Phosphorylation and RsbX-dependent dephosphorylation of RsbR in the RsbR-RsbS complex of Bacillus subtilis

In the pathway that controls sigma(B) activity, the RsbR-RsbS complex plays an important role by trapping RsbT, a positive regulator of sigma(B) of Bacillus subtilis. We have proposed that at the onset of stress, RsbR becomes phosphorylated, resulting in an enhanced activity of RsbT towards RsbS. RsbT is then free to interact with and activate RsbU, which in turn ultimately activates sigma(B). In this study with purified proteins, we used mutant RsbR proteins to analyze the role of its phosphorylatable threonine residues. The results show that the phosphorylation of either of the two RsbT-phosphorylatable threonine residues (T171 and T205) in RsbR enhanced the kinase activity of RsbT towards RsbS. However, it appeared that RsbT preferentially phosphorylates T171. We also present in vitro evidence that identifies RsbX as a potential phosphatase for RsbR T205.