Identification of novel Ssl0352 protein (NdhS), essential for efficient operation of cyclic electron transport around photosystem I, in NADPH:plastoquinone oxidoreductase (NDH-1) complexes of Synechocystis sp. PCC 6803

13C-metabolic flux analysis of respiratory chain disrupted strain ΔndhF1 of Synechocystis sp. PCC 6803

Cyanobacteria are advantageous hosts for industrial applications toward achieving sustainable society due to their unique and superior properties such as atmospheric CO2 fixation via photosynthesis. However, cyanobacterial productivities tend to be weak compared to heterotrophic microbes. To enhance them, it is necessary to understand the fundamental metabolic mechanisms unique to cyanobacteria. In cyanobacteria, NADPH and ATP regenerated by linear and cyclic electron transfers using light energy are consumed by CO2 fixation in a central metabolic pathway. The previous study demonstrated that the strain deleted a part of respiratory chain complex (ΔndhF1) perturbed NADPH levels and photosynthetic activity in Synechocystis sp. PCC 6803. It is expected that disruption of ndhF1 would result in a decrease in the function of cyclic electron transfer, which controls the ATP/NAD(P)H production ratio properly. In this study, we evaluated the effects of ndhF1 deletion on central metabolism and photosynthesis by 13C-metabolic flux analysis. As results of culturing the control and ΔndhF1 strains in a medium containing [1,2-13C] glucose and estimating the flux distribution, CO2 fixation rate by RuBisCO was decreased to be less than half in the ΔndhF1 strain. In addition, the regeneration rate of NAD(P)H and ATP by the photosystem, which can be estimated from the flux distribution, also decreased to be less than half in the ΔndhF1 strain, whereas no significant difference was observed in ATP/NAD(P)H production ratio between the control and the ΔndhF1 strains. Our result suggests that the ratio of utilization of cyclic electron transfer is not reduced in the ΔndhF1 strain unexpectedly.

Genome-wide insights into the evolutionary history of conserved photosynthetic NDH-1 in cyanobacteria

The integration of novel components into functional multi-subunit protein complexes is a key evolutionary strategy for enhancing stability, activity, and adaptation to oxidative stress. This is exemplified by the evolution of the conserved photosynthetic NDH-1 (cpNDH-1) complex, though its precise evolutionary history remains unresolved. In this study, we constructed a time-calibrated phylogenetic tree of cyanobacteria to trace the evolutionary trajectory of cpNDH-1. By mapping the orthologous of oxygenic photosynthesis-specific (OPS) subunits onto this tree, we found that the cpNDH-1 complex progressively acquired OPS subunits. Specifically, during the transition from non-photosynthetic to thylakoid-less photosynthetic cyanobacteria, cpNDH-1 incorporated OPS subunits NdhM, NdhN, NdhO, NdhP, and NdhS. Subsequently, NdhL, NdhQ, and NdhV were added as thylakoid-bearing photosynthetic cyanobacteria evolved. Our analysis reveals that the emergence of oxygenic photosynthesis was closely linked with the progressive incorporation of OPS subunits into cpNDH-1. We propose a two-step model for the evolution of these subunits, identifying potential driving factors behind this process. Genome-wide sequence analysis and structural predications further suggest that the OPS cpNDH-1 genes either evolved de novo or arose from modifications of existing genes. Collectively, these findings provide a robust framework for understanding the evolutionary emergence of OPS subunits in cyanobacterial cpNDH-1, underscoring the acquisition of new subunits as a critical adaptation to oxidative environments during the evolution of oxygenic photosynthesis.

A cyanobacteria-derived intermolecular salt bridge stabilizes photosynthetic NDH-1 and prevents oxidative stress

Throughout evolution, addition of numerous cyanobacteria-derived subunits to the photosynthetic NDH-1 complex stabilizes the complex and facilitates cyclic electron transfer around photosystem I (PSI CET), a critical antioxidant mechanism for efficient photosynthesis, but its stabilization mechanism remains elusive. Here, a cyanobacteria-derived intermolecular salt bridge is found to form between the two conserved subunits, NdhF1 and NdhD1. Its disruption destabilizes photosynthetic NDH-1 and impairs PSI CET, resulting in the production of more reactive oxygen species under high light conditions. The salt bridge and transmembrane helix 16, both situated at the C-terminus of NdhF1, collaboratively secure the linkage between NdhD1 and NdhB, akin to a cramping mechanism. The linkage is also stabilized by cyanobacteria-derived NdhP and NdhQ subunits, but their stabilization mechanisms are distinctly different. Collectively, to the best of our knowledge, this is the first study to unveil the stabilization mechanism of photosynthetic NDH-1 by incorporating photosynthetic components into its conserved subunits during evolution.

Cryo-EM structure of the NDH-PSI-LHCI supercomplex from Spinacia oleracea

The nicotinamide adenine dinucleotide phosphate (NADPH) dehydrogenase (NDH) complex is crucial for photosynthetic cyclic electron flow and respiration, transferring electrons from ferredoxin to plastoquinone while transporting H+ across the chloroplast membrane. This process boosts adenosine triphosphate production, regardless of NADPH levels. In flowering plants, NDH forms a supercomplex with photosystem I, enhancing its stability under high light. We report the cryo-electron microscopy structure of the NDH supercomplex in Spinacia oleracea at a resolution of 3.0-3.3 Å. The supercomplex consists of 41 protein subunits, 154 chlorophylls and 38 carotenoids. Subunit interactions are reinforced by 46 distinct lipids. The structure of NDH resembles that of mitochondrial complex I closely, including the quinol-binding site and an extensive internal aqueous passage for proton translocation. A well-resolved catalytic plastoquinone (PQ) occupies the PQ channel. The pronounced structural similarity to complex I sheds light on electron transfer and proton translocation within the NDH supercomplex.

Molecular Genetic Dissection of the Regulatory Network of Proton Motive Force in Chloroplasts

The proton motive force (pmf) generated across the thylakoid membrane rotates the Fo-ring of ATP synthase in chloroplasts. The pmf comprises two components: membrane potential (∆Ψ) and proton concentration gradient (∆pH). Acidification of the thylakoid lumen resulting from ∆pH downregulates electron transport in the cytochrome b6f complex. This process, known as photosynthetic control, is crucial for protecting photosystem I (PSI) from photodamage in response to fluctuating light. To optimize the balance between efficient photosynthesis and photoprotection, it is necessary to regulate pmf. Cyclic electron transport around PSI and pseudo-cyclic electron transport involving flavodiiron proteins contribute to the modulation of pmf magnitude. By manipulating the ratio between the two components of pmf, it is possible to modify the extent of photosynthetic control without affecting the pmf size. This adjustment can be achieved by regulating the movement of ions (such as K+ and Cl-) across the thylakoid membrane. Since ATP synthase is the primary consumer of pmf in chloroplasts, its activity must be precisely regulated to accommodate other mechanisms involved in pmf optimization. Although fragments of information about each regulatory process have been accumulated, a comprehensive understanding of their interactions is lacking. Here, I summarize current knowledge of the network for pmf regulation, mainly based on genetic studies.

