Polymerase that creates the 3'-poly(A) tail of mRNA's. May acquire specificity through interaction with a cleavage and polyadenylation factor. ATP + RNA(n) = diphosphate + RNA(n)-3'-adenine ribonucleotide Binds 2 magnesium ions. Also active with manganese. Mainly expressed during G1 and S phases. Belongs to the poly(A) polymerase family.