Multilevel regulation of the sporulation transcription factor sigma K in Bacillus subtilis

Gene expression in the mother-cell compartment of the Bacillus subtilis sporangium is governed in part by the sporulation transcription factor sigma K. The production of sigma K is controlled at three levels: by a chromosomal rearrangement that generates the sigma K-coding sequence (sigK), by compartment-specific transcription of sigK, and by conversion of the inactive pro-protein product of sigK (pro-sigma K) to sigma K. To investigate the function of these multiple levels of regulation, we constructed a set of strains that bypass the chromosomal rearrangement, pro-protein processing, or both levels of control. Here we show that one of the functions of the chromosomal rearrangement and pro-protein processing is to prevent inappropriate production of sigma K under nonsporulation conditions. In the absence of both of these levels of control, a low level of sigma K-directed gene expression is observed during stationary phase after growth in rich medium. The appearance of sigma K under these conditions is probably due to a low level of sigma K-directed transcription from the sigK promoter in a positive feedback loop. We also report the construction of a strain that produces high levels of sigma K during growth. Using this strain, we demonstrate that the production of sigma K during growth is sufficient to induce a cascade of gene expression that closely mimics late events in the mother-cell line of gene expression.