The Bacillus subtilis rsbU gene product is necessary for RsbX-dependent regulation of σ(B)

σ(B) is a secondary σ factor of Bacillus subtilis. σ(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 σ(B) structural gene (sigB), encode regulators of σ(B)-dependent gene expression. RsbW and RsbV have been shown to control σ(B) activity, functioning as an inhibitory σ(B) binding protein and its antagonist, respectively. Using the SPAC promoter (P(SPAC)) to control the expression of the sigB operon, a ctc::lacZ reporter system to monitor σ(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 σ(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 σ(B) from RsbW but normally prevented from doing this by RsbX.