Electron Acceptor Side Research Articles

X- and y-type thioredoxins maintain redox homeostasis on photosystem I acceptor side under fluctuating light.

Plants cope with sudden increases in light intensity through various photoprotective mechanisms. Redox regulation by thioredoxin (Trx) systems also contributes to this process. Whereas the functions of f- and m-type Trxs in response to such fluctuating light conditions have been extensively investigated, those of x- and y-type Trxs are largely unknown. Here, we analyzed the trx x single, trx y1 trx y2 double, and trx x trx y1 trx y2 triple mutants in Arabidopsis (Arabidopsis thaliana). A detailed analysis of photosynthesis revealed changes in photosystem I (PSI) parameters under low light in trx x and trx x trx y1 trx y2. The electron acceptor side of PSI was more reduced in these mutants than in the wild type. This mutant phenotype was more pronounced under fluctuating light conditions. During both low- and high-light phases, the PSI acceptor side was largely limited in trx x and trx x trx y1 trx y2. After fluctuating light treatment, we observed more severe PSI photoinhibition in trx x and trx x trx y1 trx y2 than in the wild type. Furthermore, when grown under fluctuating light conditions, trx x and trx x trx y1 trx y2 plants showed impaired growth and decreased level of PSI subunits. These results suggest that Trx x and Trx y prevent redox imbalance on the PSI acceptor side, which is required to protect PSI from photoinhibition, especially under fluctuating light. We also propose that Trx x and Trx y contribute to maintaining the redox balance even under constant low-light conditions to prepare for sudden increases in light intensity.

Structure of Chlorella ohadii Photosystem II Reveals Protective Mechanisms against Environmental Stress.

Green alga Chlorella ohadii is known for its ability to carry out photosynthesis under harsh conditions. Using cryogenic electron microscopy (cryoEM), we obtained a high-resolution structure of PSII at 2.72 Å. This structure revealed 64 subunits, which encompassed 386 chlorophylls, 86 carotenoids, four plastoquinones, and several structural lipids. At the luminal side of PSII, a unique subunit arrangement was observed to protect the oxygen-evolving complex. This arrangement involved PsbO (OEE1), PsbP (OEE2), PsbB, and PsbU (a homolog of plant OEE3). PsbU interacted with PsbO, PsbC, and PsbP, thereby stabilizing the shield of the oxygen-evolving complex. Significant changes were also observed at the stromal electron acceptor side. PsbY, identified as a transmembrane helix, was situated alongside PsbF and PsbE, which enclosed cytochrome b559. Supported by the adjacent C-terminal helix of Psb10, these four transmembrane helices formed a bundle that shielded cytochrome b559 from the surrounding solvent. Moreover, the bulk of Psb10 formed a protective cap, which safeguarded the quinone site and likely contributed to the stacking of PSII complexes. Based on our findings, we propose a protective mechanism that prevents QB (plastoquinone B) from becoming fully reduced. This mechanism offers insights into the regulation of electron transfer within PSII.

Rubredoxin 1 promotes the proper folding of D1 and is not required for heme b559 assembly in Chlamydomonas photosystem II

Photosystem II (PSII), the water:plastoquinone oxidoreductase of oxygenic photosynthesis, contains a heme b559 iron whose axial ligands are provided by histidine residues from the α (PsbE) and β (PsbF) subunits. PSII assembly depends on accessory proteins that facilitate the step-wise association of its protein and pigment components into a functional complex, a process that is challenging to study due to the low accumulation of assembly intermediates. Here, we examined the putative role of the iron[1Fe-0S]-containing protein rubredoxin 1 (RBD1) as an assembly factor for cytochrome b559, using the RBD1-lacking 2pac mutant from Chlamydomonas reinhardtii, in which the accumulation of PSII was rescued by the inactivation of the thylakoid membrane FtsH protease. To this end, we constructed the double mutant 2pac ftsh1-1, which harbored PSII dimers that sustained its photoautotrophic growth. We purified PSII from the 2pac ftsh1-1 background and found that α and β cytochrome b559 subunits are still present and coordinate heme b559 as in the WT. Interestingly, immunoblot analysis of dark- and low light-grown 2pac ftsh1-1 showed the accumulation of a 23-kDa fragment of the D1 protein, a marker typically associated with structural changes resulting from photodamage of PSII. Its cleavage occurs in the vicinity of a nonheme iron which binds to PSII on its electron acceptor side. Altogether, our findings demonstrate that RBD1 is not required for heme b559 assembly and point to a role for RBD1 in promoting the proper folding of D1, possibly via delivery or reduction of the nonheme iron during PSII assembly.

