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

在像蓝藻这样的光合生物中，二氧化碳固定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起着重要作用。由于其环境和日益增加的生物技术的重要性，了解蓝藻生理学，即无机碳（Ci）同化及其调控的潜在机制是至关重要的。然而，我们对蓝藻中初级C代谢调节的理解落后于其他细菌群。特别是对Calvin-Benson-Bassam循环酶和相关途径的调控仍然是不完整的。在我最近对蓝藻模型细菌聚囊藻sp. PCC 6803（以下简称聚囊藻）的研究活动中，我确定了两个具有强大监管潜力的候选物种。一方面，存在一种广泛保守的lysr型转录调控因子（LTTR），其功能迄今尚未被揭示。在聚胞菌中，它由sll0998基因编码，该基因显然对细胞活力至关重要。因此，我们很容易推测Sll0998是否可以作为一个或几个基本基因的激活剂。由于LTTR家族在基因调控和代谢信号感知方面的重要意义，令人惊讶的是，除了这些观察结果之外，对蓝藻中的Sll0998及其同系物一无所知。然而，由于与另外两个主要控制几种Ci摄取系统基因表达的lttr密切相关，在C同化中似乎很可能具有关键的调节功能。另一方面，我们最近发现氮胁迫诱导的RNA 4 （NsiR4）在蓝藻中作为转录后调节因子，通过调节谷氨酰胺合成酶抑制蛋白的表达参与控制氮同化。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，计算机分析强烈表明NsiR4也调节编码糖原合成、光合电子传递链和Calvin-Benson-Bassam循环的关键酶的基因。与NsiR4的n调控表达一致，所有这些基因在聚胞虫中受n限制时转录水平发生改变。所有这些酶似乎都代表了决定C代谢特定途径的中心枢纽。因此，NsiR4可能不仅参与控制N同化，还可能通过直接靶向相应的关键酶影响重要的C途径。本项目旨在利用模型菌株Synechocystis对LTTR Sll0998和调控RNA NsiR4各自的调控进行实验研究。由于其主要调控方面仍然是模糊的，所获得的结果将是一个关键的贡献，以了解调节蓝藻初级C代谢。

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.