NdhO,a Subunit of NADPH Dehydrogenase,Destabilizes Medium Size Complex of the Enzyme in Synechocystis sp. Strain PCC 6803

Two mutants that grew faster than the wild-type (WT) strain under high light conditions were isolated from Synechocystis sp. strain PCC 6803 transformed with a transposon-bearing library. Both mutants had a tag in ssl1690 encoding NdhO. Deletion of ndhO increased the activity of NADPH dehydrogenase (NDH-1)-dependent cyclic electron transport around photosystem I (NDH-CET), while overexpression decreased the activity. Although deletion and overexpression of ndhO did not have significant effects on the amount of other subunits such as NdhH, NdhI, NdhK, and NdhM in the cells, the amount of these subunits in the medium size NDH-1 (NDH-1M) complex was higher in the ndhO-deletion mutant and much lower in the overexpression strain than in the WT. NdhO strongly interacts with NdhI and NdhK but not with other subunits. NdhI interacts with NdhK and the interaction was blocked by NdhO. The blocking may destabilize the NDH-1M complex and repress the NDH-CET activity. When cells were transferred from growth light to high light, the amounts of NdhI and NdhK increased without significant change in the amount of NdhO, thus decreasing the relative amount of NdhO. This might have decreased the blocking, thereby stabilizing the NDH-1M complex and increasing the NDH-CET activity under high light conditions.     

Identification of a cyanobacterial CRR6 protein,Slr1097, required for efficient assembly of NDH‐1 complexes in Synechocystis sp. PCC 6803

Despite significant progress in clarifying the subunit compositions and functions of the multiple NADPH dehydrogenase (NDH‐1) complexes in cyanobacteria, the subunit maturation and assembly of their NDH‐1 complexes are poorly understood. By transformation of wild‐type cells with a transposon‐tagged library, we isolated three mutants of Synechocystis sp. PCC 6803 defective in NDH‐1‐mediated cyclic electron transfer and unable to grow under high light conditions. All the mutants were tagged in the same slr1097 gene, encoding an unknown protein that shares significant homology with the Arabidopsis protein chlororespiratory reduction 6 (CRR6). The slr1097 product was localized in the cytoplasm and was required for efficient assembly of NDH‐1 complexes. Analysis of the interaction of Slr1097 with 18 subunits of NDH‐1 complexes using a yeast two‐hybrid system indicated a strong interaction with NdhI but not with other Ndh subunits. Absence of Slr1097 resulted in a significant decrease of NdhI in the cytoplasm, but not of other Ndh subunits including NdhH, NdhK and NdhM; the decrease was more evident in the cytoplasm than in the thylakoid membranes. In the ?slr1097 mutant, NdhH, NdhI, NdhK and NdhM were hardly detectable in the NDH‐1M complex, whereas almost half the wild‐type levels of these subunits were present in NDH‐1L complex; similar results were observed in the NdhI‐less mutant. These results suggest that Slr1097 is involved in the maturation of NdhI, and that assembly of the NDH‐1M complex is strongly dependent on this factor. Maturation of NdhI appears not to be crucial to assembly of the NDH‐1L complex.     

Expression and activity of type 1 NAD(P)H dehydrogenase at different growth phases of the cyanobacterium, Synechocystis PCC 6803

The expression and activity of type 1 NAD(P)H dehydrogenase (NDH-1) were investigated in Synechocystis PCC 6803 cells during different growth phases (i.e. lag, logarithmic, stationary and decline phases). The relative amount of NDH-1, estimated by Western blot analysis using antibodies against NdhH, NdhI and NdhK, increased more than two-fold during growth from the lag to the logarithmic phase and then decreased after the logarithmic phase to reach lowest levels after 15 days (decline phase). The activity of light-dependent NADPH oxidation and cyclic electron flow around photosystem I (PSI) changed nearly in parallel with the amount of NdhH, NdhI and NdhK in cells across the growth phases. In contrast, the activity of photosynthetic O₂ evolution and respiratory O₂ uptake was not significantly different across phases of growth; the fluctuation of the activity at different phases was within 40%. These results suggested that the activity of light-dependent NADPH oxidation and PSI-cyclic electron flow are restricted by the amount of NDH-1 and that other factor(s) are limiting the rates of photosynthesis and respiration.     

