This domain occurs in a small set of of bacterial proteins. It has two transmembrane regions, and often occurs as tandem repeats. The are no conserved catalytic residues.

1: New Knowledge from Old: In silico discovery of novel protein domains in Streptomyces coelicolor. Yeats C, Bentley S, Bateman A; BMC Microbiol 2003;3:3-3. PMID:12625841