Two distinct mechanisms regulate the transcription of photosystem II genes in Synechocystis sp. PCC 6803

Expression and regulation of psb genes, encoding various subunits of photosystem II (PSII), were studied in the cyanobacterium Synechocystis sp. PCC 6803. Transcription of the psbA and psbD genes, encoding the PSII reaction centre proteins D1 and D2, was rapidly activated upon onset of illumination and the transcription rates were enhanced at high irradiance. Gel retardation analysis demonstrated dark‐enhanced binding of proteins to the upstream region of the psbA2 gene, pointing to a repressor‐protein‐based transcriptional regulation mechanism. Transcription of all the other psb genes also required light, but unlike the psbA and psbD genes, these psb genes did not respond specifically to high‐light. Moreover, the transcription of these psb genes was activated slowly at onset of illumination, and was strictly dependent on de novo protein synthesis. We suggest that these psb genes are up‐regulated in the light via transcriptional activator proteins, and the slow activation may be related to production of new PSII centres during growth. Apart from the two distinct mechanisms for transcriptional regulation, all psb genes shared a common regulation mechanism at the level of transcript stability, mediated by the redox poise of intersystem electron carrier(s).