NdhO, a subunit of NADPH dehydrogenase, destabilizes medium size complex of the enzyme in Synechocystis sp. strain PCC 6803

Interplay between photosynthetic electron flux and organic carbon sinks in sucrose-excreting Synechocystis sp. PCC 6803 revealed by omics approaches

BACKGROUND

Advancing the engineering of photosynthesis-based prokaryotic cell factories is important for sustainable chemical production and requires a deep understanding of the interplay between bioenergetic and metabolic pathways. Rearrangements in photosynthetic electron flow to increase the efficient use of the light energy for carbon fixation must be balanced with a strong carbon sink to avoid photoinhibition. In the cyanobacterium Synechocystis sp. PCC 6803, the flavodiiron protein Flv3 functions as an alternative electron acceptor of photosystem I and represents an interesting engineering target for reorganizing electron flow in attempts to enhance photosynthetic CO2 fixation and increase production yield.

RESULTS

We have shown that inactivation of Flv3 in engineered sucrose-excreting Synechocystis (S02:Δflv3) induces a transition from photoautotrophic sucrose production to mixotrophic growth sustained by sucrose re-uptake and the formation of intracellular carbon sinks such as glycogen and polyhydroxybutyrate. The growth of S02:Δflv3 exceeds that of the sucrose-producing strain (S02) and demonstrates unforeseen proteomic and metabolomic changes over the course of the nine-day cultivation. In the absence of Flv3, a down-regulation of proteins related to photosynthetic light reactions and CO2 assimilation occurred concomitantly with up-regulation of those related to glycolytic pathways, before any differences in sucrose production between S02 and S02:Δflv3 strains were observed. Over time, increased sucrose degradation in S02:Δflv3 led to the upregulation of respiratory pathway components, such as the plastoquinone reductase complexes NDH-11 and NDH-2 and the terminal respiratory oxidases Cyd and Cox, which transfer electrons to O2. While glycolytic metabolism is significantly up-regulated in S02:Δflv3 to provide energy for the cell, the accumulation of intracellular storage compounds and the increase in respiration serve as indirect sinks for photosynthetic electrons.

CONCLUSIONS

Our results show that the presence of strong carbon sink in the engineered sucrose-producing Synechocystis S02 strain, operating under high light, high CO2 and salt stress, cannot compensate for the lack of Flv3 by directly balancing the light transducing source and carbon fixing sink reactions. Instead, the cells immediately sense the imbalance, leading to extensive reprogramming of cellular bioenergetic, metabolic and ion transport pathways that favor mixotrophic growth rather than enhancing photoautotrophic sucrose production.

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.

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.

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.

Acute cadmium toxicity and post-stress recovery: Insights into coordinated and integrated response/recovery strategies of Anabaena sp. PCC 7120

Cyanobacteria, the first photoautotrophs have remarkable adaptive capabilities against most abiotic stresses, including Cd. A model cyanobacterium, Anabaena sp. PCC 7120 has been commonly used to understand cyanobacterial plasticity under different environmental stresses. However, very few studies have focused on the acute Cd toxicity. In this context, Anabaena was subjected to 100 μM Cd for 48 h (acute Cd stress, ACdS) and then transferred into the fresh medium for post-stress recovery (PSR). We further investigated the dynamics of morpho-ultrastructure, physiology, cytosolic proteome, thylakoidal complexes, chelators, and transporters after ACdS, as well as during early (ER), mid (MR), and late (LR) phases of PSR. The findings revealed that ACdS induced intracellular Cd accumulation and ROS production, altered morpho-ultrastructure, reduced photosynthetic pigments, and affected the structural organization of PSII, which subsequently hindered photosynthetic efficiency. Anabaena responded to ACdS and recovered during PSR by reprogramming the expression pattern of proteins/genes involved in cellular defense and repair; CO₂ access, Calvin-Benson cycle, glycolysis, and pentose phosphate pathway; protein biosynthesis, folding, and degradation; regulatory functions; PSI-based cyclic electron flow; Cd chelation; and efflux. These modulations occurred in an integrated and coordinated manner that facilitated Anabaena to detoxify Cd and repair ACdS-induced cellular damage.

