SERINE KINASE-ACTIVITY OF A BACILLUS-SUBTILIS SWITCH PROTEIN IS REQUIRED TO TRANSDUCE ENVIRONMENTAL-STRESS SIGNALS BUT NOT TO ACTIVATE ITS TARGET PP2C PHOSPHATASE

The RsbT serine kinase has two known functions in the signal transduct ion pathway that activates the general stress factor sigma(B) of Bacil lus subtilis. First, RsbT can phosphorylate and inactivate its specifi c antagonist protein, RsbS. Second, upon phosphorylation of RsbS, RsbT is released to stimulate RsbU, a PP2C phosphatase, thereby initiating a signalling cascade that ultimately activates sigma(B). Here we desc ribe a mutation that separates these two functions of RsbT. Although t he mutant RsbT protein had essentially no kinase activity, it still re tained the capacity to stimulate the RsbU phosphatase in vitro and to activate sigma(B) when overexpressed in vivo. These results support th e hypothesis that phosphatase activation is accomplished via a long-li ved interaction between RsbT and RsbU. In contrast, RsbT kinase activi ty was found to be integral for the transmission of external stimuli t o sigma(B). Thus, one route by which environmental stress signals coul d enter the sigma(B) network is by modulation of the RsbT kinase activ ity, thereby controlling the magnitude of the partner switch between t he RsbS-RsbT complex and the RsbT-RsbU complex.