Based on its similarity structurally to the RNA recognition motif this domain is thought to be RNA binding. Poly(A) polymerase, the enzyme at the heart of the polyadenylation machinery, is a template-independent RNA polymerase that specifically incorporates ATP at the 3' end of mRNA. The crystal structure of bovine poly(A) polymerase bound to an ATP analogue at 2.5 A resolution has been determined PMID:10944102. The C-terminal domain unexpectedly folds into a compact domain reminiscent of the RNA-recognition motif fold. The three invariant aspartates of the catalytic triad ligate two of the three active site metals. One of these metals also contacts the adenine ring. Furthermore, conserved, catalytically important residues contact the nucleotide. These contacts, taken together with metal coordination of the adenine base, provide a structural basis for ATP selection by poly(A) polymerase.

PFAM:PF04926,INTERPRO:IPR007010