This domain is found at the N-terminus of the sensor component of the two-component regulatory system. It includes a transmembrane region and part of the periplasmic region, which is likely to be involved in stimulus sensing [1].

1: A two-component regulatory system playing a critical role in plant pathogens and endosymbionts is present in Brucella abortus and controls cell invasion and virulence. Sola-Landa A, Pizarro-Cerda J, Grillo MJ, Moreno E, Moriyon I, Blasco JM, Gorvel JP, Lopez-Goni I; Mol Microbiol. 1998;29:125-138. PMID:9701808