The mutant of sll1961, which encodes a putative transcriptional regulator, has a defect in regulation of photosystem stoichiometry in the cyanobacterium Synechocystis sp PCC 6803

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In acclimation to changing light environments, photosynthetic organisms modulate the ratio of two photosynthetic reaction centers (photosystem I and photosystem II). Two mutants, which could not modulate photosystem stoichiometry upon the shift to high light, were isolated from mutants created by random transposon mutagenesis. Measurements of chlorophyll fluorescence and analysis of the reaction center subunits of photosystem I through Western blotting in these mutants revealed that the content of photosystem I could not be suppressed under high-light condition. In these mutants, transposon was inserted to sll1961, which is homologous to the gene of gluconate operon transcriptional repressor (GntR) in Bacillus anthracis A2012. DNA microarray analysis revealed that the expression of sll1773 was drastically induced in the sll1961 mutant upon exposure to high light for 3 h. Our results demonstrate that a novel transcriptional regulator, Sll1961, and its possible target protein, Sll1773, may be responsible for the regulation of photosystem stoichiometry in response to high light.