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

蓝藻是光自养生物，白天吸收二氧化碳，晚上通过异养途径利用积累的有机碳。在水生系统中，可用于光合作用的二氧化碳量是有限的且高度可变的，因此蓝藻已经进化出一种有效的二氧化碳浓缩机制（CCM）。在这方面，高亲和力的碳酸氢盐转运体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SbtB蛋白的基因共表达，对sbtA转运活性和二氧化碳调控基因表达有调控作用。利用蓝藻聚胞菌（Synechocystis sp. PCC 6803）模型进行的转录组研究证明，一个先前未知的基因位于sbtAB操纵子的上游，该基因编码80个氨基酸的蛋白质。在蓝藻门中，它很保守，经常与sbtAB共定位，因此，我们将该基因命名为sbtC。我们的初步研究表明，与sbtAB操纵子一样，sbtC的表达在CO2缺乏条件下受到强烈刺激，并且调控因子RbcR和SbtB在这方面发挥了积极作用。在其他蓝藻基因组中，sbtC的co2依赖性调控以及与sbtAB的共定位表明，sbtC在SbtA和蓝藻CCM中起着至关重要的作用，无论是作为结构成分还是作为调控成分。该项目旨在揭示SbtC的功能，并遵循两个主要假设：1。SbtC在SbtAB超络合物的建立中起着结构作用，而SbtAB超络合物是有效吸收碳酸氢盐所必需的。2. SbtC在不同CO2条件下对基因的协调表达起调控作用。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，而且可以帮助优化蓝藻CCM，为未来的生物技术应用提供帮助。

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.