Roles of ApcD and orange carotenoid protein in photoinduction of electron transport upon dark-light transition in the Synechocystis PCC 6803 mutant deficient in flavodiiron protein Flv1

Flavodiiron proteins Flv1/Flv3 accept electrons from photosystem (PS) I. In this work we investigated light adaptation mechanisms of Flv1-deficient mutant of Synechocystis PCC 6803, incapable to form the Flv1/Flv3 heterodimer. First seconds of dark-light transition were studied by parallel measurements of light-induced changes in chlorophyll fluorescence, P700 redox transformations, fluorescence emission at 77 K, and OCP-dependent fluorescence quenching. During the period of Calvin cycle activation upon dark-light transition, the linear electron transport (LET) in wild type is supported by the Flv1/Flv3 heterodimer, whereas in Δflv1 mutant activation of LET upon illumination is preceded by cyclic electron flow that maintains State 2. The State 2-State 1 transition and Orange Carotenoid Protein (OCP)-dependent non-photochemical quenching occur independently of each other, begin in about 10 s after the illumination of the cells and are accompanied by a short-term re-reduction of the PSI reaction center (P700+). ApcD is important for the State 2-State 1 transition in the Δflv1 mutant, but S-M rise in chlorophyll fluorescence was not completely inhibited in Δflv1/ΔapcD mutant. LET in Δflv1 mutant starts earlier than the S-M rise in chlorophyll fluorescence, and the oxidation of plastoquinol (PQH2) pool promotes the activation of PSII, transient re-reduction of P700+ and transition to State 1. An attempt to induce state transition in the wild type under high intensity light using methyl viologen, highly oxidizing P700 and PQH2, was unsuccessful, showing that oxidation of intersystem electron-transport carriers might be insufficient for the induction of State 2-State 1 transition in wild type of Synechocystis under high light.

NdhS interacts with cytochrome b6 f to form a complex in Arabidopsis

Cyclic electron transport (CET) around photosystem I (PSI) is crucial for photosynthesis to perform photoprotection and sustain the balance of ATP and NADPH. However, the critical component of CET, cyt b6 f complex (cyt b6 f), functions in CET has yet to be understood entirely. In this study, we found that NdhS, a subunit of NADPH dehydrogenase-like (NDH) complex, interacted with cyt b6 f to form a complex in Arabidopsis. This interaction depended on the N-terminal extension of NdhS, which was conserved in eukaryotic plants but defective in prokaryotic algae. The migration of NdhS was much more in cyt b6 f than in PSI-NDH super-complex. Based on these results, we suggested that NdhS and NADP+ oxidoreductase provide a docking domain for the mobile electron carrier ferredoxin to transfer electrons to the plastoquinone pool via cyt b6 f in eukaryotic photosynthesis.

Identification of a c-type heme oxygenase and its function during acclimation of cyanobacteria to nitrogen fluctuations

The mechanisms of acclimating to a nitrogen-fluctuating environment are necessary for the survival of aquatic cyanobacteria in their natural habitats, but our understanding is still far from complete. Here, the synthesis of phycobiliprotein is confirmed to be much earlier than that of photosystem components during recovery from nitrogen chlorosis and an unknown protein Ssr1698 is discovered to be involved in this synthetic process. The unknown protein is further identified as a c-type heme oxygenase (cHO) in tetrapyrrole biosynthetic pathway and catalyzes the opening of heme ring to form biliverdin IXα, which is required for phycobilin production and ensuing phycobiliprotein synthesis. In addition, the cHO-dependent phycobiliprotein is found to be vital for the growth of cyanobacterial cells during chlorosis and regreening through its nitrogen-storage and light-harvesting functions, respectively. Collectively, the cHO expressed preferentially during recovery from nitrogen chlorosis is identified in photosynthetic organisms and the dual function of this enzyme-dependent phycobiliprotein is proposed to be an important mechanism for acclimation of aquatic cyanobacteria to a nitrogen-fluctuating environment.

NDH-1L with a truncated NdhM subunit is unstable under stress conditions in cyanobacteria

Cyanobacterial NdhM, an oxygenic photosynthesis-specific NDH-1 subunit, has been found to be essential for the formation of a large complex of NDH-1 (NDH-1L). The cryo-electron microscopic (cryo-EM) structure of NdhM from Thermosynechococcus elongatus showed that the N-terminus of NdhM contains three β-sheets, while two α-helixes are present in the middle and C-terminal part of NdhM. Here, we obtained a mutant of the unicellular cyanobacterium Synechocystis 6803 expressing a C-terminal truncated NdhM subunit designated NdhMΔC. Accumulation and activity of NDH-1 were not affected in NdhMΔC under normal growth conditions. However, the NDH-1 complex with truncated NdhM is unstable under stress. Immunoblot analyses showed that the assembly process of the cyanobacterial NDH-1L hydrophilic arm was not affected in the NdhMΔC mutant even under high temperature. Thus, our results indicate that NdhM can bind to the NDH-1 complex without its C-terminal α-helix, but the interaction is weakened. NDH-1L with truncated NdhM is more prone to dissociation, and this is particularly evident under stress conditions.

Changes in intracellular energetic and metabolite states due to increased galactolipid levels in Synechococcus elongatus PCC 7942

The lipid composition of thylakoid membranes is conserved from cyanobacteria to green plants. However, the biosynthetic pathways of galactolipids, the major components of thylakoid membranes, are known to differ substantially between cyanobacteria and green plants. We previously reported on a transformant of the unicellular rod-shaped cyanobacterium Synechococcus elongatus PCC 7942, namely SeGPT, in which the synthesis pathways of the galactolipids monogalactosyldiacylglycerol and digalactosyldiacylglycerol are completely replaced by those of green plants. SeGPT exhibited increased galactolipid content and could grow photoautotrophically, but its growth rate was slower than that of wild-type S. elongatus PCC 7942. In the present study, we investigated pleiotropic effects that occur in SeGPT and determined how its increased lipid content affects cell proliferation. Microscopic observations revealed that cell division and thylakoid membrane development are impaired in SeGPT. Furthermore, physiological analyses indicated that the bioenergetic state of SeGPT is altered toward energy storage, as indicated by increased levels of intracellular ATP and glycogen. We hereby report that we have identified a new promising candidate as a platform for material production by modifying the lipid synthesis system in this way.

Cyanobacterial NDH-1 Complexes

Light reaction of photosynthesis is efficiently driven by protein complexes arranged in an orderly in the thylakoid membrane. As the 5th complex, NAD(P)H dehydrogenase complex (NDH-1) is involved in cyclic electron flow around photosystem I to protect plants against environmental stresses for efficient photosynthesis. In addition, two kinds of NDH-1 complexes participate in CO2 uptake for CO2 concentration in cyanobacteria. In recent years, great progress has been made in the understanding of the assembly and the structure of NDH-1. However, the regulatory mechanism of NDH-1 in photosynthesis remains largely unknown. Therefore, understanding the regulatory mechanism of NDH-1 is of great significance to reveal the mechanism of efficient photosynthesis. In this mini-review, the author introduces current progress in the research of cyanobacterial NDH-1. Finally, the author summarizes the possible regulatory mechanism of cyanobacterial NDH-1 in photosynthesis and discusses the research prospect.

