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.This work was supported by the National Natural Science Foundation of China (grant nos. 31370270, 31570235, 31770259, and 31700205), China Postdoctoral Science Foundation (grant nos. 2015M581643 and 2017T100304), Shanghai Science and Technology Committee (grant no. 17070502900) and Project of Shanghai Normal University (grant no. SK201705)

Study on the stress enhanced protein SEP1 in <i>Arabidopsis thaliana</i>

To investigate the biological function of the stress enhanced protein SEP1 in Arabidopsis thaliana, we generated a knockout mutant sep1-1 through CRISPR/Cas9 gene editing technology. Similar with the knockdown mutant sep1-2 purchased from Nottingham Arabidopsis Stock Centre（NASC）, sep1-1 does not exhibit any growth defects under normal light conditions. However, the maximum quantum yield （Fv/Fm） of chloroplast photosystem Ⅱ （PSⅡ） in immature leaves of the mutants was significantly decreased compared with wild type （WT） when the plants were treated with 1 200 μmol photos·m-2·s-1 high light for 8 h, suggesting reduced PSⅡ activity in the mutants. SEP1 is an intrinsic protein on the stromal thylakoids. Blue-native PAGE （BN-PAGE） and two-dimensional （2D） SDS-PAGE/western blotting analyses showed that SEP1 forms a complex with molecular mass of 100 ku, which may be involved in the PSII assembly or repair at certain stages

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

Abstract 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

CBSX2 is required for the efficient oxidation of chloroplast redox‐regulated enzymes in darkness

Abstract Thiol/disulfide‐based redox regulation in plant chloroplasts is essential for controlling the activity of target proteins in response to light signals. One of the examples of such a role in chloroplasts is the activity of the chloroplast ATP synthase (CFoCF1), which is regulated by the redox state of the CF1γ subunit and involves two cysteines in its central domain. To investigate the mechanism underlying the oxidation of CF1γ and other chloroplast redox‐regulated enzymes in the dark, we characterized the Arabidopsis cbsx2 mutant, which was isolated based on its altered NPQ (non‐photochemical quenching) induction upon illumination. Whereas in dark‐adapted WT plants CF1γ was completely oxidized, a small amount of CF1γ remained in the reduced state in cbsx2 under the same conditions. In this mutant, reduction of CF1γ was not affected in the light, but its oxidation was less efficient during a transition from light to darkness. The redox states of the Calvin cycle enzymes FBPase and SBPase in cbsx2 were similar to those of CF1γ during light/dark transitions. Affinity purification and subsequent analysis by mass spectrometry showed that the components of the ferredoxin‐thioredoxin reductase/thioredoxin (FTR‐Trx) and NADPH‐dependent thioredoxin reductase (NTRC) systems as well as several 2‐Cys peroxiredoxins (Prxs) can be co‐purified with CBSX2. In addition to the thioredoxins, yeast two‐hybrid analysis showed that CBSX2 also interacts with NTRC. Taken together, our results suggest that CBSX2 participates in the oxidation of the chloroplast redox‐regulated enzymes in darkness, probably through regulation of the activity of chloroplast redox systems in vivo

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