Functional Water Networks in Fully Hydrated Photosystem II.

Water channels and networks within photosystem II (PSII) of oxygenic photosynthesis are critical for enzyme structure and function. They control substrate delivery to the oxygen-evolving center and mediate proton transfer at both the oxidative and reductive endpoints. Current views on PSII hydration are derived from protein crystallography, but structural information may be compromised by sample dehydration and technical limitations. Here, we simulate the physiological hydration structure of a cyanobacterial PSII model following a thorough hydration procedure and large-scale unconstrained all-atom molecular dynamics enabled by massively parallel simulations. We show that crystallographic models of PSII are moderately to severely dehydrated and that this problem is particularly acute for models derived from X-ray free electron laser (XFEL) serial femtosecond crystallography. We present a fully hydrated representation of cyanobacterial PSII and map all water channels, both static and dynamic, associated with the electron donor and acceptor sides. Among them, we describe a series of transient channels and the attendant conformational gating role of protein components. On the acceptor side, we characterize a channel system that is absent from existing crystallographic models but is likely functionally important for the reduction of the terminal electron acceptor plastoquinone QB. The results of the present work build a foundation for properly (re)evaluating crystallographic models and for eliciting new insights into PSII structure and function.

An Insight into the Bicarbonate Effect in Photosystem II through the Prism of the JIP Test

Photosystem II (PSII) is the unique pigment–protein complex that is capable of evolving molecular oxygen using solar energy. The activity of PSII determines the overall productivity of all oxygenic photosynthetic organisms. It is well known that the absence of HCO3− induces a drop in the activity of PSII. However, it is not yet clear what type of photochemical reaction, single turn-over or multiple turn-over, HCO3− is involved in. Kinetic parameters of this (these) involvement(s) are almost unexplored now. This work addresses these issues. Using the JIP test, being the perspective noninvasive method for measuring PSII activity in plants, this paper describes how HCO3− deficiency affects the electron transfer on the oxidizing as well as the reducing sides of PSII in thylakoids and in PSII preparations from the leaves of pea plants. HCO3− was found to be simultaneously involved both in single turn-over and in multiple turn-over events (“dynamical processes”). Moreover, the involvement of HCO3− in dynamical photochemical processes was revealed to be associated with both sides of PSII, being the rate limiting on the reducing side, which follows from obtained kinetic parameters. The involvement of HCO3− in dynamical processes as the constant exchangeable ligand is discussed for both the electron donor and acceptor sides of PSII.

Redox properties and regulatory mechanism of the iron-quinone electron acceptor in photosystem II as revealed by FTIR spectroelectrochemistry.

Photosystem II (PSII) performs oxidation of water and reduction of plastoquinone through light-induced electron transfer. Electron transfer reactions at individual redox cofactors are controlled by their redox potentials, and the forward and backward electron flows in PSII are regulated by tuning them. It is, thus, crucial to accurately estimate the redox potentials of the cofactors and their shifts by environmental changes to understand the regulatory mechanisms in PSII. Fourier-transform infrared (FTIR) spectroelectrochemistry combined with a light-induced difference technique is a powerful method to investigate the mechanisms of the redox reactions in PSII. In this review, we introduce the methodology and the application of this method in the studies of the iron-quinone complex, which consists of two plastoquinone molecules, QA and QB, and the non-heme iron, on the electron-acceptor side of PSII. It is shown that FTIR spectroelectrochemistry is a useful method not only for estimating the redox potentials but also for detecting the reactions of nearby amino-acid residues coupled with the redox reactions.