New insights into the effect of NdhO levels on cyanobacterial cell death triggered by high temperature

NdhO, a regulatory oxygenic photosynthesis-specific subunit, is close to the ferredoxin-binding site of cyanobacterial NDH-1, and its levels are negatively associated with the rates of cyclic electron transfer around PSI mediated by NDH-1 (NDH-CET). However, the effect of NdhO levels on cyanobacterial cell death triggered by high temperature remains elusive. Here, our results uncovered a synergistic effect of NdhO levels on the cell death and reactive oxygen species (ROS) accumulation when cyanobacterial cells grown at 30°C for 1 day were transferred to 45°C for 2 days. Such synergistic effect was found to be closely associated with the activities of NDH-CET and CO2 assimilation during high temperature. Collectively, we propose that the effect of NdhO levels on the cyanobacterial cell bleaching and cell death triggered by high temperature is a result of influencing production of ROS by NDH-CET, which is considered to be vital to balance the ATP/NADPH ratio and improve the Calvin-Benson cycle.

Cyclic Electron Flow-Coupled Proton Pumping in Synechocystis sp. PCC6803 Is Dependent upon NADPH Oxidation by the Soluble Isoform of Ferredoxin:NADP-Oxidoreductase

Ferredoxin:NADP-oxidoreductase (FNR) catalyzes the reversible exchange of electrons between ferredoxin (Fd) and NADP(H). Reduction of NADP+ by Fd via FNR is essential in the terminal steps of photosynthetic electron transfer, as light-activated electron flow produces NADPH for CO2 assimilation. FNR also catalyzes the reverse reaction in photosynthetic organisms, transferring electrons from NADPH to Fd, which is important in cyanobacteria for respiration and cyclic electron flow (CEF). The cyanobacterium Synechocystis sp. PCC6803 possesses two isoforms of FNR, a large form attached to the phycobilisome (FNRL) and a small form that is soluble (FNRS). While both isoforms are capable of NADPH oxidation or NADP+ reduction, FNRL is most abundant during typical growth conditions, whereas FNRS accumulates under stressful conditions that require enhanced CEF. Because CEF-driven proton pumping in the light–dark transition is due to NDH-1 complex activity and they are powered by reduced Fd, CEF-driven proton pumping and the redox state of the PQ and NADP(H) pools were investigated in mutants possessing either FNRL or FNRS. We found that the FNRS isoform facilitates proton pumping in the dark–light transition, contributing more to CEF than FNRL. FNRL is capable of providing reducing power for CEF-driven proton pumping, but only after an adaptation period to illumination. The results support that FNRS is indeed associated with increased cyclic electron flow and proton pumping, which is consistent with the idea that stress conditions create a higher demand for ATP relative to NADPH.

Screening of mutants using chlorophyll fluorescence

Chlorophyll fluorescence has been widely used for the estimation of photosynthesis or its regulatory mechanisms. Chlorophyll fluorescence measurements are the methods with non-destructive nature and do not require contact between plant materials and fluorometers. Furthermore, the measuring process is very rapid. These characteristics of chlorophyll fluorescence measurements make them a suitable tool to screen mutants of photosynthesis-related genes. Furthermore, it has been shown that genes with a wide range of functions can be also analyzed by chlorophyll fluorescence through metabolic interactions. In this short review, we would like to first introduce the basic principle of the chlorophyll fluorescence measurements, and then explore the advantages and limitation of various screening methods. The emphasis is on the possibility of chlorophyll fluorescence measurements to screen mutants defective in metabolisms other than photosynthesis.

The Significance of Chloroplast NAD(P)H Dehydrogenase Complex and Its Dependent Cyclic Electron Transport in Photosynthesis

Chloroplast NAD(P)H dehydrogenase (NDH) complex, a multiple-subunit complex in the thylakoid membranes mediating cyclic electron transport, is one of the most important alternative electron transport pathways. It was identified to be essential for plant growth and development during stress periods in recent years. The NDH-mediated cyclic electron transport can restore the over-reduction in stroma, maintaining the balance of the redox system in the electron transfer chain and providing the extra ATP needed for the other biochemical reactions. In this review, we discuss the research history and the subunit composition of NDH. Specifically, the formation and significance of NDH-mediated cyclic electron transport are discussed from the perspective of plant evolution and physiological functionality of NDH facilitating plants' adaptation to environmental stress. A better understanding of the NDH-mediated cyclic electron transport during photosynthesis may offer new approaches to improving crop yield.

Genetic, Genomics, and Responses to Stresses in Cyanobacteria: Biotechnological Implications

Cyanobacteria are widely-diverse, environmentally crucial photosynthetic prokaryotes of great interests for basic and applied science. Work to date has focused mostly on the three non-nitrogen fixing unicellular species Synechocystis PCC 6803, Synechococcus PCC 7942, and Synechococcus PCC 7002, which have been selected for their genetic and physiological interests summarized in this review. Extensive "omics" data sets have been generated, and genome-scale models (GSM) have been developed for the rational engineering of these cyanobacteria for biotechnological purposes. We presently discuss what should be done to improve our understanding of the genotype-phenotype relationships of these models and generate robust and predictive models of their metabolism. Furthermore, we also emphasize that because Synechocystis PCC 6803, Synechococcus PCC 7942, and Synechococcus PCC 7002 represent only a limited part of the wide biodiversity of cyanobacteria, other species distantly related to these three models, should be studied. Finally, we highlight the need to strengthen the communication between academic researchers, who know well cyanobacteria and can engineer them for biotechnological purposes, but have a limited access to large photobioreactors, and industrial partners who attempt to use natural or engineered cyanobacteria to produce interesting chemicals at reasonable costs, but may lack knowledge on cyanobacterial physiology and metabolism.

Role of type I NADH dehydrogenase in Synechocystis sp. PCC 6803 under phycobilisome excited red light

Cyanobacterial type I NADH dehydrogenase (NDH-1) is involved in various bioenergetic reactions including respiration, cyclic electron transport (CET), and CO₂ uptake. The role of NDH-1 is usually minor under normal growth conditions and becomes important under stress conditions. However, in our previous study, flux balance analysis (FBA) simulation predicted that the drive of NDH-1 as CET pathway with a photosystem (PS) I/PSII excitation ratio around 1.0 contributes to achieving an optimal specific growth rate. In this study, to experimentally elucidate the predicted functions of NDH-1, first, we measured the PSI/PSII excitation ratios of Synechocystis sp. PCC 6803 grown under four types of spectral light conditions. The specific growth rate was the highest and PSI/PSII excitation ratio was with 0.88 under the single-peak light at 630 nm (Red1). Considering this measured excitation ratios, FBA simulation predicted that NDH-1-dependent electron transport was the major pathway under Red1. Moreover, in actual culture, an NDH-1 deletion strain had slower growth rate than that of wild type only under Red1 light condition. Therefore, we experimentally demonstrated that NDH-1 plays an important role under optimal light conditions such as Red1 light, where Synechocystis exhibits the highest specific growth rate and PSI/PSII excitation ratio of around 1.0.

