Separate mechanisms activate sigma B of Bacillus subtilis in response to environmental and metabolic stresses

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Abstract
Sigma B is a secondary sigma factor that controls the general stress response of Bacillus subtilis. sigma B-dependent transcription is induced by the activation of sigma B itself, a process that involves release of sigma B from an inhibitory complex with its primary regulator, RsbW. sigma B becomes available to RNA polymerase when RsbW forms a complex with an additional regulatory protein (RsbV) and, because of this, fails to bind sigma B. Using Western blot (immunoblot) analyses, reporter gene fusion assays, and measurements of nucleotide pool sizes, we provide evidence for two independent processes that promote the binding of RsbW to RsbV. The first occurs during carbon limitation or entry into stationary phase. Activation of sigma B under these circumstances correlates with a drop in the intracellular levels of ATP and may be a direct consequence of ATP levels on RsbW's binding preference. The second activation process relies on the product of a third regulatory gene, rsbU. RsbU is dispensable for sigma B activation during carbon limitation or stationary phase but is needed for activation of sigma B in response to any of a number of different environmental insults (ethanol treatment, salt or acid shock, etc.). RsbU, or a process dependent on it, alters RsbW binding without regard for intracellular levels of ATP. In at least some instances, the effects of multiple inducing stimuli are additive. The data are consistent with RsbW being a regulator at which distinct signals from separate effectors can be integrated to modulate sigma B activity.

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