The cAMP receptor protein, SyCRP1 acts as a transcriptional repressor of CO2-concentrating mechanism genes at high inorganic carbon levels in Synechocystis PCC 6803.

Cyanobacteria utilize a CO2-concentrating mechanism (CCM) to enhance photosynthetic efficiency by accumulating CO2 around RuBisCO, a process crucial for adapting to fluctuating environmental CO2 levels. While the upregulation of CCM genes under low inorganic carbon (Ci) conditions is known, the precise Ci sensing and regulatory mechanisms governing CCM gene expression remain incompletely understood. We show that a membrane-bound SyCRP1 senses high Ci levels through cAMP binding to its low- and high-affinity sites. We demonstrate its interaction with membrane lipids and liposomes, and a subsequent reversal of its membrane localization upon cAMP binding. Comprehensive ChIP-seq analysis reveals direct binding of SyCRP1 to regulatory elements of core CCM genes. Our findings establish SyCRP1 as a key transcriptional repressor of these genes under high Ci conditions, significantly advancing our understanding of the molecular mechanisms governing CCM genes' expression in cyanobacteria.