Elucidating the role of SbtC in the assimilation of inorganic carbon in cyanobacteria

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Cyanobacteria are photoautotrophic organisms, which assimilate CO2 during the day and use accumulated organic carbon by heterotrophic pathways during the night. In aquatic systems, the amount of CO2 available for photosynthesis is limited and highly variable, so cyanobacteria have evolved an efficient CO2-concentrating mechanism (CCM). In this regard, the high-affinity bicarbonate transporter SbtA plays a crucial role, which is expressed under CO2-limited conditions. In many cyanobacteria, the sbtA gene is co-expressed with another gene encoding the protein SbtB, which exerts a regulatory effect on SbtA transport activity and CO2-regulated gene expression. Transcriptome studies using the model cyanobacterium Synechocystis sp. PCC 6803 provided evidence that a previously unknown gene, which encodes a protein of 80 amino acids, is located upstream of the sbtAB operon. In the cyanobacterial phylum it is well conserved and frequently co-localized with sbtAB and hence, we named this gene sbtC. Our preliminary investigations showed that the expression of sbtC, like the sbtAB operon, is strongly stimulated under CO2 deficiency conditions and that the regulators RbcR and SbtB play a positive role in this regard. The CO2-dependent regulation of sbtC and the co-localization with sbtAB in other cyanobacterial genomes indicate that SbtC plays a crucial function associated with SbtA and the cyanobacterial CCM, either as structural or regulatory component. The project aims to reveal the function of SbtC and follows two major hypotheses: 1. SbtC plays a structural role in the establishment of a SbtAB supercomplex required for efficient bicarbonate uptake. 2. SbtC plays a regulatory role in coordinated gene expression under different CO2 conditions. These investigations should not only enable a better understanding of the cyanobacterial carbon turnover and in particular the role of the new protein SbtC, but can also help to optimize the cyanobacterial CCM for future biotechnological applications.