Electron flow through NDH-1 complexes is the major driver of cyclic electron flow-dependent proton pumping in cyanobacteria

Cyclic electron flow (CEF) around photosystem I is vital to balancing the photosynthetic energy budget of cyanobacteria and other photosynthetic organisms. The coupling of CEF to proton pumping has long been hypothesized to occur, providing proton motive force (PMF) for the synthesis of ATP with no net cost to [NADPH]. This is thought to occur largely through the activity of NDH-1 complexes, of which cyanobacteria have four with different activities. While a much work has been done to understand the steady-state PMF in both the light and dark, and fluorescent probes have been developed to observe these fluxes in vivo, little has been done to understand the kinetics of these fluxes, particularly with regard to NDH-1 complexes. To monitor the kinetics of proton pumping in Synechocystis sp. PCC 6803, the pH sensitive dye Acridine Orange was used alongside a suite of inhibitors in order to observe light-dependent proton pumping. The assay was demonstrated to measure photosynthetically driven proton pumping and used to measure the rates of proton pumping unimpeded by dark ΔpH. Here, the cyanobacterial NDH-1 complexes are shown to pump a sizable portion of proton flux when CEF-driven and LEF-driven proton pumping rates are observed and compared in mutants lacking some or all NDH-1 complexes. It is also demonstrated that PSII and LEF are responsible for the bulk of light induced proton pumping, though CEF and NDH-1 are capable of generating ~40% of the proton pumping rate when LEF is inactivated.

Recent advances on the structure and function of NDH-1: The complex I of oxygenic photosynthesis

Photosynthetic NADH dehydrogenase-like complex type-1 (a.k.a, NDH, NDH-1, or NDH-1L) is a multi-subunit, membrane-bound oxidoreductase related to the respiratory complex I. Although originally discovered 30 years ago, a number of recent advances have revealed significant insight into the structure, function, and physiology of NDH-1. Here, we highlight progress in understanding the function of NDH-1 in the photosynthetic light reactions of both cyanobacteria and chloroplasts from biochemical and structural perspectives. We further examine the cyanobacterial-specific forms of NDH-1 that possess vectorial carbonic anhydrase (vCA) activity and function in the CO₂-concentrating mechanism (CCM). We compare the proposed mechanism for the cyanobacterial NDH-1 vCA-activity to that of the DAB (DABs accumulates bicarbonate) complex, another putative vCA. Finally, we discuss both new and remaining questions pertaining to the mechanisms of NDH-1 complexes in light of these recent advances.

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 CO₂, both FDPs Flv1 and Flv3 are indispensable for oxygen reduction. We performed a detailed in vivo kinetic study of wild-type (WT) and Δflv1/3 strains of Synechocystis using light-induced NADPH fluorescence and near-infrared absorption of iron-sulfur clusters from ferredoxin and the PSI acceptors (F_AF_B), collectively named FeS. These measurements were performed under conditions where the Calvin-Benson cycle is inactive or poorly activated. Under such conditions, the NADPH decay following a short illumination decays in parallel in both strains and exhibits a time lag which is correlated to the presence of reduced FeS. On the contrary, reduced FeS decays much faster in WT than in Δflv1/3 (13 vs 2 s^-1). These data unambiguously show that reduced ferredoxin, or possibly reduced F_AF_B, is the direct electron donor to the Flv1/Flv3 heterodimer. Evidences for large reduction of (F_AF_B) and recombination reactions within PSI were also provided by near-infrared absorption. Mutants lacking either the NDH1-L complex, the homolog of complex I of respiration, or the Pgr5 protein show no difference with WT in the oxidation of reduced FeS following a short illumination. These observations question the participation of a significant cyclic electron flow in cyanobacteria during the first seconds of the induction phase of photosynthesis.

Structural insights into NDH-1 mediated cyclic electron transfer

NDH-1 is a key component of the cyclic-electron-transfer around photosystem I (PSI CET) pathway, an important antioxidant mechanism for efficient photosynthesis. Here, we report a 3.2-Å-resolution cryo-EM structure of the ferredoxin (Fd)-NDH-1L complex from the cyanobacterium Thermosynechococcus elongatus. The structure reveals three β-carotene and fifteen lipid molecules in the membrane arm of NDH-1L. Regulatory oxygenic photosynthesis-specific (OPS) subunits NdhV, NdhS and NdhO are close to the Fd-binding site whilst NdhL is adjacent to the plastoquinone (PQ) cavity, and they play different roles in PSI CET under high-light stress. NdhV assists in the binding of Fd to NDH-1L and accelerates PSI CET in response to short-term high-light exposure. In contrast, prolonged high-light irradiation switches on the expression and assembly of the NDH-1MS complex, which likely contains no NdhO to further accelerate PSI CET and reduce ROS production. We propose that this hierarchical mechanism is necessary for the survival of cyanobacteria in an aerobic environment.

NDH-1 is a key component of the cyclic-electron-transfer around photosystem I pathway, an antioxidant mechanism for efficient photosynthesis. Here, authors report a cryo-EM structure of the ferredoxin (Fd)-NDH-1L complex from the cyanobacterium Thermosynechococcus elongatus.

Structural basis for electron transport mechanism of complex I-like photosynthetic NAD(P)H dehydrogenase

NAD(P)H dehydrogenase-like (NDH) complex NDH-1L of cyanobacteria plays a crucial role in cyclic electron flow (CEF) around photosystem I and respiration processes. NDH-1L couples the electron transport from ferredoxin (Fd) to plastoquinone (PQ) and proton pumping from cytoplasm to the lumen that drives the ATP production. NDH-1L-dependent CEF increases the ATP/NADPH ratio, and is therefore pivotal for oxygenic phototrophs to function under stress. Here we report two structures of NDH-1L from Thermosynechococcus elongatus BP-1, in complex with one Fd and an endogenous PQ, respectively. Our structures represent the complete model of cyanobacterial NDH-1L, revealing the binding manner of NDH-1L with Fd and PQ, as well as the structural elements crucial for proper functioning of the NDH-1L complex. Together, our data provides deep insights into the electron transport from Fd to PQ, and its coupling with proton translocation in NDH-1L.

NAD(P)H dehydrogenase-like complex NDH-1L couples the electron transport from ferredoxin (Fd) to plastoquinone (PQ) and proton pumping from cytoplasm to the lumen. Here authors report two structures of NDH-1L from Thermosynechococcus elongatus BP-1, in complex with one Fd and an endogenous PQ, respectively.

Flux balance analysis of cyanobacteria reveals selective use of photosynthetic electron transport components under different spectral light conditions

Cyanobacteria acclimate and adapt to changing light conditions by controlling the energy transfer between photosystem I (PSI) and II (PSII) and pigment composition. Photosynthesis is driven by balancing the excitation between PSI and PSII. To predict the detailed electron transfer flux of cyanobacteria, we refined the photosynthesis-related reactions in our previously reconstructed genome-scale model. Two photosynthetic bacteria, Arthrospira and Synechocystis, were used as models. They were grown under various spectral light conditions and flux balance analysis (FBA) was performed using photon uptake fluxes into PSI and PSII, which were converted from each light spectrum by considering the photoacclimation of pigments and the distribution ratio of phycobilisome to PSI and PSII. In Arthrospira, the FBA was verified with experimental data using six types of light-emitting diodes (White, Blue, Green, Yellow, Red1, and Red2). FBA predicted the cell growth of Synechocystis for the LEDs, excepting Red2. In an FBA simulation, cells used respiratory terminal oxidases and two NADH dehydrogenases (NDH-1 and NDH-2) to balance the PSI and PSII excitations depending on the light conditions. FBA simulation with our refined model functionally implicated NDH-1 and NDH-2 as a component of cyclic electron transport in the varied light environments.