The plastid NAD(P)H dehydrogenase-like complex: structure, function and evolutionary dynamics

The thylakoid NAD(P)H dehydrogenase-like (NDH) complex is a large protein complex that reduces plastoquinone and pumps protons into the lumen generating protonmotive force. In plants, the complex consists of both nuclear and chloroplast-encoded subunits. Despite its perceived importance for stress tolerance and ATP generation, chloroplast-encoded NDH subunits have been lost numerous times during evolution in species occupying seemingly unrelated environmental niches. We have generated a phylogenetic tree that reveals independent losses in multiple phylogenetic lineages, and we use this tree as a reference to discuss possible evolutionary contexts that may have relaxed selective pressure for retention of ndh genes. While we are still yet unable to pinpoint a singular specific lifestyle that negates the need for NDH, we are able to rule out several long-standing explanations. In light of this, we discuss the biochemical changes that would be required for the chloroplast to dispense with NDH functionality with regards to known and proposed NDH-related reactions.

Low-Temperature Adaptation of the Snow Alga Chlamydomonas nivalis Is Associated With the Photosynthetic System Regulatory Process

The alga Chlamydomonas nivalis thrives in polar snow fields and on high-altitude mountain tops, and contributes significantly on primary production in the polar regions, however, the mechanisms underlying this adaptation to low temperatures are unknown. Here, we compared the growth, photosynthetic activity, membrane lipid peroxidation, and antioxidant activity of C. nivalis with those of the model alga C. reinhardtii, under grow temperature and low temperatures. C. nivalis maintained its photosynthetic activity in these conditions by reducing the light-harvesting ability of photosystem II and enhancing the cyclic electron transfer around photosystem I, both of which limited damage to the photosystem from excess light energy and resulted in ATP production, supporting cellular growth and other physiological processes. Furthermore, the increased cyclic electron transfer rate, carotenoid content, and antioxidant enzyme activities jointly regulated the reactive oxygen species levels in C. nivalis, enabling recovery from excess excitation energy and reduced photooxidative damage to the cell. Therefore, we propose a model in which adaptive mechanisms related to photosynthetic regulation promote the survival and even blooming of C. nivalis under polar environment, suggesting that C. nivalis can provide organic carbon sources as an important primary producer for other surrounding life in the polar regions for maintaining ecosystem.

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.

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.

Rational engineering of photosynthetic electron flux enhances light-powered cytochrome P450 activity

In this study, we exploited a modified photosynthetic electron transfer chain (PET) in the model cyanobacterium Synechococcus PCC 7002, where electrons derived from water-splitting are used to power reactions catalyzed by a heterologous cytochrome P450 (CYP1A1). A simple in vivo fluorescent assay for CYP1A1 activity was employed to determine the impact of rationally engineering of photosynthetic electron flow. This showed that knocking out a subunit of the type I NADH dehydrogenase complex (NDH-1), suggested to be involved in cyclic photosynthetic electron flow (ΔndhD2), can double the activity of CYP1A1, with a concomitant increase in the flux of electrons from photosynthesis. This also resulted in an increase in cellular adenosine triphosphate (ATP) and the ATP/nicotinamide adenine dinucleotide phosphate (NADPH) ratio, suggesting that expression of a heterologous electron sink in photosynthetic organisms can be used to modify the bioenergetic landscape of the cell. We therefore demonstrate that CYP1A1 is limited by electron supply and that photosynthesis can be re-engineered to increase heterologous P450 activity for the production of high-value bioproducts. The increase in cellular ATP achieved could be harnessed to support metabolically demanding heterologous processes. Furthermore, this experimental system could provide valuable insights into the mechanisms of photosynthesis.

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.

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.

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.

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.

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.