxin complex has been measured by backscattering interferometry (BSI) with cyanobacterial PSI (350 kDa) and ferredoxin (10.5 kDa). The BSI signal, consisting of shifts for interference fringes resulting from a change in refractive index due to complex formation, was monitored as ferredoxin concentration was titrated.