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

蓝藻是一种光自养生物，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CO2量是有限且高度可变的，因此蓝藻进化出了一种高效的CO2浓缩机制(CCM)。在这方面，高亲和力的碳酸氢盐转运蛋白SbtA起着至关重要的作用，它在二氧化碳限制的条件下表达。在许多蓝藻中，SbtA基因与编码SbtB蛋白的另一个基因共表达，SbtB蛋白对SbtA的转运活性和CO2调节的基因表达起着调节作用。模式蓝藻聚球藻的转录组研究。PCC 6803提供了证据，表明一个以前未知的基因位于sbtAB操纵子的上游，该基因编码一个80个氨基酸的蛋白质。在蓝藻门中，它被很好地保守，并经常与sbtAB共定位，因此，我们将其命名为SBTC。我们的初步研究表明，与sbtAB操纵子一样，SBTC的表达在二氧化碳缺乏条件下受到强烈刺激，调控因子RbcR和SbtB在这方面发挥了积极作用。SBTC的CO2依赖性调控以及与sbtAB在其他蓝藻基因组中的共存表明，无论是作为结构成分还是作为调节成分，SBTC在与SbtA和蓝藻CCM相关的关键功能中发挥着重要作用。该项目旨在揭示SBTC的功能，并遵循两个主要假设：1.SBTC在建立有效吸收碳酸氢盐所需的SbtAB超复合体中发挥结构作用。2.SBTC在不同CO2条件下对协调基因的表达起着调节作用。这些研究不仅有助于更好地了解蓝藻的碳周转，特别是新蛋白SBTC的作用，而且还有助于优化蓝藻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.