The Bacillus subtilis rsbU gene product is necessary for RsbX-dependent regulation of sigma B.

sigma B is a secondary sigma factor of Bacillus subtilis. sigma B-dependent transcription is induced when B. subtilis enters the stationary phase of growth or is exposed to any of a number of different environmental stresses. Three genes (rsbV, rsbW, and rsbX), which are cotranscribed with the sigma B structural gene (sigB), encode regulators of sigma B-dependent gene expression. RsbW and RsbV have been shown to control sigma B activity, functioning as an inhibitory sigma B binding protein and its antagonist, respectively. Using the SPAC promoter (PSPAC) to control the expression of the sigB operon, a ctc::lacZ reporter system to monitor sigma B activity, and monoclonal antibodies to determine the levels of sigB operon products, we have now obtained evidence that RsbX is an indirect regulator of sigma B activity. Genetic data and in vivo measurements argue that RsbX negatively regulates an extension of the RsbV-RsbW pathway that requires the product of an additional regulatory gene (rsbU) which lies immediately upstream of the sigB operon. The results are consistent with RsbU, or a process dependent on RsbU, being able to facilitate the RsbV-dependent release of sigma B from RsbW but normally prevented from doing this by RsbX.