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 σ factor of the RNA polymerase holoenzyme is responsible for the recognition of a promoter sequence. In the cyanobacterium Synechocystis sp. PCC 6803, the primary σ 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 σ factors. According to the models, the overall three-dimensional structures of group 1 and 2 σ factors are similar, the SigB and SigD factors being the most similar ones. In addition, we have constructed a complete set of group 2 σ factor double inactivation strains, ΔsigBC, ΔsigBD, ΔsigBE, ΔsigCD, ΔsigCE, and ΔsigDE. 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 ΔsigBD strain, and all strains lacking the SigD factor were found to be sensitive to bright light. Furthermore, all group 2 σ factors were found to be involved in acclimation to salt- or sorbitol-induced osmotic stresses.