Oxidative stress and photoinhibition can be separated in the cyanobacterium Synechocystis sp. PCC 6803

Abstract

Roles of oxidative stress and photoinhibition in high light acclimation were studied using a regulatory mutant of the cyanobacterium Synechocystis sp. PCC 6803. The mutant strain ΔsigCDE contains the stress responsive SigB as the only functional group 2 σ factor. The ∆sigCDE strain grew more slowly than the control strain in methyl-viologen-induced oxidative stress. Furthermore, a fluorescence dye detecting H2O2, hydroxyl and peroxyl radicals and peroxynitrite, produced a stronger signal in ∆sigCDE than in the control strain, and immunological detection of carbonylated residues showed more protein oxidation in ∆sigCDE than in the control strain. These results indicate that ∆sigCDE suffers from oxidative stress in standard conditions. The oxidative stress may be explained by the findings that ∆sigCDE had a low content of glutathione and low amount of Flv3 protein functioning in the Mehler-like reaction. Although ∆sigCDE suffers from oxidative stress, up-regulation of photoprotective carotenoids and Flv4, Sll2018, Flv2 proteins protected PSII against light induced damage by quenching singlet oxygen more efficiently in ∆sigCDE than in the control strain in visible and in UV-A/B light. However, in UV-C light singlet oxygen is not produced and PSII damage occurred similarly in the ∆sigCDE and control strains. According to our results, resistance against the light-induced damage of PSII alone does not lead to high light tolerance of the cells, but in addition efficient protection against oxidative stress would be required.

Keywords: 2,6-dichloro-p-benzoquinone; 3-(3,4-dichlorophenyl)-1,1-dimethylurea Photosystems I, PSI; 5-(and-6)-chloromethyl-2′,7′-dichlorodihydrofluorescein diacetate; CM-H(2)DCFDA; CS; Chl; DCBQ; DCMU; High light; Oxidative stress; PPFD; PSII; Photoinhibition; Photosystem II; ROS; Reactive oxygen species (ROS); Sigma factor; chlorophyll; glucose tolerant control strain of Synechocystis sp. PCC 6803; k(PI); photosynthetic photon flux density; rate constant of photoinhibition; reactive oxygen species.