Nucleotidyltransferase (NT) domain of ClassII CCA-adding enzymes.
CCA-adding enzymes add the sequence [cytidine(C)-cytidine-adenosine (A)], one nucleotide at a time, onto the 3' end of tRNA, in a template-independent reaction. This Class II group is comprised mainly of eubacterial and eukaryotic enzymes and includes Bacillus stearothermophilus CCAase, Escherichia coli poly(A) polymerase I, human mitochondrial CCAase, and Saccharomyces cerevisiae CCAase (CCA1). CCA-adding enzymes have a single catalytic pocket, which recognizes both ATP and CTP substrates. Included in this subgroup are CC- and A-adding enzymes from various ancient species of bacteria such as Aquifex aeolicus; these enzymes collaborate to add CCA to tRNAs. 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. These carboxylate residues are fairly well conserved in this family. Escherichia coli CCAase is related to this group but has not been included in this alignment as this enzyme lacks the N-terminal helix conserved in the remainder of the NT superfamily.