Ssl3451 is Important for Accumulation of NDH-1 Assembly Intermediates in the Cytoplasm of Synechocystis sp. Strain PCC 6803

Two mutants sensitive to high light for growth and impaired in NDH-1 activity were isolated from a transposon-tagged library of Synechocystis sp. strain PCC 6803. Both mutants were tagged in the ssl3451 gene encoding a hypothetical protein, which shares a significant homology with the Arabidopsis (Arabidopsis thaliana) CHLORORESPIRATORY REDUCTION 42 (CRR42). In Arabidopsis, CRR42 associates only with an NDH-1 hydrophilic arm assembly intermediate (NAI) of about 400 kDa (NAI400), one of total three NAIs (NAI800, NAI500 and NAI400), and its deletion has little, if any, effect on accumulation of any NAIs in the stroma. In comparison, the ssl3451 product was localized mainly in the cytoplasm and associates with two NAIs of about 300 kDa (NAI300) and 130 kDa (NAI130). Deletion of Ssl3451 reduced the abundance of the NAI300 complex to levels no longer visible on gels and of the NAI130 complex to a low level, thereby impeding the assembly process of NDH-1 hydrophilic arm. Further, Ssl3451 interacts with another assembly factor Ssl3829 and they have a similar effect on accumulation of NAIs and NdhI maturation factor Slr1097 in the cytoplasm. We thus propose that Ssl3451 plays an important role in accumulation of the NAI300 and NAI130 complexes in the cytoplasm via its interacting protein Ssl3829.

Molecular dynamics and structural models of the cyanobacterial NDH-1 complex

NDH-1 is a gigantic redox-driven proton pump linked with respiration and cyclic electron flow in cyanobacterial cells. Based on experimentally resolved X-ray and cryo-EM structures of the respiratory complex I, we derive here molecular models of two isoforms of the cyanobacterial NDH-1 complex involved in redox-driven proton pumping (NDH-1L) and CO2-fixation (NDH-1MS). Our models show distinct structural and dynamic similarities to the core architecture of the bacterial and mammalian respiratory complex I. We identify putative plastoquinone-binding sites that are coupled by an electrostatic wire to the proton pumping elements in the membrane domain of the enzyme. Molecular simulations suggest that the NDH-1L isoform undergoes large-scale hydration changes that support proton-pumping within antiporter-like subunits, whereas the terminal subunit of the NDH-1MS isoform lacks such structural motifs. Our work provides a putative molecular blueprint for the complex I-analogue in the photosynthetic energy transduction machinery and demonstrates that general mechanistic features of the long-range proton-pumping machinery are evolutionary conserved in the complex I-superfamily.

Heterocyst Thylakoid Bioenergetics

Heterocysts are specialized cells that differentiate in the filaments of heterocystous cyanobacteria. Their role is to maintain a microoxic environment for the nitrogenase enzyme during diazotrophic growth. The lack of photosynthetic water oxidation in the heterocyst puts special constraints on the energetics for nitrogen fixation, and the electron transport pathways of heterocyst thylakoids are slightly different from those in vegetative cells. During recent years, there has been a growing interest in utilizing heterocysts as cell factories for the production of fuels and other chemical commodities. Optimization of these production systems requires some consideration of the bioenergetics behind nitrogen fixation. In this overview, we emphasize the role of photosynthetic electron transport in providing ATP and reductants to the nitrogenase enzyme, and provide some examples where heterocysts have been used as production facilities.

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 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.

NDH-1 Is Important for Photosystem I Function of Synechocystis sp. Strain PCC 6803 under Environmental Stress Conditions

Cyanobacterial NDH-1 interacts with photosystem I (PSI) to form an NDH-1-PSI supercomplex. Here, we observed that absence of NDH-1 had little, if any, effect on the functional fractions of PSI under growth conditions, but significantly reduced the functional fractions of PSI when cells of Synechocystis sp. strain PCC 6803 were moved to conditions of multiple stresses. The significant reduction in NDH-1-dependent functional fraction of PSI was initiated after PSII activity was impaired. This finding is consistent with our observation that the functional fraction of PSI under growth conditions was rapidly and significantly decreased with increasing concentrations of DCMU, which rapidly and significantly suppressed PSII activity by blocking the transfer of electrons from QA to QB in the PSII reaction center. Furthermore, absence of NDH-1 resulted in the PSI limitation at the functionality of PSI itself but not its donor-side and acceptor-side under conditions of multiple stresses. This was supported by the result of a significant destabilization of the PSI complex in the absence of NDH-1 but the presence of multiple stresses. Based on the above results, we propose that NDH-1 is important for PSI function of Synechocystis sp. strain PCC 6803 mainly via maintaining stabilization of PSI under conditions of environmental stresses.

Septal protein SepJ from the heterocyst-forming cyanobacterium Anabaena forms multimers and interacts with peptidoglycan

Heterocyst‐forming cyanobacteria grow as filaments that can be hundreds of cells long. Proteinaceous septal junctions provide cell–cell binding and communication functions in the filament. In Anabaena sp. strain PCC 7120, the SepJ protein is important for the formation of septal junctions. SepJ consists of integral membrane and extramembrane sections – the latter including linker and coiled‐coil domains. SepJ (predicted MW, 81.3 kDa) solubilized from Anabaena membranes was found in complexes of about 296–334 kDa, suggesting that SepJ forms multimeric complexes. We constructed an Anabaena strain producing a double‐tagged SepJ protein (SepJ‐GFP‐His₁₀) and isolated the tagged protein by a two‐step affinity chromatography procedure. Analysis of the purified protein preparation provided no indication of the presence of specific SepJ partners, but suggested that SepJ is processed to remove an N‐terminal fragment. Additionally, pull‐down experiments showed that His₆‐tagged versions of SepJ and of the SepJ coiled‐coil domain interact with Anabaena peptidoglycan (PG). Our results indicate that SepJ forms multimers, that it interacts with PG, and that the coiled‐coil domain is involved in this interaction. These observations support the idea that SepJ is a component of the septal junctions that join the cells in the Anabaena filament.

Stromal Loop of Lhca6 is Responsible for the Linker Function Required for the NDH-PSI Supercomplex Formation

The light-harvesting complex I (LHCI) proteins in Arabidopsis thaliana are encoded by six genes. Major LHCI proteins (Lhca1-Lhca4) harvest light energy and transfer the resulting excitation energy to the PSI core by forming a PSI supercomplex. In contrast, the minor LHCI proteins Lhca5 and Lhca6 contribute to supercomplex formation between the PSI supercomplex and the chloroplast NADH dehydrogenase-like (NDH) complex, although Lhca5 is also solely associated with the PSI supercomplex. Lhca6 was branched from Lhca2 during the evolution of land plants. In this study, we focused on the molecular evolution involved in the transition from a major LHCI, Lhca2, to the linker protein Lhca6. To elucidate the domains of Lhca6 responsible for linker function, we systematically swapped domains between the two LHCI proteins. To overcome problems due to the low stability of chimeric proteins, we employed sensitive methods to evaluate supercomplex formation: we monitored NDH activity by using Chl fluorescence analysis and detected NDH-PSI supercomplex formation by using protein blot analysis in the form of two-dimensional blue-native (BN)/SDS-PAGE. The stromal loop of Lhca6 was shown to be necessary and sufficient for linker function. Chimeric Lhca6, in which the stromal loop was substituted by that of Lhca2, was not functional as a linker and was detected at the position of the PSI supercomplex in the BN-polyacrylamide gel. The stromal loop of Lhca6 is likely to be necessary for the interaction with chloroplast NDH, rather than for the association with the PSI supercomplex.

