Temporal regulation of σ by partner-switching mechanism at a distinct growth stage in Bacillus cereus.

The alternative transcription factor σ in Bacillus cereus governs the transcription of a number of genes that confer protection against general stress. This transcription factor is regulated by protein-protein interactions among RsbV, RsbW, σ, RsbY, RsbM and RsbK, all encoded in the sigB cluster. Among these regulatory proteins, RsbV, RsbW and σ comprise a partner-switching mechanism. Under normal conditions, σ remains inactive by associating with anti-sigma factor RsbW, which prevents σ from binding to the core RNA polymerase. During environmental stress, RsbK activates RsbY to hydrolyze phosphorylated RsbV, and the dephosphorylated RsbV then sequesters RsbW to liberate σ from RsbW. Although the σ partner-switching module is thought to be the core mechanism for σ regulation, the actual protein-protein interactions among these three proteins in the cell remain to be investigated. In the current study, we show that RsbW and RsbV form a long-lived complex under transient stress treatment, resulting in high persistent expression of RsbV, RsbW and σ from mid-log phase to stationary phase. Full sequestration of RsbW by excess RsbV and increased RsbW:RsbV complex stability afforded by cellular ADP contribute to the prolonged activation of σ. Interestingly, the high expression levels of RsbV, RsbW and σ were dramatically decreased beginning from the transition stage to the stationary phase. Thus, protein interactions among σ partner-switching components are required for the continued induction of σ during environmental stress in the log phase and significant down-regulation of σ is observed in the stationary phase. Our data show that σ is temporally regulated in B. cereus.