This domain is found in Sensor protein TorS from Escherichia coli and similar proteins predominantly found in proteobacteria. TorS is a sensor histidine kinase that forms a complex with TorT, and TorR, to form the trimethylamine-N-oxide (TMAO) reductase (Tor) pathway, which plays a role in the regulation of this terminal electron receptor in anaerobic respiration. This domain folds into a four-helical bundle [1-3].

1: Structural analysis of sensor domains from the TMAO-responsive histidine kinase receptor TorS. Moore JO, Hendrickson WA; Structure. 2009;17:1195-1204. PMID:19748340

2: An asymmetry-to-symmetry switch in signal transmission by the histidine kinase receptor for TMAO. Moore JO, Hendrickson WA; Structure. 2012;20:729-741. PMID:22483119