Unique attributes of cyanobacterial metabolism revealed by improved genome-scale metabolic modeling and essential gene analysis

The model cyanobacterium, Synechococcus elongatus PCC 7942, is a genetically tractable obligate phototroph that is being developed for the bioproduction of high-value chemicals. Genome-scale models (GEMs) have been successfully used to assess and engineer cellular metabolism; however, GEMs of phototrophic metabolism have been limited by the lack of experimental datasets for model validation and the challenges of incorporating photon uptake. Here, we develop a GEM of metabolism in S. elongatus using random barcode transposon site sequencing (RB-TnSeq) essential gene and physiological data specific to photoautotrophic metabolism. The model explicitly describes photon absorption and accounts for shading, resulting in the characteristic linear growth curve of photoautotrophs. GEM predictions of gene essentiality were compared with data obtained from recent dense-transposon mutagenesis experiments. This dataset allowed major improvements to the accuracy of the model. Furthermore, discrepancies between GEM predictions and the in vivo dataset revealed biological characteristics, such as the importance of a truncated, linear TCA pathway, low flux toward amino acid synthesis from photorespiration, and knowledge gaps within nucleotide metabolism. Coupling of strong experimental support and photoautotrophic modeling methods thus resulted in a highly accurate model of S. elongatus metabolism that highlights previously unknown areas of S. elongatus biology.

The NDH-1L-PSI Supercomplex Is Important for Efficient Cyclic Electron Transport in Cyanobacteria

Two mutants isolated from a tagging library of Synechocystis sp. strain PCC 6803 were sensitive to high light and had a tag in sll1471 encoding CpcG2, a linker protein for photosystem I (PSI)-specific antenna. Both mutants demonstrated strongly impaired NDH-1-dependent cyclic electron transport. Blue native-polyacrylamide gel electrophoresis followed by immunoblotting and mass spectrometry analyses of the wild type and a mutant containing CpcG2 fused with yellow fluorescent protein-histidine6 indicated the presence of a novel NDH-1L-CpcG2-PSI supercomplex, which was absent in the cpcG2 deletion mutant, the PSI-less mutant, and several other strains deficient in NDH-1L and/or NDH-1M. Coimmunoprecipitation and pull-down analyses on CpcG2-yellow fluorescent protein-histidine6, using antibody against green fluorescent protein and nickel column chromatography, confirmed the association of CpcG2 with the supercomplex. Conversely, the use of antibodies against NdhH or NdhK after blue native-polyacrylamide gel electrophoresis and in coimmunoprecipitation experiments verified the necessity of CpcG2 in stabilizing the supercomplex. Furthermore, deletion of CpcG2 destabilized NDH-1L as well as its degradation product NDH-1M and significantly decreased the number of functional PSI centers, consistent with the involvement of CpcG2 in NDH-1-dependent cyclic electron transport. The CpcG2 deletion, however, had no effect on respiration. Thus, we propose that the formation of an NDH-1L-CpcG2-PSI supercomplex in cyanobacteria facilitates PSI cyclic electron transport via NDH-1L.

NdhV subunit regulates the activity of type-1 NAD(P)H dehydrogenase under high light conditions in cyanobacterium Synechocystis sp. PCC 6803

The cyanobacterial NAD(P)H dehydrogenase (NDH-1) complexes play crucial roles in variety of bioenergetic reactions. However, the regulative mechanism of NDH-1 under stressed conditions is still unclear. In this study, we detected that the NDH-1 activity is partially impaired, but the accumulation of NDH-1 complexes was little affected in the NdhV deleted mutant (ΔndhV) at low light in cyanobacterium Synechocystis sp. PCC 6803. ΔndhV grew normally at low light but slowly at high light under inorganic carbon limitation conditions (low pH or low CO2), meanwhile the activity of CO2 uptake was evidently lowered than wild type even at pH 8.0. The accumulation of NdhV in thylakoids strictly relies on the presence of the hydrophilic subcomplex of NDH-1. Furthermore, NdhV was co-located with hydrophilic subunits of NDH-1 loosely associated with the NDH-1L, NDH-1MS' and NDH-1M complexes. The level of the NdhV was significantly increased at high light and deletion of NdhV suppressed the up-regulation of NDH-1 activity, causing the lowered the photosynthetic oxygen evolution at pH 6.5 and high light. These data indicate that NdhV is an intrinsic subunit of hydrophilic subcomplex of NDH-1, required for efficient operation of cyclic electron transport around photosystem I and CO2 uptake at high lights.

A Cytoplasmic Protein Ssl3829 Is Important for NDH-1 Hydrophilic Arm Assembly in Synechocystis sp. Strain PCC 6803

Despite significant progress in clarifying the subunit compositions and functions of the multiple NDH-1 complexes in cyanobacteria, the assembly factors and their roles in assembling these NDH-1 complexes remain elusive. Two mutants sensitive to high light for growth and impaired in NDH-1-dependent cyclic electron transport around photosystem I were isolated from Synechocystis sp. strain PCC 6803 transformed with a transposon-tagged library. Both mutants were tagged in the ssl3829 gene encoding an unknown protein, which shares significant similarity with Arabidopsis (Arabidopsis thaliana) CHLORORESPIRATORY REDUCTION7. The ssl3829 product was localized in the cytoplasm and associates with an NDH-1 hydrophilic arm assembly intermediate (NAI) of about 300 kD (NAI300) and an NdhI maturation factor, Slr1097. Upon deletion of Ssl3829, the NAI300 complex was no longer visible on gels, thereby impeding the assembly of the NDH-1 hydrophilic arm. The deletion also abolished Slr1097 and consequently reduced the amount of mature NdhI in the cytoplasm, which repressed the dynamic assembly process of the NDH-1 hydrophilic arm because mature NdhI was essential to stabilize all functional NAIs. Therefore, Ssl3829 plays an important role in the assembly of the NDH-1 hydrophilic arm by accumulating the NAI300 complex and Slr1097 protein in the cytoplasm.

Functional Characterization of the Subunits N, H, J, and O of the NAD(P)H Dehydrogenase Complexes in Synechocystis sp. Strain PCC 6803

The cyanobacterial NAD(P)H dehydrogenase (NDH-1) complexes play crucial roles in variety of bioenergetic reactions such as respiration, CO2 uptake, and cyclic electron transport around PSI. Recently, substantial progress has been made in identifying the composition of subunits of NDH-1 complexes. However, the localization and the physiological roles of several subunits in cyanobacteria are not fully understood. Here, by constructing fully segregated ndhN, ndhO, ndhH, and ndhJ null mutants in Synechocystis sp. strain PCC 6803, we found that deletion of ndhN, ndhH, or ndhJ but not ndhO severely impaired the accumulation of the hydrophilic subunits of the NDH-1 in the thylakoid membrane, resulting in disassembly of NDH-1MS, NDH-1MS', as well as NDH-1L, finally causing the severe growth suppression phenotype. In contrast, deletion of NdhO affected the growth at pH 6.5 in air. In the cytoplasm, either NdhH or NdhJ deleted mutant, but neither NdhN nor NdhO deleted mutant, failed to accumulate the NDH-1 assembly intermediate consisting of NdhH, NdhJ, NdhK, and NdhM. Based on these results, we suggest that NdhN, NdhH, and NdhJ are essential for the stability and the activities of NDH-1 complexes, while NdhO for NDH-1 functions under the condition of inorganic carbon limitation in Synechocystis sp. strain PCC 6803. We discuss the roles of these subunits and propose a new NDH-1 model.

