Abstract
Small CAB-like proteins (SCPs) are single-helix light-harvesting-like proteins found in all organisms performing oxygenic photosynthesis. We investigated the effect of growth in moderate salt stress on these stress-induced proteins in the cyanobacterium Synechocystis sp. PCC 6803 depleted of Photosystem I (PSI), which expresses SCPs constitutively, and compared these cells with a PSI-less/ScpABCDE− mutant. SCPs, by stabilizing chlorophyll-binding proteins and Photosystem II (PSII) assembly, protect PSII from photoinhibitory damages, and in their absence electrons accumulate and will lead to ROS formation. The presence of 0.2 M NaCl in the growth medium increased the respiratory activity and other PSII electron sinks in the PSI-less/ScpABCDE− strain. We postulate that this salt-induced effect consumes the excess of PSII-generated electrons, reduces the pressure of the electron transport chain, and thereby prevents 1O2 production.
Highlights
In oxygenic photosynthesis, light energy is used to drive two specialized protein complexes called Photosystem I (PSI) and Photosystem II (PSII), which are embedded in a special membrane system, the thylakoid membrane
Cells grown in BG-11 supplemented with NaCl evolved more oxygen; while more than five times O2 h−1 was produced per cell by the PSIless/ScpABCDE− strain grown in the presence of NaCl (p < 0.001, Table 1), oxygen evolution of the PSI-less culture only slightly increased
Proteins were extracted from the same number of cells of the PSI-less/ScpABCDE− and the PSI-less control strain, which had been grown in the presence or absence of NaCl for 4 days
Summary
Light energy is used to drive two specialized protein complexes called Photosystem I (PSI) and Photosystem II (PSII), which are embedded in a special membrane system, the thylakoid membrane. PCC 6803 (hereafter Synechocystis), PQ can be reduced either by PSII or by the NADP(H) dehydrogenase-like complex type I (NDH-1), the succinate dehydrogenase (SDH), and different NDH-2s (Cooley and Vermaas 2001). Electrons are transferred directly from plastoquinol (PQH2) to the bd quinol oxidases (Cyd; Berry et al 2002) or via Cyt b6f to PSI or an aa3-type cytochrome c oxidase (Cox) complex (Howitt and Vermaas 1998; Ermakova et al 2016). The NDH-1, SDH, and NDH-2 complexes and Cyd are ubiquitous in the thylakoid and the cytoplasmic membranes, while Cox and ARTO have been located only in the thylakoid or cytoplasmic membrane, respectively
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