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

蓝藻是光合自养生物，白天吸收CO2，夜间通过异养途径利用积累的有机碳。在水生系统中，可用于光合作用的CO2的量是有限的且高度可变的，因此蓝藻已经进化出有效的CO2浓缩机制（CCM）。在这方面，高亲和力碳酸氢盐转运蛋白SbtA起着至关重要的作用，它在CO2限制条件下表达。在许多蓝藻中，sbtA基因与另一个编码蛋白SbtB的基因共表达，该蛋白对SbtA转运活性和CO2调节的基因表达产生调节作用。转录组的研究，使用模式蓝藻集胞藻属PCC 6803提供的证据表明，一个以前未知的基因，编码80个氨基酸的蛋白质，位于上游的sbtAB操纵子。在蓝藻门中，它是非常保守的，经常与sbtAB共定位，因此，我们命名为sbtC。我们的初步研究表明，sbtC的表达，如sbtAB操纵子，强烈刺激CO2缺乏条件下，调节RbcR和SbtB在这方面发挥积极作用。sbtC的CO2依赖性调节以及与sbtAB在其他蓝藻基因组中的共定位表明，SbtC作为结构或调节组分与SbtA和蓝藻CCM相关，发挥着至关重要的功能。本课题旨在揭示SbtC的功能，主要有两个假设：1. SbtC在建立有效的碳酸氢盐吸收所需的SbtAB超复合物中起着结构性作用。2. SbtC在不同CO2条件下协调基因表达中起调节作用。这些研究不仅可以更好地了解蓝藻碳周转，特别是新蛋白SbtC的作用，而且还可以帮助优化蓝藻CCM，以用于未来的生物技术应用。

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.