Threonine phosphorylation of modulator protein RsbR governs its ability to regulate a serine kinase in the environmental stress signaling pathway of Bacillus subtilis.

The sigmaB transcription factor of the bacterium Bacillus subtilis controls the synthesis of over 100 general stress proteins that are induced by growth-limiting conditions. Genetic evidence suggests that RsbR modulates the phosphorylation state of the RsbS antagonist in the signaling pathway that regulates sigmaB activity in response to environmental stresses that limit growth. According to the current model, the phosphorylated RsbS antagonist is unable to complex RsbT, which is then released to initiate a signaling cascade that ultimately activates sigmaB. Here, we show that the RsbR protein itself has no kinase activity but instead stimulates RsbS phosphorylation by the RsbT serine kinase in vitro. We further show that in addition to its previously known serine kinase activity directed toward the RsbS antagonist, purified RsbT also possesses a threonine kinase activity directed toward residues 171 and 205 of the RsbR modulator. Threonine residues 171 and 205 were each found to be important for RsbR function in vivo, and phosphorylation of these residues abolished the ability of RsbR to stimulate RsbT kinase activity in vitro. These results are consistent with a model in which RsbR modulates the kinase activity of RsbT directed toward its RsbS antagonist in vivo, either specifically in response to environmental signals or as part of a feedback mechanism to prevent continued signaling.