Abstract
Genes encoding subunits of photosystem I (PSI genes) in the cyanobacterium Synechocystis sp. PCC 6803 are actively transcribed under low-light conditions, whereas their transcription is coordinately and rapidly down-regulated upon the shift to high-light conditions. To identify the molecular mechanism, we searched for common light-responsive elements in the promoter region of PSI genes. First, by the deletion analysis of the psaAB and psaD promoters, we found that these promoters possessed a common light-responsive element located just upstream of the basal promoter region. These AT-rich upstream sequences enhanced the basal promoter activity under low-light conditions and their activity was transiently suppressed upon the shift to high-light conditions. Successive analysis of the other PSI promoters revealed that their light response was also achieved by AT-rich sequences located at the (-70 to -46) region. Thus, AT-rich upstream elements are responsible for the coordinated high-light response of PSI genes dispersed throughout Synechocystis genome.
