This is non catalytic domain B of beta-galactosidase enzymes belong to the glycosyl hydrolase 42 family. This domain is related to glutamine amidotransferase enzymes, but the catalytic residues are replaced by non functional amino acids. This domain is involved in trimerisation [1].

1: Sequence analysis of flanking regions of the pfoA gene of Clostridium perfringens: beta-galactosidase gene (pbg) is located in the 3'-flanking region. Shimizu T, Kobayashi T, Ba-Thein W, Ohtani K, Hayashi H; Microbiol Immunol 1995;39:677-686. PMID:8577281

2: Trimeric crystal structure of the glycoside hydrolase family 42 beta-galactosidase from Thermus thermophilus A4 and the structure of its complex with galactose. Hidaka M, Fushinobu S, Ohtsu N, Motoshima H, Matsuzawa H, Shoun H, Wakagi T; J Mol Biol 2002;322:79-91. PMID:12215416