Distance and Potential Dependence of Charge Transport Through the Reaction Center of Individual Photosynthetic Complexes

AbstractCharge separation and transport through the reaction center of photosystem I (PSI) is an essential part of the photosynthetic electron transport chain. A strategy is developed to immobilize and orient PSI complexes on gold electrodes allowing to probe the complex's electron acceptor side, the chlorophyll special pair P700. Electrochemical scanning tunneling microscopy (ECSTM) imaging and current–distance spectroscopy of single protein complex shows lateral size in agreement with its known dimensions, and a PSI apparent height that depends on the probe potential revealing a gating effect in protein conductance. In current–distance spectroscopy, it is observed that the distance‐decay constant of the current between PSI and the ECSTM probe depends on the sample and probe electrode potentials. The longest charge exchange distance (lowest distance‐decay constant β) is observed at sample potential 0 mV/SSC (SSC: reference electrode silver/silver chloride) and probe potential 400 mV/SSC. These potentials correspond to hole injection into an electronic state that is available in the absence of illumination. It is proposed that a pair of tryptophan residues located at the interface between P700 and the solution and known to support the hydrophobic recognition of the PSI redox partner plastocyanin, may have an additional role as hole exchange mediator in charge transport through PSI.

Rovibrational spectra of Ar–SO2 and (SO2)2 van der Waals complexes in the v1 region of SO2

Rovibrational absorption spectra of Ar–SO2 and (SO2)2 van der Waals complexes generated in a slit supersonic jet expansion have been recorded in the v1 symmetric stretching region of SO2 monomer using an external-cavity quantum cascade laser. Two vibrational bands for (SO2)2 and one for Ar–SO2 are assigned and analyzed. All these vibrational bands are of a/c hybrid type. The observed infrared transitions are fitted together with previous radio frequency and microwave spectra, yielding precise molecular constants in both the ground and excited vibrational states for these two complexes. The band origins of (SO2)2 are determined to be 1150.30319(13) cm−1 for vibration on the electron-donor side and 1153.40911(10) cm−1 for vibration on the electron-acceptor side, which is shifted from that of the SO2v1 band by about −1.41 cm-1 and + 1.70 cm−1, respectively. The vibrational energies of the two tunneling components of Ar–SO2 are determined to be 1151.61315(11) cm−1 and 1151.64840(11) cm−1. The tunneling splitting of Ar–SO2 increases from 982.881(32) MHz in the ground state to 1056.7(34) MHz in the excited vibrational state, indicating a decrease of the tunneling barrier upon excitation of the SO2 symmetric stretching vibration.

Cover Feature: Tailoring Quantum Dot Sizes for Optimal Photoinduced Catalytic Activation of Nitrogenase (ChemSusChem 24/2021)

The Cover Feature shows how nanomaterial sizes affect the light-induced electron transfer process with the nitrogenase enzyme. When 2.3 nm CdSe quantum dots are utilized for the photocatalytic hydrogen generation process, an 8-fold increase can be gained as compared with larger-sized quantum dots. Additionally, cysteine-mutated nitrogenase at the electron acceptor side (P-cluster) does not lead to enhanced catalytic performance. More information can be found in the Communication by M. M. Meirovich et al.

