A membrane-bound cAMP receptor protein, SyCRP1 mediates inorganic carbon response in Synechocystis sp. PCC 6803.

The availability of inorganic carbon (C i) as the source for photosynthesis is fluctuating in aquatic environments. Despite the involvement of transcriptional regulators CmpR and NdhR in regulating genes encoding C i transporters at limiting CO 2 , the C i -sensing mechanism is largely unknown among cyanobacteria. Here we report that a cAMP-dependent transcription factor SyCRP1 mediates C i response in Synechocystis. The mutant ∆ sycrp1 exhibited a slow-growth phenotype and reduced maximum rate of bicarbonate-dependent photosynthetic electron transport (V max) compared to wild-type at the scarcity of CO 2. The number of carboxysomes was decreased significantly in the ∆ sycrp1 at low CO 2 consistent with its reduced V max. The DNA microarray analysis revealed the upregulation of genes encoding C i transporters in ∆sycrp1. The membrane-localized SyCRP1 was released into the cytosol in wild-type cells shifted from low to high CO 2 or upon cAMP treatment. Soluble His-tagged SyCRP1 was shown to target DNA-binding sites upstream of the C i -regulated genes sbtA and ccmK 3. In addition, cAMP enhanced the binding of SyCRP1 to its target sites. Our data collectively suggest that the C i is sensed through the second messenger cAMP releasing membrane-bound SyCRP1 into cytoplasm under sufficient CO 2 conditions. Hence, SyCRP1 is a possible regulator of carbon concentrating mechanism, and such a regulation might be mediated via sensing C i levels through cAMP in Synechocystis. • It has been known that cAMP receptor proteins (CRP) play a crucial role in regulation of gene expression in bacteria. • Transcription regulators of CCM genes were decribed previously, however C i -sensing mechanism in cyanobacteria is unclear. • A membrane associated Synechocystis CRP (SyCRP1), releases to cytosol when bound to CAMP to regulate CCM. • Thus, SyCRP1 mediates C i response through the signalling molecule cAMP in Synechocytis. [ABSTRACT FROM AUTHOR]