Regulation der bidirektionalen NiFe-Hydrogenase im Cyanobaterium Synechocystis sp. PCC 6803
In this thesis the regulation of the bidirectional NiFe-hydrogenase in the cyanobacterium Synechocystis sp. PCC 6803 was investigated. Influences of external signals on enzyme and promoter activity were measured in vivo. In order to search for signaltransduction pathways involved in the regulation of the bidirectional NiFe-hydrogenase several mutants with deleted histidine kinases were studied. The point of main effort lay on investigations on the transcriptional level. The transcription start point of the hydrogenase genes (hoxEFUYH) was located by 5`RACE at -168 bp relative to the ATG of hoxE. Several promoter probe vectors carrying different promoter fragments revealed that the promoter covers approximately 700 nucleotides and needs sequences far upstream from the transcription start point. This is unusual for cyanobacteria who possess a compact genome. It was shown in band-shift-assays that the transcription factor LexA binds to the examined promoter. Measurements of the promoter activity revealed that LexA activates the transcription of the hydrogenase genes. In agreement with this finding the hydrogenase activity of a merodiploid lexA mutant was decreased. The results presented in this thesis support the idea, that LexA, which functions as a repressor in E.coli regulating genes involved in DNA repair might have a different function in cyanobacteria (Domain et al. 2004). In this thesis it was demonstrated for the first time that LexA might function as a transcription activator. Furthermore LexA is the first transcription factor identified to be involved in the expression of a bidirectional hydrogenase in a cyanobacterium.