Members of this family adopt a structure consisting of a five helical bundle core. They are predominantly found in Archaeal tRNA nucleotidyltransferase, following the catalytic nucleotidyltransferase domain [1].

1: Crystal structures of an archaeal class I CCA-adding enzyme and its nucleotide complexes. Xiong Y, Li F, Wang J, Weiner AM, Steitz TA; Mol Cell. 2003;12:1165-1172. PMID:14636575