A ubiquitin superfamily domain that is often present in multiple tandem copies in the same polypeptide. Members of this family are associated in gene neighborhoods, or on occasions fused to, bacterial homologs of components of ubiquitin-dependent modification system such as the E1, E2 and JAB metallopeptidase enzymes and a distinct metal-binding domain [1]. The E2/UBC fold domain appears to be inactive. The JAB domain in these operons is usually fused to the E1 domain [1].

1: The prokaryotic antecedents of the ubiquitin-signaling system and the early evolution of ubiquitin-like beta-grasp domains. Iyer LM, Burroughs AM, Aravind L; Genome Biol. 2006;7:R60. PMID:16859499