Provides the crucial link between the upstream module (communication of environmental stress) and the downstream module (integration of the environmental signals with signals of energy stress) that compose the signal transduction pathway controlling the sigma-B factor. Phosphorylates and inactivates its specific antagonist protein RsbS thanks to its serine kinase activity. Upon phosphorylation of RsbS, RsbT is released to stimulate RsbU, a PP2C phosphatase, thereby initiating the signaling cascade that ultimately activates sigma-B. The activity of the RsbU phosphatase may be stimulated by a long-lived interaction with RsbT and the serine kinase function of RsbT is not required to directly modify RsbU. Also phosphorylates RsbR thanks to its threonine kinase activity, preventing it to phosphorylate RsbT. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein]