In cyanobacteria, transcription of genes encoding subunits of PSI is tightly repressed under high-light conditions. To elucidate the molecular mechanism, we examined the promoter architecture of the psaAB genes encoding reaction center subunits of PSI in a cyanobacterium Synechocystis sp. PCC 6803. Primer extension analysis showed the existence of two promoters, P1 and P2, both of which are responsible for the light intensity-dependent transcription of the psaAB genes. Deletion analysis of the upstream region of psaAB fused to bacterial luciferase reporter genes (luxAB) indicated that the light response of these promoters is achieved in a totally different manner. The cis-element required for the light response of P1, designated as PE1, was located just upstream of the -35 element of P1 and was comprised of AT-rich sequence showing significant homology to the upstream promoter (UP)-element often found in strong bacterial promoters. PE1 activated P1 under low-light conditions, and the down-regulation of P1 was achieved by rapid inactivation of PE1 upon the shift to high-light conditions. On the other hand, the cis-element required for the light response of P2, designated as HNE2, was located upstream of the P1 region, far from the basal promoter of P2. The down-regulation of P2 seemed to be attained through the negative regulation by HNE2 activated only under high-light conditions. DNA gel mobility shift assays showed that at least five regions in psaAB promoters were responsible for the binding of putative regulatory protein factors.
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