FIG. 6.
Model of σB regulation and σB operon. (A) σB is present in prestressed cells but held inactive in a complex with an anti-σB protein, RsbW (6, 18). σB is released from its anti-sigma factor, RsbW, by the binding of the anti-sigma factor antagonist, RsbV, to RsbW (18). In unstressed cells RsbV is phosphorylated by RsbW and inactive (3, 18, 51). Physical and nutritional stress activate novel phosphatases to dephosphorylate and reactivate RsbV-P. RsbU, the phosphatase that responds to physical stress, requires an additional protein, RsbT, for activity. RsbT is both a kinase and an RsbU activator. In unstressed B. subtilis cells, RsbT is held inactive by RsbS (56) in a complex with RsbR and a family of homologous proteins (RsbRB, RsbRC, and RsbRD) that facilitate the RsbT-RsbS interactions (1, 2, 23, 30). Upon exposure to physical stress (e.g., ethanol or osmotic shock), RsbT becomes empowered to phosphorylate RsbR and RsbS, freeing itself to interact with RsbU, to induce the dephosphorylation of RsbV-P. Once dephosphorylated, RsbV can sequester RsbW, freeing σB to activate the GSR (14, 30, 56). Negative regulation is reestablished by RsbX, a phosphatase that dephosphorylates RsbR-P and RsbS-P, allowing RsbR/S to again sequester RsbT (14, 49, 56). Nutritional stress (e.g., glucose or phosphate limitation) generates an unknown signal to activate a separate phosphatase (RsbP/Q) that is able to dephosphorylate RsbV-P (10, 28, 47, 51). (B) σB (sigB) is coexpressed in an eight-gene operon (rsbR, rsbS, rsbT, rsbU, rsbV, rsbW, and rsbX) from a promoter (PA) likely recognized by σA-containing RNA polymerase (27, 41, 53). A σB-dependent promoter (PB) within the operon upregulates the distal four genes upon σB activation (5, 9).