S4 protein Sll1252 is necessary for energy balancing in photosynthetic electron transport in Synechocystis sp. PCC 6803

Sll1252 was identified as a novel protein in photosystem II complexes from Synechocystis sp. PCC 6803. To investigate the function of Sll1252, the corresponding gene, sll1252, was deleted in Synechocystis 6803. Despite the homology of Sll1252 to YlmH, which functions in the cell division machinery in Streptococcus, the growth rate and cell morphology of the mutant were not affected in normal growth medium. Instead, it seems that cells lacking this polypeptide have increased sensitivity to Cl^- depletion. The growth and oxygen evolving activity of the mutant cells was highly suppressed compared with those of wild-type cells when Cl^- and/or Ca²⁺ was depleted from the medium. Recovery of photosystem II from photoinhibition was suppressed in the mutant. Despite the defects in photosystem II, in the light, the acceptor side of photosystem II was more reduced and the donor side of photosystem I was more oxidized compared with wild-type cells, suggesting that functional impairments were also present in cytochrome b₆/f complexes. The amounts of cytochrome c₅₅₀ and cytochrome f were smaller in the mutant in the Ca²⁺- and Cl^--depleted medium. Furthermore, the amount of IsiA protein was increased in the mutant, especially in the Cl^--depleted medium, indicating that the mutant cells perceive environmental stress to be greater than it is. The amount of accompanying cytochrome c₅₅₀ in purified photosystem II complexes was also smaller in the mutant. Overall, the Sll1252 protein appears to be closely related to redox sensing of the plastoquinone pool to balance the photosynthetic electron flow and the ability to cope with global environmental stresses.