NDH-1 and NDH-2 Plastoquinone Reductases in Oxygenic Photosynthesis

Oxygenic photosynthesis converts solar energy into chemical energy in the chloroplasts of plants and microalgae as well as in prokaryotic cyanobacteria using a complex machinery composed of two photosystems and both membrane-bound and soluble electron carriers. In addition to the major photosynthetic complexes photosystem II (PSII), cytochrome b6f, and photosystem I (PSI), chloroplasts also contain minor components, including a well-conserved type I NADH dehydrogenase (NDH-1) complex that functions in close relationship with photosynthesis and likewise originated from the endosymbiotic cyanobacterial ancestor. Some plants and many microalgal species have lost plastidial ndh genes and a functional NDH-1 complex during evolution, and studies have suggested that a plastidial type II NADH dehydrogenase (NDH-2) complex substitutes for the electron transport activity of NDH-1. However, although NDH-1 was initially thought to use NAD(P)H as an electron donor, recent research has demonstrated that both chloroplast and cyanobacterial NDH-1s oxidize reduced ferredoxin. We discuss more recent findings related to the biochemical composition and activity of NDH-1 and NDH-2 in relation to the physiology and regulation of photosynthesis, particularly focusing on their roles in cyclic electron flow around PSI, chlororespiration, and acclimation to changing environments.

Oscillation Kinetics of Post-illumination Increase in Chl Fluorescence in Cyanobacterium Synechocystis PCC 6803

After termination of longer-illumination (more than 30 s), the wild type of Synechocystis PCC 6803 showed the oscillation kinetics of post-illumination increase in Chl fluorescence: a fast phase followed by one or two slow phases. Unlike the wild type, ndh-B defective mutant M55 did not show any post-illumination increase under the same conditions, indicating that not only the fast phase, but also the slow phases were related to the NDH-mediated cyclic electron flow around photosystem I (PS I) to plastoquinone (PQ). The fast phase was stimulated by dark incubation or in the presence of Calvin cycle inhibitor, iodoacetamide (IA) or cyclic photophosphorylation cofactor, phenazine methosulphate (PMS), implying the redox changes of PQ by electrons generated at PS I reduced side, probably NAD(P)H or ferredoxin (Fd). In contrast, the slow phases disappeared after dark starvation or in the presence of IA or PMS, and reappeared by longer re-illumination, suggesting that they are related to the redox changes of PQ by the electrons from the photoreductants produced in carbon assimilation process. Both the fast phase and slow phases were stimulated at high temperature and the slow phase was promoted by response to high concentration of NaCl. The mutant M55 without both phases could not survive under the stressed conditions.

NdhV Is a Subunit of NADPH Dehydrogenase Essential for Cyclic Electron Transport in Synechocystis sp. Strain PCC 6803

Two mutants sensitive to heat stress for growth and impaired in NADPH dehydrogenase (NDH-1)-dependent cyclic electron transport around photosystem I (NDH-CET) were isolated from the cyanobacterium Synechocystis sp. strain PCC 6803 transformed with a transposon-bearing library. Both mutants had a tag in the same sll0272 gene, encoding a protein highly homologous to NdhV identified in Arabidopsis (Arabidopsis thaliana). Deletion of the sll0272 gene (ndhV) did not influence the assembly of NDH-1 complexes and the activities of CO2 uptake and respiration but reduced the activity of NDH-CET. NdhV interacted with NdhS, a ferredoxin-binding subunit of cyanobacterial NDH-1 complex. Deletion of NdhS completely abolished NdhV, but deletion of NdhV had no effect on the amount of NdhS. Reduction of NDH-CET activity was more significant in ΔndhS than in ΔndhV. We therefore propose that NdhV cooperates with NdhS to accept electrons from reduced ferredoxin.

NdhM Subunit Is Required for the Stability and the Function of NAD(P)H Dehydrogenase Complexes Involved in CO2 Uptake in Synechocystis sp. Strain PCC 6803

The cyanobacterial type I NAD(P)H dehydrogenase (NDH-1) complexes play a crucial role in a variety of bioenergetic reactions such as respiration, CO2 uptake, and cyclic electron transport around photosystem I. Two types of NDH-1 complexes, NDH-1MS and NDH-1MS', are involved in the CO2 uptake system. However, the composition and function of the complexes still remain largely unknown. Here, we found that deletion of ndhM caused inactivation of NDH-1-dependent cyclic electron transport around photosystem I and abolishment of CO2 uptake, resulting in a lethal phenotype under air CO2 condition. The mutation of NdhM abolished the accumulation of the hydrophilic subunits of the NDH-1, such as NdhH, NdhI, NdhJ, and NdhK, in the thylakoid membrane, resulting in disassembly of NDH-1MS and NDH-1MS' as well as NDH-1L. In contrast, the accumulation of the hydrophobic subunits was not affected in the absence of NdhM. In the cytoplasm, the NDH-1 subcomplex assembly intermediates including NdhH and NdhK were seriously affected in the ΔndhM mutant but not in the NdhI-deleted mutant ΔndhI. In vitro protein interaction analysis demonstrated that NdhM interacts with NdhK, NdhH, NdhI, and NdhJ but not with other hydrophilic subunits of the NDH-1 complex. These results suggest that NdhM localizes in the hydrophilic subcomplex of NDH-1 complexes as a core subunit and is essential for the function of NDH-1MS and NDH-1MS' involved in CO2 uptake in Synechocystis sp. strain PCC 6803.

NDH-1L interacts with ferredoxin via the subunit NdhS in Thermosynechococcus elongatus

The large size complex of cyanobacterial NAD(P)H dehydrogenase (NDH-1) complex (NDH-1L) plays crucial role in a variety of bioenergetic reactions such as respiration and cyclic electron flow around photosystem I. Although the complex has been isolated and identified, its biochemical function still remains to be clarified. Here, we highly purified the NDH-1L complex from the cells of Thermosynechococcus elongatus by Ni(2+) affinity chromatography and size-exclusion chromatography. The purified NDH-1L complex has an apparent total molecular mass of approximately 500 kDa. 14 known subunits were identified by mass spectrometry and immunoblotting, including the NdhS subunit containing ferredoxin (Fd)-docking site domain. Surface plasmon resonance measurement demonstrates that the NDH-1L complex could bind to Fd with the binding constant (K D) of 59 µM. Yeast two-hybrid system assay further confirmed the interaction of Fd with NdhS and indicated that NdhH is involved in the interaction. Our results provide direct biochemical evidence that the cyanobacterial NDH-1 complex catalyzes the electron transport from reduced Fd to plastoquinone via NdhS and NdhH.