Multistep assembly of chloroplast NADH dehydrogenase-like subcomplex A requires several nucleus-encoded proteins, including CRR41 and CRR42, in Arabidopsis

Chloroplast NADH dehydrogenase-like complex (NDH) mediates photosystem I cyclic electron transport and chlororespiration in thylakoids. Recently, substantial progress has been made in understanding the structure of NDH, but our knowledge of its assembly has been limited. In this study, a series of interactive proteomic analyses identified several stroma-localized factors required for the assembly of a stroma-protruding arm of NDH (subcomplex A). In addition to further characterization of the previously identified CHLORORESPIRATORY REDUCTION1 (CRR1), CRR6, and CRR7, two novel stromal proteins, CRR41 and CRR42, were discovered. Arabidopsis thaliana mutants lacking these proteins are specifically defective in the accumulation of subcomplex A. A total of 10 mutants lacking subcomplex A, including crr27/cpn60β4, which is specifically defective in the folding of NdhH, and four mutants lacking NdhL-NdhO subunits, were extensively characterized. We propose a model for subcomplex A assembly: CRR41, NdhO, and native NdhH, as well as unknown factors, are first assembled to form an NDH subcomplex A assembly intermediate (NAI500). Subsequently, NdhJ, NdhM, NdhK, and NdhI are incorporated into NAI500 to form NAI400. CRR1, CRR6, and CRR42 are involved in this process. CRR7 is likely to be involved in the final step, in which the fully assembled NAI, including NdhN, is inserted into thylakoids.     

Effects of low CO2 on NAD(P)H dehydrogenase,a mediator of cyclic electron transport around photosystem I in the cyanobacterium synechocystis PCC6803

The expression and activity of type-1 NAD(P)H dehydrogenase (NDH-1) was compared between cells of Synechocystis PCC6803 grown in high (H-cells) and low (L-cells) CO(2) conditions. Western analysis indicated that L-cells contain higher amounts of the NDH-1 subunits, NdhH, NdhI and NdhK. An NADPH-specific subcomplex of NDH-1 showed higher NADPH-nitroblue tetrazolium oxidoreductase activity in L-cells. The activities of both NADPH-menadione oxidoreductase and light-dependent NADPH oxidation driven by photosystem I were much higher in L-cells than in H-cells. The initial rate of re-reduction of P700(+) following actinic light illumination in the presence of DCMU under background far-red light was enhanced in L-cells. In addition, rotenone, a specific inhibitor of NDH-1, suppressed the relative rate of post-illumination increase in Chl fluorescence of L-cells more than that of H-cells, suggesting that the involvement of NDH-1 in cyclic electron flow around photosystem I was enhanced by low CO(2). Taken together, these results suggest that NDH-1 complex and NDH-1-mediated cyclic electron transport are stimulated by low CO(2) and function in the acclimation of cyanobacteria to low CO(2).     

Cyanobacterial NADPH dehydrogenase complexes

Cyanobacteria possess functionally distinct multiple NADPH dehydrogenase (NDH-1) complexes that are essential to CO₂ uptake, photosystem-1 cyclic electron transport and respiration. The unique nature of cyanobacterial NDH-1 complexes is the presence of subunits involved in CO₂ uptake. Other than CO₂ uptake, chloroplastic NDH-1 complex has a similar role as cyanobacterial NDH-1 complexes in photosystem-1 cyclic electron transport and respiration (chlororespiration). In this mini-review we focus on the structure and function of cyanobacterial NDH-1 complexes and their phylogeny. The function of chloroplastic NDH-1 complex and characteristics of plants defective in NDH-1 are also described for comparison.     

