The stress-induced SCP/HLIP family of small light-harvesting-like proteins (ScpABCDE) protects Photosystem II from photoinhibitory damages in the cyanobacterium Synechocystis sp. PCC 6803

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.

Keywords: Photosystem II photoinhibition; Salt stress; Singlet oxygen; Small CAB-like proteins (SCPs); Terminal oxidases.

Fig. 1

Color appearance of the PSI-less and PSI-less/ScpABCDE⁻ mutants grown in the presence or absence of 0.2 M NaCl. Cell cultures of PSI-less and PSI-less/ScpABCDE⁻ strains were in BG-11 supplemented with glucose in the presence or absence of 0.2 M NaCl as described in Material and Methods. The photo was taken after dilution at OD₇₃₀ 0.4–0.5. Representative cultures of three biological replicates are shown

Fig. 2

Immunodetection of the PSII proteins D1, CP47, and PsbH, as well as of ScpC/D and ScpE, in the PSI-less and the PSI-less/ScpABCDE⁻ strains, when grown in the presence or absence of 0.2 M NaCl. Proteins extracted from the same number of cells were loaded in each lane. A representative immunoblot of three biological replicates is shown

Fig. 3

77 K fluorescence spectra of the PSI-less (left) and the PSI-less/ScpABCDE⁻ mutant (right). Spectra were recorded from 600 to 750 nm after growth in the absence (black line) or presence (gray line) of 0.2 M NaCl and normalized to the 685 nm peak. a Excitation of chlorophyll at 435 nm. b Excitation of phycobilisomes at 580 nm

Fig. 4

Measurement of ¹O₂ production expressed as the inverse rate of His-mediated oxygen uptake in the PSI-less and the PSI-less/ScpABCDE⁻ strains grown in the presence or absence of NaCl. The measurements were performed in the presence of 5 mM His at 2300 μmol photon m⁻² s⁻¹ light intensity. The results are a mean (± SD) of three independent experiments. P values are classified as: *0.01 to 0.05 significant; **0.001 to 0.01 very significant; ***< 0.001 extremely significant

Fig. 5

Measurements of photodamage and PSII repair of the PSI-less (upper panels) and PSI-less/ScpABCDE⁻ (lower panels) mutants grown in the presence (right, open symbols) or absence (left, closed symbols) of 0.2 M NaCl. Photoinhibitory treatment was performed in the presence (down triangle symbol, solid line and circle symbol, dotted lines) or absence (square symbol, solid line and upper triangle symbol, dotted lines) of 300 μg/mL lincomycin, as well as in the presence (dotted lines) or absence (solid lines) of 5 μM gabaculine

Fig. 6

Schematic diagram of the thylakoid membrane-localized photosynthetic and respiratory electron transport chains in PSI-less/ScpABCDE⁻ in the absence (a) or presence (b) of NaCl. Lines indicate the electron transport, filled lines indicate sustained electron transfer, and dotted lines indicate poor electron transfer. Cox, cytochrome c oxidase; Cyd, bd quinol oxidases; Cyt c ₆, cytochrome c ₆; PC, plastocyanin; PQ, plastoquinone; PQH₂, plastoquinol; OEC, oxygen evolving complex; PSII, photosystem II