Isolation and characterization of Bacillus subtilis sigB operon mutations that suppress the loss of the negative regulator RsbX

sigma B, a transcription factor that controls the Bacillus subtilis general stress response regulon, is activated by either a drop in intracellular ATP or exposure to environmental stress. RsbX, one of seven sigma(B) regulators (Rsb proteins) whose genes are cotranscribed with sigma(B), is a negative regulator in the stress-dependent activation pathway. To better define the interactions that take place among the Rsb proteins, we analyzed sigB operon mutations which suppress the high-level sigma(B) activity that normally accompanies the loss of RsbX. Each of these mutations was in one of three genes (rsbT, -U, and -V) which encode positive regulators of sigma(B), and they all defined amino acid changes which either compromised the activities of the mutant Rsbs or affected their ability to accumulate. sigma(B) activity remained inducible by ethanol in several of the RsbX(-) suppressor strains. This finding supports the notion that RsbX is not needed as the target for sigma(B) activation by at least some stresses, sigma(B) activity in several RsbX- strains with suppressor mutations in rsbT or -U was high during growth and underwent a continued, rather than a transient, increase following stress. Thus, RsbX is likely responsible for maintaining low sigma(B) activity during balanced grow th and for reestablishing sigma(B) activity at prestress levels following induction. Although RsbX likely participates in limiting the sigma(B) induction response, a second mechanism for curtailing unrestricted sigma(B) activation was suggested by the sigma(B) induction profile in two suppressor strains with mutations in rsbV sigma(B) activity in these mutants was stress inducible but transient, even in the absence of RsbX.