TMT-based quantitative proteomic analysis of the effects of Pseudomonas syringae pv. tabaci (Pst) infection on photosynthetic function and the response of the MAPK signaling pathway in tobacco leaves

To reveal the mechanism of photosynthesis inhibition by infection and the response of the MAPK signaling pathway to pathogen infection, tobacco leaves were inoculated with Pseudomonas syringae pv. tabaci (Pst), and the effects of Pst infection on photosynthesis of tobacco leaves were studied by physiological and proteomic techniques, with a focus on MAPK signaling pathway related proteins. Pst infection was observed to lead to the degradation of chlorophyll (especially Chl b) in tobacco leaves and the down-regulation of light harvesting antenna proteins expression, thus limiting the light harvesting ability. The photosystem II and I (PSII and PSI) activities were also decreased, and Pst infection inhibited the utilization of light and CO2. Proteomic analyses showed that the number of differentially expressed proteins (DEPs) under Pst infection at 3 d were significantly higher than at 1 d, especially the number of down-regulated proteins. The KEGG enrichment of DEPs was mainly enriched in the energy metabolism processes such as photosynthesis antenna proteins and photosynthesis. The down-regulation of chlorophyll a-b binding protein, photosynthetic electron transport related proteins (e.g., PSII and PSI core proteins, the Cytb6/f complex, PC, Fd, FNR), ATP synthase subunits, and key enzymes in the Calvin cycle were the key changes associated with Pst infection that may inhibit tobacco photosynthesis. The effect of Pst infection on the PSII electron acceptor side was significantly greater than that on the PSII donor side. The main factor that decreased the photosynthetic ability of tobacco leaves with Pst infection at 1 d may be the inhibition of photochemical reactions leading to an insufficient supply of ATP, rather than decreased expression of enzymes involved in the Calvin cycle. At 1 d into Pst infection, the PSII regulated energy dissipation yield Y(NPQ) may play a role in preventing photosynthetic inhibition in tobacco leaves, but the long-term Pst infection significantly inhibited Y(NPQ) and the expression of PsbS proteins. Proteins involved in the MAPK signaling pathway were up-regulated, suggesting the MAPK signaling pathway was activated to respond to Pst infection. However, at the late stage of Pst infection (at 3 d), MAPK signaling pathway proteins were degraded, and the defense function of the MAPK signaling pathway in tobacco leaves was damaged.

Spatiotemporal Heterogeneity of Chlorophyll Content and Fluorescence Response Within Rice (Oryza sativa L.) Canopies Under Different Nitrogen Treatments.

Accurate acquisition of plant phenotypic information has raised long-standing concerns in support of crop breeding programs. Different methods have been developed for high throughput plant phenotyping, while they mainly focused on the canopy level without considering the spatiotemporal heterogeneity at different canopy layers and growth stages. This study aims to phenotype spatiotemporal heterogeneity of chlorophyll (Chl) content and fluorescence response within rice leaves and canopies. Multipoint Chl content and high time-resolved Chl a fluorescence (ChlF) transient (OJIP transient) of rice plants were measured at different nitrogen levels and growth stages. Results showed that the Chl content within the upper leaves exhibited an increasing trend from the basal to the top portions but a decreasing pattern within the lower leaves at the most growth stages. Leaf Chl content within the rice canopy was higher in the lower leaves in the vegetative phase, while from the initial heading stage the pattern gradually reversed with the highest Chl content appearing in the upper leaves. Nitrogen supply mainly affects the occurrence time of the reverse vertical pattern. This could be the result of different nutritional demands of leaves transforming from sinks to sources, and it was further confirmed by the fall of the JI phase of OJIP transient in the vegetative phase and the rise in the reproductive phase. We further deduced that the vertical distribution of Chl content could have a defined pattern at a specific growth stage. Furthermore, the reduction of end acceptors at photosystem I (PSI) electron acceptor side per cross section (RE0/CS) was found to be a potential sensitive predictor for identifying the vertical heterogeneity of leaf Chl content. These findings provide prior knowledge on the vertical profiles of crop physiological traits, which explore the opportunity to develop more efficient plant phenotyping tools for crop breeding.

Achieving small non-radiative energy loss through synergically non-fullerene electron acceptor selection and side chain engineering in benzo[1,2-b:4,5-b′]difuran polymer-based organic solar cells

The energy losses of benzo[1,2-b:4,5-b′]difuran polymers (P-FT and P-FP) with different side chains were systemically investigated by applying two representative electron acceptors (m-ITIC and Y6) in organic solar cells.

