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

• Applicant: Privatdozent Dr. Stephan Klähn
• Subject Area: Metabolism, Biochemistry and Genetics of Microorganisms
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 387876635

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.

• DFG Programme: Research Grants