Transcriptional regulation of genes encoding subunits of photosystem I during acclimation to high-light conditions in Synechocystis sp PCC 6803

Cyanobacteria, algae and plants reduce photosystem contents and modulate photosystem stoichiometry upon acclimation to high-light (HL) conditions to avoid the damage due to excess light energy. In order to elucidate the molecular mechanism of HL acclimation, mRNA levels of genes encoding subunits of photosystems were examined in response to change of photon flux density in the wild-type strain of Synechocystis sp. PCC 6803. Transcript levels and promoter activities of photosystem I (PSI) genes rapidly decreased upon the shift to HL to less than 10% of the initial level within I h, whereas responses of photosystem 11 (PSII) transcript levels were not coordinated. The prompt change in promoter activities of PSI genes, but not PSII genes, seems important for Synechocystis cells to regulate their photosystem contents in response to changes in photon flux density. The mRNA stabilities of PSI genes also decreased during HL incubation. The down-regulation of PSI transcripts under HL did not depend on de novo protein synthesis, contrasting with the requirement of newly synthesized protein factor(s) for the accumulation of PSI transcripts under low light. When gene expression profiles of wild-type cells and a pmgA-disrupted mutant that cannot modulate photosystem stoichiometry under HL were compared, a large increase in the psaAB and psaA transcript levels was observed in the mutant under prolonged exposure to HL for 6 h. Repression of the promoter activity of the psaA gene by a pmgA-dependent mechanism was shown to be essential for the adjustment of photosystem stoichiometry under HL conditions.