Cyanobacterial Alkanes Modulate Photosynthetic Cyclic Electron Flow to Assist Growth under Cold Stress

All cyanobacterial membranes contain diesel-range C15-C19 hydrocarbons at concentrations similar to chlorophyll. Recently, two universal but mutually exclusive hydrocarbon production pathways in cyanobacteria were discovered. We engineered a mutant of Synechocystis sp. PCC 6803 that produces no alkanes, which grew poorly at low temperatures. We analyzed this defect by assessing the redox kinetics of PSI. The mutant exhibited enhanced cyclic electron flow (CEF), especially at low temperature. CEF raises the ATP:NADPH ratio from photosynthesis and balances reductant requirements of biosynthesis with maintaining the redox poise of the electron transport chain. We conducted in silico flux balance analysis and showed that growth rate reaches a distinct maximum for an intermediate value of CEF equivalent to recycling 1 electron in 4 from PSI to the plastoquinone pool. Based on this analysis, we conclude that the lack of membrane alkanes causes higher CEF, perhaps for maintenance of redox poise. In turn, increased CEF reduces growth by forcing the cell to use less energy-efficient pathways, lowering the quantum efficiency of photosynthesis. This study highlights the unique and universal role of medium-chain hydrocarbons in cyanobacterial thylakoid membranes: they regulate redox balance and reductant partitioning in these oxygenic photosynthetic cells under stress.

Proteomic approaches in research of cyanobacterial photosynthesis

Oxygenic photosynthesis in cyanobacteria, algae, and plants is carried out by a fabulous pigment-protein machinery that is amazingly complicated in structure and function. Many different approaches have been undertaken to characterize the most important aspects of photosynthesis, and proteomics has become the essential component in this research. Here we describe various methods which have been used in proteomic research of cyanobacteria, and demonstrate how proteomics is implemented into on-going studies of photosynthesis in cyanobacterial cells.

The Antarctic Psychrophile Chlamydomonas sp. UWO 241 Preferentially Phosphorylates a Photosystem I-Cytochrome b6/f Supercomplex

Chlamydomonas sp. UWO 241 (UWO 241) is a psychrophilic green alga isolated from Antarctica. A unique characteristic of this algal strain is its inability to undergo state transitions coupled with the absence of photosystem II (PSII) light-harvesting complex protein phosphorylation. We show that UWO 241 preferentially phosphorylates specific polypeptides associated with an approximately 1,000-kD pigment-protein supercomplex that contains components of both photosystem I (PSI) and the cytochrome b₆/f (Cyt b₆/f) complex. Liquid chromatography nano-tandem mass spectrometry was used to identify three major phosphorylated proteins associated with this PSI-Cyt b₆/f supercomplex, two 17-kD PSII subunit P-like proteins and a 70-kD ATP-dependent zinc metalloprotease, FtsH. The PSII subunit P-like protein sequence exhibited 70.6% similarity to the authentic PSII subunit P protein associated with the oxygen-evolving complex of PSII in Chlamydomonas reinhardtii. Tyrosine-146 was identified as a unique phosphorylation site on the UWO 241 PSII subunit P-like polypeptide. Assessment of PSI cyclic electron transport by in vivo P700 photooxidation and the dark relaxation kinetics of P700(+) indicated that UWO 241 exhibited PSI cyclic electron transport rates that were 3 times faster and more sensitive to antimycin A than the mesophile control, Chlamydomonas raudensis SAG 49.72. The stability of the PSI-Cyt b₆/f supercomplex was dependent upon the phosphorylation status of the PsbP-like protein and the zinc metalloprotease FtsH as well as the presence of high salt. We suggest that adaptation of UWO 241 to its unique low-temperature and high-salt environment favors the phosphorylation of a PSI-Cyt b₆/f supercomplex to regulate PSI cyclic electron transport rather than the regulation of state transitions through the phosphorylation of PSII light-harvesting complex proteins.

Subunit Q Is Required to Stabilize the Large Complex of NADPH Dehydrogenase in Synechocystis sp. Strain PCC 6803

Two major complexes of NADPH dehydrogenase (NDH-1) have been identified in cyanobacteria. A large complex (NDH-1L) contains NdhD1, NdhF1, and NdhP, which are absent in a medium size complex (NDH-1M). They play important roles in respiration, NDH-1-dependent cyclic electron transport around photosystem I, and CO2 uptake. Two mutants sensitive to high light for growth and impaired in cyclic electron transport around photosystem I were isolated from the cyanobacterium Synechocystis sp. strain PCC 6803 transformed with a transposon-bearing library. Both mutants had a tag in an open reading frame encoding a product highly homologous to NdhQ, a single-transmembrane small subunit of the NDH-1L complex, identified in Thermosynechococcus elongatus by proteomics strategy. Deletion of ndhQ disassembled about one-half of the NDH-1L to NDH-1M and consequently impaired respiration, but not CO2 uptake. During prolonged incubation of the thylakoid membrane with n-dodecyl-β-D-maltoside at room temperature, the rest of the NDH-1L in ΔndhQ was disassembled completely to NDH-1M and was much faster than in the wild type. In the ndhP-deletion mutant (ΔndhP) background, absence of NdhQ almost completely disassembled the NDH-1L to NDH-1M, similar to the results observed in the ΔndhD1/ΔndhD2 mutant. We therefore conclude that both NdhQ and NdhP are essential to stabilize the NDH-1L complex.

(1)H, (13)C and (15)N chemical shift assignments of the NDH-1 complex subunit CupS

The cyanobacterial NDH-1 complex is involved in respiratory as well as in cyclic electron transfer around photosystem I. Here, we report both backbone and side chain chemical shift assignments of CupS, a small subunit of the multisubunit membrane protein complex NDH-1 from Thermosynechococcus elongatus. The construct contains 159 amino acids including a Strep-tag and two additional amino acids.

The NdhV subunit is required to stabilize the chloroplast NADH dehydrogenase-like complex in Arabidopsis

The chloroplast NADH dehydrogenase-like (NDH) complex is involved in cyclic electron transport around photosystem I (PSI) and chlororespiration. Although the NDH complex was discovered more than 20 years ago, its low abundance and fragile nature render it recalcitrant to analysis, and it is thought that some of its subunits remain to be identified. Here, we identified the NDH subunit NdhV that readily disassociates from the NDH complex in the presence of detergent, salt and alkaline solutions. The Arabidopsis ndhv mutant is partially defective in the accumulation of NDH subcomplex A (SubA) and SubE, resulting in impaired NDH activity. NdhV was mainly detected in the wild-type thylakoid membrane, and its accumulation in thylakoids strictly depended on the presence of the NDH complex. Quantitative immunoblot analysis revealed that NdhV and NdhN occur at close to equimolar concentrations. Furthermore, several NDH subunits were co-immunopurified with NdhV using a combination of chemical crosslinking and an affinity chromatography assay. These data indicate that NdhV is an intrinsic subunit of NDH. We found that NdhV did not directly affect NDH activity, but that NDH SubA and SubE were more rapidly degraded in ndhv than in the wild type under high-light treatment. We propose that NdhV is an NDH subunit that stabilizes this complex, especially under high-light conditions.