Mechanism of transcriptional regulation in cyanobacteria upon acclimation to high-light conditions

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Previously, we analyzed the promoter architecture of the psaAB genes encoding reaction center subunits of photosystem I (PSI) in the cyanobacterium Synechocystis sp. PCC 6803. There exist two promoters, P1 and P2, both of which show typical high-light (HL) response of PSI genes ; their activities are high under low-light (LL) conditions but rapidly downregulated upon the shift to HL conditions. In this study, it was shown that a response regulator RpaB binds to multiple high-light regulatory 1 (HLR1) sequences in the upstream region of the psaAB genes. We explored the regulatory role of cis-elements, including these HLR1 sequences on the individual activity of P1 and P2. Our results suggest that cis-elements, including multiple HLR1 sequences, differently regulate the activities of dual promoters of the psaAB genes to achieve the fine-tuning of the gene expression.