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

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 Delta sigCDE contains the stress responsive SigB as the only functional group 2 sigma factor. The Delta 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 Delta sigCDE than in the control strain, and immunological detection of carbonylated residues showed more protein oxidation in Delta sigCDE than in the control strain. These results indicate that Delta sigCDE suffers from oxidative stress in standard conditions. The oxidative stress may be explained by the findings that Delta sigCDE had a low content of glutathione and low amount of Flv3 protein functioning in the Mehler-like reaction. Although Delta sigCDE suffers from oxidative stress, up-regulation of photoprotective carotenoids and Flv4, SII2018, Flv2 proteins protected PSII against light induced damage by quenching singlet oxygen more efficiently in Delta 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 Delta 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. (C) 2013 Elsevier B.V. All rights reserved