Structural characterization of NDH-1 complexes of Thermosynechococcus elongatus by single particle electron microscopy

The structure of the multifunctional NAD(P)H dehydrogenase type 1 (NDH-1) complexes from cyanobacteria was investigated by growing the wild type and specific ndh His-tag mutants of Thermosynechococcus elongatus BP-1 under different CO₂ conditions, followed by an electron microscopy (EM) analysis of their purified membrane protein complexes. Single particle averaging showed that the complete NDH-1 complex (NDH-1L) is L-shaped, with a relatively short hydrophilic arm. Two smaller complexes were observed, differing only at the tip of the membrane-embedded arm. The smallest one is considered to be similar to NDH-1M, lacking the NdhD1 and NdhF1 subunits. The other fragment, named NDH-1I, is intermediate between NDH-1L and NDH-1M and only lacks a mass compatible with the size of the NdhF1 subunit. Both smaller complexes were observed under low- and high-CO₂ growth conditions, but were much more abundant under the latter conditions. EM characterization of cyanobacterial NDH-1 further showed small numbers of NDH-1 complexes with additional masses. One type of particle has a much longer peripheral arm, similar to the one of NADH: ubiquinone oxidoreductase (complex I) in E. coli and other organisms. This indicates that Thermosynechococcus elongatus must have protein(s) which are structurally homologous to the E. coli NuoE, -F, and -G subunits. Another low-abundance type of particle (NDH-1U) has a second labile hydrophilic arm at the tip of the membrane-embedded arm. This U-shaped particle has not been observed before by EM in a NDH-I preparation.     

The 5 kDa Protein NdhP Is Essential for Stable NDH-1L Assembly in Thermosynechococcus elongatus

The cyanobacterial NADPH:plastoquinone oxidoreductase complex (NDH-1), that is related to Complex I of eubacteria and mitochondria, plays a pivotal role in respiration as well as in cyclic electron transfer (CET) around PSI and is involved in a unique carbon concentration mechanism (CCM). Despite many achievements in the past, the complex protein composition and the specific function of many subunits of the different NDH-1 species remain elusive. We have recently discovered in a NDH-1 preparation from Thermosynechococcus elongatus two novel single transmembrane peptides (NdhP, NdhQ) with molecular weights below 5 kDa. Here we show that NdhP is a unique component of the ∼450 kDa NDH-1L complex, that is involved in respiration and CET at high CO₂ concentration, and not detectable in the NDH-1MS and NDH-1MS'' complexes that play a role in carbon concentration. C-terminal fusion of NdhP with his-tagged superfolder GFP and the subsequent analysis of the purified complex by electron microscopy and single particle averaging revealed its localization in the NDH-1L specific distal unit of the NDH-1 complex, that is formed by the subunits NdhD1 and NdhF1. Moreover, NdhP is essential for NDH-1L formation, as this type of NDH-1 was not detectable in a ΔndhP::Km mutant.     

Identification of NdhL and Ssl1690 (NdhO) in NDH-1L and NDH-1M complexes of Synechocystis sp. PCC 6803

The subunit compositions of two types of NAD(P)H dehydrogenase complexes of Synechocystis sp. PCC 6803, NDH-1L and NDH-1M, were studied by two-dimensional blue-native/SDS-PAGE followed by electrospray tandem mass spectrometry. Fifteen proteins were observed in NDH-1L including hydrophilic subunits (NdhH, -K, -I, -J, -M, and -N) and hydrophobic subunits (NdhA, -B, -E, -G, -D1, and -F1). In addition, NdhL and a novel subunit, Ssl1690 (designated NdhO), were shown to be components of this complex. All subunits mentioned above were present in the NDH-1M complex except NdhD1 and NdhF1. NdhL and Ssl1690 (NdhO) were homologous to hypothetical proteins encoded by genomic DNA in higher plants, suggesting that chloroplast NDH-1 complexes contain related subunits. Diagnostic sequence motifs were found for both NdhL and NdhO homologous proteins. Analysis of ndhL deletion mutant (M9) revealed the presence of assembled NDH-1L and NDH-1M complexes, but these complexes appear to be functionally impaired in the absence of NdhL. Both NDH-1 complexes were absent in the ndhB deletion mutant (M55).     

Possibilities of subunit localization with fluorescent protein tags and electron microscopy examplified by a cyanobacterial NDH-1 study

Cyanobacterial NDH-1 is a multisubunit complex involved in proton translocation, cyclic electron flow around photosystem I and CO₂ uptake. The function and location of several of its small subunits are unknown. In this work, the location of the small subunits NdhL, -M, -N, -O and CupS of Synechocystis 6803 NDH-1 was established by electron microscopy (EM) and single particle analysis. To perform this, the subunits were enlarged by fusion with the YFP protein. After classification of projections, the position of the YFP tag was revealed; all five subunits are integrated in the membrane domain. The results on NDH-1 demonstrate that a GFP tag can be revealed after data processing of EM data sets of moderate size, thus showing that this way of labeling is a fast and reliable way for subunit mapping in multisubunit complexes after partial purification.     