How dorice seedlings of landrace Pokkali survive in saline fields after transplantation? Physiology, biochemistry, and photosynthesis.

Rice, one of the most important staple food crops in the world, is highly sensitive to soil salinity at the seedling stage. The ultimate yield of this crop is a function of the number of seedlings surviving after transplantation in saline water. Oryza sativa cv. IR64 is a high-yielding salinity-sensitive variety, while Pokkali is a landrace traditionally cultivated by the local farmers in the coastal regions in India. However, the machinery responsible for the seedling-stage tolerance in Pokkali is not understood. To bridge this gap, we subjected young seedlings of these contrasting genotypes to salinity and performed detailed investigations about their growth parameters, ion homeostasis, biochemical composition, and photosynthetic parameters after every 24h of salinity for three days. Taken together, all the physiological and biochemical indicators, such as proline accumulation, K+/Na+ ratio, lipid peroxidation, and electrolyte leakage, clearly revealed significant differences between IR64 and Pokkali under salinity, establishing their contrasting nature at this stage. In response to salinity, the Fv/Fm ratio (maximum quantum efficiency of Photosystem II as inferred from Chl a fluorescence) and the energy conserved for the electron transport after the reduction of QA (the primary electron acceptor of PSII), to QA-, and reduction of the end electron acceptor molecules towards the PSI (Photosystem I) electron acceptor side was higher in Pokkali than IR64 plants. These observations reflect a direct contribution of photosynthesis towards seedling-stage salinity tolerance in rice. These findings will help to breed high-yielding crops for salinity prone agricultural lands.

Does the water-oxidizing Mn4CaO5 cluster regulate the redox potential of the primary quinone electron acceptor QA in photosystem II? A study by Fourier transform infrared spectroelectrochemistry

Redox titration using fluorescence measurements of photosystem II (PSII) has long shown that impairment of the water-oxidizing Mn4CaO5 cluster upshifts the redox potential (Em) of the primary quinone electron acceptor QA by more than 100 mV, which has been proposed as a photoprotection mechanism of PSII. However, the molecular mechanism of this long-distance interaction between the Mn4CaO5 cluster and QA in PSII remains unresolved. In this study, we reinvestigated the effect of depletion of the Mn4CaO5 cluster on Em(QA−/QA) using Fourier transform infrared (FTIR) spectroelectrochemistry, which can directly monitor the redox state of QA at an intended potential. Light-induced FTIR difference measurements at a series of electrode potentials for intact and Mn-depleted PSII preparations from spinach and Thermosynechococcus elongatus showed that depletion of the Mn4CaO5 cluster hardly affected the Em(QA−/QA) values. In contrast, fluorescence spectroelectrochemical measurement using the same PSII sample, electrochemical cell, and redox mediators reproduced a large upshift of apparent Em upon Mn depletion, whereas a smaller shift was observed when weaker visible light was used for fluorescence excitation. Thus, the possibility was suggested that the measuring light for fluorescence disturbed the titration curve in Mn-depleted PSII, in contrast to no interference of infrared light with the PSII reactions in FTIR measurements. From these results, it was concluded that the Mn4CaO5 cluster does not directly regulate Em(QA−/QA) to control the redox reactions on the electron acceptor side of PSII.

Effects of directional planting on light environment and leaf photosynthesis of summer maize population

