FIG. 1.
Model of the σB signal transduction network. (A) Two independent signaling pathways converge on the RsbV anti-anti-σ and the RsbW anti-σ, the direct regulators of σB activity. The energy pathway terminates with the RsbP phosphatase (Energy PP2C), which contains a PAS domain implicated in energy sensing; the environmental pathway terminates with the RsbU phosphatase (Environmental PP2C), which is activated by upstream signaling elements. Phosphorylated RsbV (RsbV-P) is the antagonist form found in unstressed cells. When activated by stress, either RsbP or RsbU can dephosphorylate RsbV-P, allowing it to bind and inactivate the RsbW anti-σ. (B) In the environmental signaling pathway, RsbS and RsbT are paralogs of RsbV and RsbW, respectively. RsbS is the antagonist form in unstressed cells, and RsbRA, RsbRB, RsbRC, and RsbRD are redundant coantagonists that function with RsbS to bind the RsbT kinase in an inactive complex. Following environmental stress, RsbT phosphorylates RsbRA and RsbS, releasing RsbT to bind and activate the RsbU phosphatase. The RsbX feedback phosphatase returns the system to its prestress condition. Phosphorylation of RsbRB, RsbRC, and RsbRD is not shown but is thought to resemble that of RsbRA. (C) RsbR coantagonist proteins share a carboxyl-terminal domain (shaded) with the smaller RsbS antagonist (1, 25). In the RsbR family, this domain contains two conserved threonine (T) residues, and RsbT is known to phosphorylate RsbRA on T171 and T205 in vitro (12). In contrast, RsbS bears an aspartate (D) and a serine (S) at these corresponding positions. Genetic evidence suggests that phosphorylation of S59 is required to relieve RsbS antagonist function (16, 30).