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.Cyanobacterial NADPH dehydrogenase (NDH-1) complexes are localized in the thylakoid membrane (Ohkawa et al., 2001, 2002; Zhang et al., 2004; Xu et al., 2008; Battchikova et al., 2011a) and participate in a variety of bioenergetic reactions, such as respiration, cyclic electron transport around PSI, and CO₂ uptake (Ogawa, 1991; Mi et al., 1992; Ohkawa et al., 2000). Structurally, the cyanobacterial NDH-1 complexes closely resemble energy-converting complex I in eubacteria and the mitochondrial respiratory chain, regardless of the absence of homologs of three subunits in cyanobacterial genomes that constitute the catalytically active core of complex I (Friedrich et al., 1995; Friedrich and Scheide, 2000; Arteni et al., 2006). Over the past few years, significant achievements have been made in resolving the subunit compositions and functions of the multiple NDH-1 complexes in several cyanobacterial strains (for review, see Battchikova and Aro, 2007; Ogawa and Mi, 2007; Ma, 2009; Battchikova et al., 2011b; Ma and Ogawa, 2015). Four types of NDH-1 have been identified in the cyanobacterium Synechocystis sp. strain PCC 6803 (hereafter, Synechocystis 6803), and all four types of NDH-1 are involved in NDH-1-dependent cyclic electron transport (CET) around PSI (NDH-CET; Bernát et al., 2011). The NDH-CET plays an important role in coping with various environmental stresses, regardless of its elusive mechanism. For example, this function can greatly alleviate high light-sensitive growth phenotypes (Endo et al., 1999; Battchikova et al., 2011a; Dai et al., 2013; Zhang et al., 2014; Zhao et al., 2014). Therefore, high light strategy can help in identifying the proteins essential to NDH-CET.Proteomics studies revealed the presence of three major NDH-1 complexes in cyanobacteria: a large complex (NDH-1L), a medium size complex (NDH-1M), and a small complex (NDH-1S) with molecular masses of about 460, 350, and 200 kD, respectively (Herranen et al., 2004). NDH-1M consists of 14 subunits (i.e. NdhA–NdhC, NdhE, NdhG–NdhO, and NdhS). In addition to these subunits, the NDH-1L complex contains NdhD1, NdhF1, NdhP, and NdhQ (Prommeenate et al., 2004; Battchikova et al., 2005, 2011b; Zhang et al., 2005, 2014; Nowaczyk et al., 2011; Wulfhorst et al., 2014; Ma and Ogawa, 2015) and is involved in respiration (Zhang et al., 2004). NDH-1S is composed of NdhD3, NdhF3, CO₂ uptake A (CupA), and CupS (Ogawa and Mi, 2007) and is considered to be associated with NDH-1M in the cells as a functional complex NDH-1MS (Zhang et al., 2004, 2005) participating in CO₂ uptake. Among the several copies of ndhD and ndhF genes found in cyanobacterial genomes, ndhD1 and ndhF1 show the highest homology to chloroplast ndhD and ndhF genes, respectively, and CupA and CupS subunits of the cyanobacteria have no counterparts in higher plants. These facts suggest that the structure and composition of NDH-1L, but not the NDH-1MS complex, are similar to those of the chloroplast NDH-1 complex (Battchikova and Aro, 2007; Ogawa and Mi, 2007; Shikanai, 2007; Ma, 2009; Suorsa et al., 2009; Battchikova et al., 2011b; Ifuku et al., 2011; Peng et al., 2011a; Ma and Ogawa, 2015). Despite their similarity, a large number of subunits that constitute the chloroplast NDH-1 complex, including ferredoxin-binding subcomplex subunits NdhT and NdhU and all the subunits of subcomplex B and lumen subcomplex, are absent in the cyanobacterial NDH-1L complex (Battchikova et al., 2011b; Ifuku et al., 2011; Peng et al., 2011a). This implies that the stabilization strategies for the cyanobacterial NDH-1L complex and chloroplastic NDH-1 complex might be significantly different.Recently, a new oxygenic photosynthesis-specific small subunit NdhQ was identified in the NDH-1L complex purified by Ni²⁺ affinity chromatography from Thermosynechococcus elongatus (Nowaczyk et al., 2011). NdhQ is extensively present in cyanobacteria, but its homolog is absent in higher plants (Nowaczyk et al., 2011). In this study, we demonstrate that deletion of NdhQ disassembled the NDH-1L into NDH-1M, but not NDH-1MS, in Synechocystis 6803 and consequently impaired respiration, but not CO₂ uptake. NdhQ and NdhP stabilize the NDH-1L complex. Thus, the stabilization strategy of cyanobacterial NDH-1L is distinctly different from that of the chloroplastic NDH-1 complex.     

