The redox state of photosynthetic electron transport chain regulates chlorophyll biosynthesis in the cyanobacterium <i>Synechocystis</i> PCC 6803

Cyanobacteria are widely distributed in various environmental conditions with dynamic changes in photosynthetic activity and chlorophyll content. However, it remains elusive how their chlorophyll biosynthesis is regulated by the redox state of photosynthetic electron transport chain (PETC). In this study, the effects of light-dark transition and inhibition of different sites within PETC on the transcriptional expression of chlorophyll biosynthesis related genes were investigated in Synechocystis PCC 6803. It was demonstrated that the transcript levels of chlorophyll biosynthesis related genes were generally stimulated upon light exposure and suppressed in darkness. The introduction of electron transport inhibitors changed the pattern of light-induced expression of these genes, suggesting the redox state of PETC governs the chlorophyll biosynthesis. Meanwhile, the transcription regulation pattern of chlorophyll biosynthesis related genes can be categorized into three groups based on their different transcriptional responses to the inhibition of PETC. The low-oxygen inducible gene chlAII, which encodes magnesium-protoporphyrin IX monomethyl ester (oxidative) cyclase, could be activated by over-reduced PQ pool. Several redox-sensitive transcription factors were identified to play some roles in regulating chlorophyll biosynthesis by constructing and screening a yeast one-hybrid library. The transcription factor cyAbrB1 was shown to bind to the promoters of most chlorophyll biosynthesis related genes, implying its global transcriptional regulatory role in chlorophyll biosynthesis. Our research provides a systematic understanding of the transcription regulation mechanism of chlorophyll biosynthesis triggered by the redox state of PETC in cyanobacteria.