This domain appears to be the N-terminus of the RelB antitoxin of toxin-antitoxin stability system or prevent-host death system. Together RelE toxin and the RelB antitoxin form a non-toxic complex. Although toxin-antitoxin gene cassettes were first found in plasmids, it is clear that these loci are abundant in free-living prokaryotes, including many pathogenic bacteria, and these toxin-antitoxin loci provide a control mechanism that helps free-living prokaryotes cope with nutritional stress [1,2].

1: New connections in the prokaryotic toxin-antitoxin network: relationship with the eukaryotic nonsense-mediated RNA decay system. Anantharaman V, Aravind L; Genome Biol 2003;4:R81. PMID:14659018

2: Prokaryotic toxin-antitoxin stress response loci. Gerdes K, Christensen SK, Lobner-Olesen A; Nat Rev Microbiol. 2005;3:371-382. PMID:15864262