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 ge neral stress response regulon, is activated by either a drop in intrac ellular ATP or exposure to environmental stress. RsbX, one of seven si gma(B) regulators (Rsb proteins) whose genes are cotranscribed with si gma(B), is a negative regulator in the stress-dependent activation pat hway. To better define the interactions that take place among the Rsb proteins, we analyzed sigB operon mutations which suppress the high-le vel 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 ac id changes which either compromised the activities of the mutant Rsbs or affected their ability to accumulate. sigma(B) activity remained in ducible 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 se veral RsbX- strains with suppressor mutations in rsbT or -U was high d uring growth and underwent a continued, rather than a transient, incre ase 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 Rs bX likely participates in limiting the sigma(B) induction response, a second mechanism for curtailing unrestricted sigma(B) activation was s uggested by the sigma(B) induction profile in two suppressor strains w ith mutations in rsbV sigma(B) activity in these mutants was stress in ducible but transient, even in the absence of RsbX.