To improve light environment, photosynthetic capacity, and thus the yield of maize, the effects of directional planting on light distribution in canopy and photosynthetic characteristics of ear leaves, as well as the performance of PSII that closely related with photosynthetic characteristics and reflected by the rapid chlorophyll fluorescence kinetic curves were examined in Zhengdan 958 maize variety. The results showed that the orientation of leaves remarkably affected photosynthetically active radiation (PAR) interception of ear leaves, with PAR interception of ear leaves in southward treatment being 271.8% higher than that under northward treatment. The orientation of leaves affec-ted photosynthetic light use efficiency of ear leaves under high and low light conditions. The southward treatment increased net photosynthetic rate (Pn) under saturated light in ear leaves, indicating that the use efficiency to high light was enhanced in leaves of southward treatment. In contrast, the northward treatment increased the apparent quantum yield (α) of ear leaves, indicating leaves in southward treatment adapted the light-limited environment. During the early stage after anthesis, the performance of PSII electron donor side and electron acceptor side was significantly improved, and thus enhanced the performance of PSII reaction center (PIABS) and fluorescence photochemical quenching coefficient (Ψo) in ear leaves of southward treatment. The increase of quantum yield of electron transfer (φEo) indicated the enhancement of transfer performance of electrons from photosystem 2 (PSII) to photosystem 1 (PSI) in leaves of southward treatment. The photosynthetic performance of ear leaves showed a trend of southward > eastward > westward > northward during the early stage after anthesis. Forty days after anthesis, the use efficiency to high light decreased in ear leaves of southward treatment, but the ear leaves of southward treatment showed high use efficiency to low light, which changed the trend of photosynthetic performance of ear leaves to northward > westward > eastward > southward. In summary, northward and eastward treatments improved the light distribution in canopy, the PAR interception of ear leaves, the capacity of photosynthesis and dry matter production, and consequently increased the yield of summer maize.

Oxidation of P700 Ensures Robust Photosynthesis.

In the light, photosynthetic cells can potentially suffer from oxidative damage derived from reactive oxygen species. Nevertheless, a variety of oxygenic photoautotrophs, including cyanobacteria, algae, and plants, manage their photosynthetic systems successfully. In the present article, we review previous research on how these photoautotrophs safely utilize light energy for photosynthesis without photo-oxidative damage to photosystem I (PSI). The reaction center chlorophyll of PSI, P700, is kept in an oxidized state in response to excess light, under high light and low CO2 conditions, to tune the light utilization and dissipate the excess photo-excitation energy in PSI. Oxidation of P700 is co-operatively regulated by a number of molecular mechanisms on both the electron donor and acceptor sides of PSI. The strategies to keep P700 oxidized are diverse among a variety of photoautotrophs, which are evolutionarily optimized for their ecological niche.

A sixty-year tryst with photosynthesis and related processes: an informal personal perspective.

After briefly describing my early collaborative work at the University of Allahabad, that had laid the foundation of my research life, I present here some of our research on photosynthesis at the University of Illinois at Urbana-Champaign, randomly selected from light absorption to NADP+ reduction in plants, algae, and cyanobacteria. These include the fact that (i) both the light reactions I and II are powered by light absorbed by chlorophyll (Chl) a of different spectral forms; (ii) light emission (fluorescence, delayed fluorescence, and thermoluminescence) by plants, algae, and cyanobacteria provides detailed information on these reactions and beyond; (iii) primary photochemistry in both the photosystems I (PS I) and II (PS II) occurs within a few picoseconds; and (iv) most importantly, bicarbonate plays a unique role on the electron acceptor side of PS II, specifically at the two-electron gate of PS II. Currently, the ongoing research around the world is, and should be, directed towards making photosynthesis better able to deal with the global issues (such as increasing population, dwindling resources, and rising temperature) particularly through genetic modification. However, basic research is necessary to continue to provide us with an understanding of the molecular mechanism of the process and to guide us in reaching our goals of increasing food production and other chemicals we need for our lives.