Synthetic DNA system for structure-function studies of the high affinity CO2 uptake NDH-13 protein complex in cyanobacteria

The CO₂-concentrating mechanism (CCM) in cyanobacteria supports high rates of photosynthesis by greatly increasing the concentration of CO₂ around the major carbon fixing enzyme, Rubisco. However, the CCM remains poorly understood, especially in regards to the enigmatic CO₂-hydration enzymes which couple photosynthetically generated redox energy to the hydration of CO₂ to bicarbonate. This CO₂-hydration reaction is catalysed by specialized forms of NDH-1 thylakoid membrane complexes that contain phylogenetically unique extrinsic proteins that appear to couple CO₂ hydration to NDH-1 proton pumping. The development of the first molecular genetic system to probe structure-function relationships of this important enzyme system is described. A CO₂-hydration deficient strain was constructed as a recipient for DNA constructs containing different forms of the CO₂-hydration system. This was tested by introducing a construct to an ectopic location that gives constitutive expression, rather than native inducible expression, of the ndhF3-ndhD3-cupA-cupS, (cupA operon) encoding high affinity CO₂-hydration complex, NDH-1₃. Uptake assays show the restoration of high affinity for CO₂ uptake, but demonstrate that the CupA complex can drive only modest uptake fluxes, underlining the importance of its tandem operation with the CupB-containing complex NDH-1₄, the complementary high flux, low affinity CO₂ hydration system. Experiments with the carbonic anhydrase inhibitor, ethoxyzolamide, indicate that the NDH-1₃ complex is strongly inhibited, yet the remaining NDH-1₄ activity in the wild-type is less so, suggesting structural differences between the low affinity and high affinity CO₂–hydration systems. This new construct will be an important tool to study and better understand cyanobacterial CO₂ uptake systems.     

NdhP Is an Exclusive Subunit of Large Complex of NADPH Dehydrogenase Essential to Stabilize the Complex 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 and NdhF1, which are absent in a medium size complex (NDH-1M). They play important roles in respiration, cyclic electron transport around photosystem I, and CO₂ acquisition. Two mutants sensitive to high light for growth and impaired in NDH-1-mediated cyclic electron transfer were isolated from Synechocystis sp. strain PCC 6803 transformed with a transposon-bearing library. Both mutants had a tag in sml0013 encoding NdhP, a single transmembrane small subunit of the NDH-1 complex. During prolonged incubation of the wild type thylakoid membrane with n-dodecyl β-d-maltoside (DM), about half of the NDH-1L was disassembled to NDH-1M and the rest decomposed completely without forming NDH-1M. In the ndhP deletion mutant (ΔndhP), disassembling of NDH-1L to NDH-1M occurred even on ice, and decomposition to a small piece occurred at room temperature much faster than in the wild type. Deletion of the C-terminal tail of NdhP gave the same result. The C terminus of NdhP was tagged by YFP-His₆. Blue native gel electrophoresis of the DM-treated thylakoid membrane of this strain and Western analysis using the antibody against GFP revealed that NdhP-YFP-His₆ was exclusively confined to NDH-1L. During prolonged incubation of the thylakoid membrane of the tagged strain with DM at room temperature, NDH-1L was partially disassembled to NDH-1M and the 160-kDa band containing NdhP-YFP-His₆ and possibly NdhD1 and NdhF1. We therefore conclude that NdhP, especially its C-terminal tail, is essential to assemble NdhD1 and NdhF1 and stabilize the NDH-1L complex.     

