Role of multiple HLR1 sequences in the regulation of the dual promoters of the psaAB genes in Synechocystis sp. PCC 6803.

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 suggested 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. Under LL conditions, the most influential cis-element is HLR1C (-62 to -45, relative to the transcriptional starting point of P1) working for positive regulation of P1. The other HLR1 sequences also affect the promoter activity under LL conditions; HLR1A (-255 to -238) is involved in repression of P1, whereas HLR1B (-153 to -126) works for activation of P2. Upon the shift to HL conditions, regulation via HNE2 located within the region from -271 to -177 becomes active in order to downregulate both P1 and P2 activities. A positive effect of HLR1B on P2 may persist under HL. These 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.