Gene expression control of carbon assimilation in cyanobacteria - new insights into an old story?

在光合生物如蓝藻中，CO2固定是生长和生物量生产的基础。蓝藻对全球CO2的固定有很大贡献，因此在地球化学循环中发挥着重要作用。由于它们的环境以及越来越多的生物技术的重要性，它是至关重要的了解蓝藻生理学，即无机碳（Ci）同化及其调节的基本机制。然而，我们对蓝藻中初级碳代谢调节的理解落后于其他细菌群体。尤其是Calvin-Benson-Bassam循环酶及其相关途径的调控仍然是零碎的，在我最近对模式蓝藻集胞藻PCC 6803（以下简称集胞藻）的研究活动中，我已经确定了两个具有很强调控潜力的候选者。一方面，存在一种广泛保守的LysR型转录调节因子（LTTR），其功能至今尚未被揭示。在集胞藻中，它由sll 0998基因编码，该基因显然对细胞活力至关重要。因此，推测Sll 0998是否可能作为一个或几个必需基因的激活剂是诱人的。由于LTTR家族在基因调控和代谢信号感测中的重要意义，令人惊讶的是，除了这些观察之外，对Sll 0998及其在蓝藻中的同源物一无所知。然而，在C同化中的关键调节功能似乎很可能是由于与两个额外的LTTR的密切关系，这两个LTTR主要控制几个Ci吸收系统的基因表达。我们最近鉴定了氮胁迫诱导的RNA 4（NsiR 4）作为后-蓝细菌中的转录调节因子，其通过调节抑制蛋白的表达来参与控制氮同化，谷氨酰胺合成酶。然而，蓝藻的转录后控制知之甚少，需要进一步关注。有趣的是，计算机模拟分析强烈表明，NsiR 4还调节编码糖原合成、光合电子传递链和Calvin-Benson-Bassam循环的关键酶的基因。与NsiR 4的N调节表达一致，所有这些基因在集胞藻中的N限制后显示改变的转录水平。所有这些酶似乎代表了决定C代谢特定途径的中心枢纽。因此，NsiR 4可能不仅参与控制氮同化作用，而且可能通过直接靶向相应的关键酶来影响重要的碳代谢途径。本项目旨在以集胞藻为模型菌株，对LTTR Sll 0998和调控RNA NsiR 4各自的调节子进行实验研究。由于其主要的调控方面仍然是模糊的，所获得的结果将是一个至关重要的贡献，为了解调节初级碳代谢蓝藻。

In photosynthetic organisms such as cyanobacteria, CO2 fixation is the basis for growth and biomass production. Cyanobacteria largely contribute to global CO2 fixation and thus play major roles in biogeochemical cycles. Due to their environmental as well as increasing biotechnological importance it is crucial to understand cyanobacterial physiology, i.e. the underlying mechanisms of inorganic carbon (Ci) assimilation and its regulation. However, our understanding of the regulation of primary C metabolism in cyanobacteria is staying behind other bacterial groups. In especially the regulation of Calvin-Benson-Bassam cycle enzymes and those of associated pathways is still fragmentary.During my recent research activities on the model cyanobacterium Synechocystis sp. PCC 6803 (hereafter Synechocystis) I have identified two candidates with strong regulatory potential. On the one hand, there is a widely conserved LysR-type transcriptional regulator (LTTR) whose function has not been revealed so far. In Synechocystis, it is encoded by the sll0998 gene which is apparently crucial for cell viability. Hence, it is tempting to speculate whether Sll0998 might work as activator for one or several essential genes. Due to the crucial meaning of the LTTR family in both, gene regulation and sensing of metabolic signals it is somewhat surprising that beyond these observations nothing is known about Sll0998 and its homologs in cyanobacteria. Nevertheless, a crucial regulatory function in C assimilation seems very likely due to the close relation to two additional LTTRs that mainly control the expression of genes for several Ci uptake systems.On the other hand, we recently identified the nitrogen stress induced RNA 4 (NsiR4) as post-transcriptional regulator in cyanobacteria which is involved in controlling nitrogen assimilation by regulating the expression of an inhibitory protein for the glutamine synthetase. However, post-transcriptional control in cyanobacteria is poorly understood and needs further attention. Interestingly, in silico analyses strongly suggest that NsiR4 also regulates genes encoding key enzymes for glycogen synthesis, the photosynthetic electron transport chain and the Calvin-Benson-Bassam cycle. Consistent with a N-regulated expression of NsiR4 all these genes show altered transcript levels upon N-limitation in Synechocystis. All these enzymes appear to represent central hubs determining specific routes of C metabolism. Consequently, NsiR4 might not only be involved in controlling N assimilation but also could influence important C routes by targeting corresponding key enzymes directly.The projects aims in the experimental investigation of the respective regulons of the LTTR Sll0998 and the regulatory RNA NsiR4 using the model strain Synechocystis. Since its main regulatory aspects are still ambiguous the obtained results will be a crucial contribution for the understanding of regulating primary C metabolism in cyanobacteria.