Nucleotidyltransferase (NT) domain of poly(A) polymerases and terminal uridylyl transferases.
Poly(A) polymerases (PAPs) catalyze mRNA poly(A) tail synthesis, and terminal uridylyl transferases (TUTases) uridylate RNA. PAPs in this subgroup include human PAP alpha, mouse testis-specific cytoplasmic PAP beta, human nuclear PAP gamma, Saccharomyces cerevisiae PAP1, TRF4 and-5, Schizosaccharomyces pombe caffeine-induced death proteins -1, and -14, Caenorhabditis elegans Germ Line Development-2, and Chlamydomonas reinhardtii MUT68. This family also includes human U6 snRNA-specific TUTase1, and Trypanosoma brucei 3'-TUTase-1,-2, and 4. This family belongs to the Pol beta-like NT superfamily. In the majority of enzymes in this superfamily, two carboxylates, Dx[D/E], together with a third more distal carboxylate, coordinate two divalent metal cations involved in a two-metal ion mechanism of nucleotide addition. For the majority of proteins in this family, these carboxylate residues are conserved.