Modeling and mutagenesis of amino acid residues critical for CO2 hydration by specialized NDH-1 complexes in cyanobacteria

The uptake of inorganic carbon in cyanobacteria is facilitated by an energetically intensive CO₂-concentrating mechanism (CCM). This includes specialized Type-1 NDH complexes that function to couple photosynthetic redox energy to CO₂ hydration forming the bicarbonate that accumulates to high cytoplasmic concentrations during the operation of the CCM, required for effective carbon fixation. Here we used a Synechococcus PCC7942 expression system to investigate the role of conserved histidine and cysteine residues in the CupB (also designated, ChpX) protein, which has been hypothesized to participate in a vectoral CO₂ hydration reaction near the interface between CupB protein and the proton-pumping subunits of the NDH-1 complex. A homology model has been constructed and most of the targeted conserved residues are in the vicinity of a Zn ion modeled to form the catalytic site of deprotonation and CO₂ hydration. Growth and CO₂ uptake assays show that the most severe defects in activity among the targeted residues are due to a substitution of the predicted Zn ligand, CupB-His86. Mutations at other sites produced intermediate effects. Proteomic analysis revealed that some amino acid substitution mutations of CupB caused the induction of bicarbonate uptake proteins to a greater extent than complete deletion of CupB, despite growth under CO₂-enriched conditions. The results are discussed in terms of hypotheses on the catalytic function of this unusual enzyme.     

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.Cyanobacterial NADPH dehydrogenase (NDH-1) complexes are localized in the thylakoid membrane (Ohkawa et al., 2001, 2002; Zhang et al., 2004; Xu et al., 2008; Battchikova et al., 2011b) and participate in a variety of bioenergetic reactions, such as respiration, cyclic electron transport around photosystem I (NDH-CET), and CO₂ uptake (Ogawa, 1991; Mi et al., 1992; Ohkawa et al., 2000). Structurally, the cyanobacterial NDH-1 complexes closely resemble energy-converting complex I in eubacteria and the mitochondrial respiratory chain regardless of the absence of homologs of three subunits in cyanobacterial genomes that constitute the catalytically active core of complex I (Friedrich et al., 1995; Friedrich and Scheide, 2000; Arteni et al., 2006). Over the past decade, new subunits of NDH-1 complexes specific to oxygenic photosynthesis have been identified in several cyanobacterial strains. They are NdhM to NdhQ and NdhS (Prommeenate et al., 2004; Battchikova et al., 2005, 2011b; Nowaczyk et al., 2011; Wulfhorst et al., 2014; Zhang et al., 2014; Zhao et al., 2014b, 2015), in addition to NdhL first identified in the cyanobacterium Synechocystis sp. strain PCC 6803 (hereafter Synechocystis 6803) about 20 years ago (Ogawa, 1992). Among them, NdhS possesses a ferredoxin (Fd)-binding motif and was shown to bind Fd, which suggested that Fd is one of the electron donors to NDH-1 complexes (Mi et al., 1995; Battchikova et al., 2011b; Ma and Ogawa, 2015). Deletion of NdhS strongly reduced the activity of NDH-CET but had no effect on respiration and CO₂ uptake (Battchikova et al., 2011b; Ma and Ogawa, 2015). The NDH-CET plays an important role in coping with various environmental stresses regardless of its elusive mechanism. For example, this function can greatly alleviate heat-sensitive growth phenotypes (Wang et al., 2006a; Zhao et al., 2014a). Thus, heat treatment strategy can help in identifying the proteins essential to NDH-CET.Here, a new oxygenic photosynthesis-specific (OPS) subunit NdhV was identified in Synechocystis 6803 with the help of heat treatment strategy, and its deletion did not influence the assembly of NDH-1L and NDH-1MS complexes and the activities of CO₂ uptake and respiration but impaired the NDH-CET activity. We give evidence that NdhV interacts with NdhS and is another component of Fd-binding domain of cyanobacterial NDH-1 complex. A possible role of NdhV on the NDH-CET activity is discussed.     

