Characterization of single and double inactivation strains reveals new physiological roles for group 2 σ factors in the cyanobacterium Synechocystis sp PCC 6803

Cyanobacteria are eubacteria that perform oxygenic photosynthesis like plants. The initiation of transcription, mediated by the RNA polymerase holoenzyme, is the main determinant of gene regulation in eubacteria. The sigma factor of the RNA polymerase holoenzyme is responsible for the recognition of a promoter sequence. In the cyanobacterium Synechocystis sp. PCC 6803, the primary sigma factor, SigA, is essential for cell viability. The SigB, SigC, SigD, and SigE factors show significant sequence similarity with the SigA factor but are nonessential. In this study, we have used homology modeling to construct a three-dimensional model of Synechocystis RNA polymerase holoenzyme and all group 1 and 2 sigma factors. According to the models, the overall three-dimensional structures of group 1 and 2 sigma factors are similar, the SigB and SigD factors being the most similar ones. In addition, we have constructed a complete set of group 2 sigma factor double inactivation strains, Delta sigBC, Delta sigBD, Delta sigBE, Delta sigCD, Delta sigCE, and Delta sigDE. All double mutants grow well under standard conditions, but differences are observed in stress conditions. The transition from lag phase to exponential growth is slow in the DsigBD strain, and all strains lacking the SigD factor were found to be sensitive to bright light. Furthermore, all group 2 sigma factors were found to be involved in acclimation to salt- or sorbitol-induced osmotic stresses.