The small protein SbtC is a functional component of the CO2 concentrating mechanism in <i>Synechocystis</i> sp. PCC 6803

Oxygenic phototrophs fix CO2 via the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO), which shows relatively low CO2 affinity and specificity. To circumvent low and fluctuating CO2 concentrations in aquatic systems, cyanobacteria and algae have evolved sophisticated inorganic carbon (Ci) concentrating mechanisms (CCMs). Bicarbonate transporters such as SbtA play a crucial role in the cyanobacterial CCM and hence display multiple layers of tight regulation. Control of sbtA gene expression and corresponding transporter activity involves the PII-like protein SbtB, whose gene is frequently co-transcribed with sbtA. A previously non-annotated gene located upstream of the sbtAB operon in the model Synechocystis sp. PCC 6803 encodes the small protein SbtC, composed of 80 amino acids. Presence of SbtC was confirmed by immunoblotting of the sbtC-coding sequence fused to a Flag-tag. Similar to sbtAB, transcription of the sbtC locus is induced by low CO2 availability; however, it is controlled independently. Mutation of the sbtC locus in a wild-type background produced only a mild phenotype, even under low CO2, but impaired diurnal growth resembled that of the mutant ΔsbtB. Biochemical analysis indicated a trimeric SbtABC complex in the membrane. Bicarbonate leakage from cells was strongly elevated when either sbtB or sbtC was deleted from recombinant Synechocystis strains harboring only SbtA as single Ci uptake system. Our results provide evidence that SbtC contributes to the formation of the SbtAB complex, thereby regulating bicarbonate exchange at the cytoplasmic membrane. Well-conserved SbtC-like proteins encoded in the neighborhood of sbtAB exist in many cyanobacterial genomes, pointing towards an important role in the cyanobacterial CCM.