Response of NAD(P)H dehydrogenase complex to the alteration of CO2 concentration in the cyanobacterium Synechocystis PCC6803

An NADPH-specific NDH-1 sub-complex was separated by native-polyacrylamide gel electrophoresis and detected by activity staining from the whole cell extracts of Synechocystis PCC6803. Low CO2 caused an increase in the activity of this sub-complex quickly, accompanied by an evident increase in the expression of NdhK and PSI-driven NADPH oxidation activity that can reflect the activity of NDH-1-mediated cyclic electron transport. During incubation with high CO2, the activities of NDH-1 sub-complex and PSI-driven NADPH oxidation as well as the protein level of NdhK slightly increased at the beginning, but decreased evidently in various degrees along with incubation time. These results suggest that CO2 concentration in vitro as a signal can control the activity of NDH-1 complex, and NDH-1 complex may in turn function in the regulation of CO2 uptake.     

Cyanobacterial NDH-1 complexes: multiplicity in function and subunit composition

In cyanobacteria, the NAD(P)H:quinone oxidoreductase (NDH-1) is involved in a variety of functions like respiration, cyclic electron flow around PSI and CO₂ uptake. Several types of NDH-1 complexes, which differ in structure and are responsible for these functions, exist in cyanobacterial membranes. This minireview is based on data obtained by reverse genetics and proteomics studies and focuses on the structural and functional differences of the two types of cyanobacterial NDH-1 complexes: NDH-1L, important for respiration and PSI cyclic electron flow, and NDH-1MS, the low-CO₂ inducible complex participating in CO₂ uptake. The NDH-1 complexes in cyanobacteria share a common NDH-1M 'core' complex and differ in the composition of the distal membrane domain composed of specific NdhD and NdhF proteins, which in complexes involved in CO₂ uptake is further associated with the hydrophilic carbon uptake (CUP) domain. At present, however, very important questions concerning the nature of catalytically active subunits that constitute the electron input device (like NADH dehydrogenase module of the eubacterial 'model' NDH-1 analogs), the substrate specificity and reaction mechanisms of cyanobacterial complexes remain unanswered and are shortly discussed here.     

A qualitative analysis of the regulation of cyclic electron flow around photosystem I from the post-illumination chlorophyll fluorescence transient in Arabidopsis: a new platform for the in vivo investigation of the chloroplast redox state

A transient in chlorophyll fluorescence after cessation of actinic light illumination, which has been ascribed to electron donation from stromal reductants to plastoquinone (PQ) by the NAD(P)H-dehydrogenase (NDH) complex, was investigated in Arabidopsis thaliana. The transient was absent in air in a mutant lacking the NDH complex (ndhM). However, in ndhM, the transient was detected in CO₂-free air containing 2% O₂. To investigate the reason, ndhM was crossed with a pgr5 mutant impaired in ferredoxin (Fd)-dependent electron donation from NADPH to PQ, which is known to be redundant for NDH-dependent PQ reduction in the cyclic electron flow around photosystem I (PSI). In ndhM pgr5, the transient was absent even in CO₂-free air with 2% O₂, demonstrating that the post-illumination transient can also be induced by the Fd- (or PGR5)-dependent PQ reduction. On the other hand, the transient increase in chlorophyll fluorescence was found to be enhanced in normal air in a mutant impaired in plastid fructose-1,6-bisphosphate aldolase (FBA) activity. The mutant, termed fba3-1, offers unique opportunities to examine the relative contribution of the two paths, i.e., the NDH- and Fd- (or PGR5)-dependent paths, on the PSI cyclic electron flow. Crossing fba3-1 with either ndhM or pgr5 and assessing the transient suggested that the main route for the PSI cyclic electron flow shifts from the NDH-dependent path to the Fd-dependent path in response to sink limitation of linear electron flow.     

Photorespiration in Green Plants During Photosynthesis Estimated by Use of Isotopic CO(2)

Photosynthetic re-absorption of photorespired CO₂ causes underestimation in measured photorespiration and turnover rate of the substrate for photorespiration. Actual values of photorespiration exceed the measured by a factor greater than 1 + R′_w/r_p + [C_L]_x/(r_p·L_x). (R′_w and r_p are the partial resistances to CO₂ uptake between atmosphere, mesophyll evaporating surface, and photosynthetic sink, respectively; L_x is the measured flux of photorespired CO₂ and [C_L]_x is the ambient conc of photorespired CO₂). In 8 species, 1 + R′_w/r_p alone amounted to a correction ranging between 148% and 233%.