Increase of nitrogen to promote growth of poplar seedlings and enhance photosynthesis under NaCl stress

The solution culture method was used to study the effect of increasing nitrogen on the growth and photosynthesis of poplar seedlings under 100 mmol L−1 NaCl stress. I Increase in nitrogen reduced stomatal limitation of leaves under NaCl stress, improved utilization of CO2 by mesophyll cells, enhanced photosynthetic carbon assimilation capacity, significantly alleviated saline damage of NaCl, and promoted the accumulation of aboveground and root biomass. I Increased nitrogen enhanced photochemical efficiency (ФPSII) and electron transport rates, relieved the reduction of maximum photochemical efficiency (Fv/Fm) under NaCl, and reduced the degree of photoinhibition caused by NaCl stress. Increased nitrogen applications reduced the proportion of energy dissipating in the form of ineffective heat energy and hence a greater proportion of light energy absorbed by leaves was allocated to photochemical reactions. Under treatment with increased nitrogen, the synergistic effect of heat dissipation and the xanthophyll cycle in the leaves effectively protected photosynthetic PSII and enhanced light energy utilization of leaves under NaCl stress. The increased nitrogen promoted photosynthetic electron supply and transport ability under NaCl stress evident in enhanced functioning of the oxygen-evolving complex on the electron donor side of PS II. It increased the ability of the receptor pool to accept electrons on the PSII electron acceptor side and improved the stability of thylakoid membranes under NaCl stress. Therefore, increasing nitrogen applications under NaCl stress can promote poplar growth by improving the efficiency of light energy utilization.

Changing frequency of fluctuating light reveals the molecular mechanism for P700 oxidation in plant leaves.

Natural sunlight exceeds the demand of photosynthesis such that it can cause plants to produce reactive oxygen species (ROS), which subsequently cause photo‐oxidative damage. Because photosystem I (PSI) is a major source of ROS, plants actively maintain the reaction center chlorophyll of PSI(P700) oxidized under excessive light conditions to alleviate the ROS production. P700 oxidation is universally recognized in photosynthetic organisms as a physiological response to excessive light. However, it is still poorly understood how P700 oxidation is induced in response to fluctuating light with a variety of frequencies. Here, we investigated the relationships of photosynthetic parameters with P700 oxidation in Arabidopsis thaliana under a sine fluctuating light with different frequencies. As the photon flux density of the light increased, P700 was oxidized concurrently with the chlorophyll fluorescence parameter qL unless the electron acceptor side of PSI was limited. Conversely, we did not observe a proportional relationship of non‐photochemical quenching with P700 oxidation. The mutant crr‐2, which lacks chloroplast NADPH dehydrogenase, was impaired in P700 oxidation during light fluctuation at high, but not low frequency, unlike the pgrl1 mutant deficient in PGR5 and PGRL1 proteins, which could not oxidize P700 during light fluctuation at both high and low frequencies. Taken together, our findings suggested that the changing frequency of fluctuating light reveals the tracking performance of molecular mechanisms underlying P700 oxidation.

Consequences of structural modifications in cytochrome b559 on the electron acceptor side of Photosystem II.

Cytb559 in Photosystem II is a heterodimeric b-type cytochrome. The subunits, PsbE and PsbF, consist each in a membrane α-helix. Mutants were previously designed and studied in Thermosynechococcus elongatus (Sugiura et al., Biochim Biophys Acta 1847:276-285, 2015) either in which an axial histidine ligand of the haem-iron was substituted for a methionine, the PsbE/H23M mutant in which the haem was lacking, or in which the haem environment was modified, the PsbE/Y19F and PsbE/T26P mutants. All these mutants remained active showing that the haem has no structural role provided that PsbE and PsbF subunits are present. Here, we have carried on the characterization of these mutants. The following results were obtained: (i) the Y19F mutation hardly affect the Em of Cytb559, whereas the T26P mutation converts the haem into a form with a Em much below 0mV (so low that it is likely not reducible by QB-) even in an active enzyme; (ii) in the PsbE/H23M mutant, and to a less extent in PsbE/T26P mutant, the electron transfer efficiency from QA- to QB is decreased; (iii) the lower Em of the QA/QA- couple in the PsbE/H23M mutant correlates with a higher production of singlet oxygen; (iv) the superoxide and/or hydroperoxide formation was not increased in the PsbE/H23M mutant lacking the haem, whereas it was significantly larger in the PsbE/T26P. These data are discussed in view of the literature to discriminate between structural and redox roles for the haem of Cytb559 in the production of reactive oxygen species.