Member of the two-component regulatory system TorS/TorR involved in the anaerobic utilization of trimethylamine-N-oxide (TMAO). Detects the presence of TMAO in the medium and, in response, activates TorR via a four-step phosphorelay. When TMAO is removed, TorS can dephosphorylate TorR, probably by a reverse phosphorelay involving His-860 and Asp-733 (By similarity). ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Inhibited by TorC apocytochrome. May form homomultimers. Seems to interact with TorT and TorC apocytochrome (By similarity). Activation requires a sequential transfer of a phosphate group from a His in the primary transmitter domain, to an Asp in the receiver domain and to a His in the secondary transmitter domain.