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

在蓝藻等光合作用生物中，固定二氧化碳是生长和生物量生产的基础。蓝藻在很大程度上促进了全球二氧化碳的固定，因此在生物地球化学循环中发挥着重要作用。由于它们在环境和生物技术方面的重要性，了解蓝藻的生理学，即无机碳(Ci)同化的潜在机制及其调控是至关重要的。然而，我们对蓝藻初级碳代谢调节的了解落后于其他细菌群。尤其是对Calvin-Benson-Bassam循环酶及其相关途径的调控仍然是零散的。在我最近对模型蓝藻聚胞藻的研究活动中。PCC6803(以下简称聚球藻)我已经确定了两个具有很强调控潜力的候选基因。一方面，有一种广泛保守的LysR-型转录调节因子(LTTR)，其功能至今尚未被揭示。在聚球藻中，它由sll0998基因编码，该基因显然对细胞活力至关重要。因此，人们很容易推测S110998是否可能作为一个或几个必需基因的激活剂。由于LTTR家族在基因调控和代谢信号感知中的重要意义，令人惊讶的是，除了这些观察之外，关于S110998及其在蓝藻中的同源物的任何了解都是令人惊讶的。另一方面，我们最近发现氮胁迫诱导的RNA4(NsiR4)是蓝藻中的转录后调控因子，它通过调节谷氨酰胺合成酶抑制蛋白的表达来调控氮的同化。然而，对蓝藻的转录后调控知之甚少，需要进一步关注。有趣的是，在计算机分析中，强烈表明NsiR4还调节编码糖原合成、光合作用电子传输链和Calvin-Benson-Bassam循环的关键酶的基因。与NsiR4的N调控表达一致，所有这些基因在集胞藻中都表现出在N限制下的转录水平的变化。所有这些酶似乎都代表着决定C代谢特定途径的中心枢纽。因此，NsiR4可能不仅参与控制N的同化，还可能通过直接靶向相应的关键酶来影响重要的C途径。本项目旨在利用模式菌株集胞藻对LTTRS110998和调控RNA NsiR4的各自规律进行实验研究。由于其主要调控方面仍不明确，本研究结果将对理解蓝藻初级碳代谢的调控具有重要意义。

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.