Essential protein (PubMed:19956626). Polymerase that creates the 3'-poly(A) tail of mRNA's (PubMed:15297145). Also required for the endoribonucleolytic cleavage reaction at some polyadenylation sites. May acquire specificity through interaction with a cleavage and polyadenylation specificity factor (CPSF) at its C-terminus (By similarity). ATP + RNA(n) = diphosphate + RNA(n)-3'-adenine ribonucleotide Binds 2 magnesium ions. Also active with manganese. Monomer (By similarity). Forms a complex with cleavage and polyadenylation specificity factor (CPSF) subunits FIPS5 and CPSF30 (PubMed:18479511). Expressed in leaves (mostly in petioles and tips), cotyledon, roots (tips, vascular tissue of the radicle, and throughout the root tissue excluding the elongation zone), stems, and flowers (restricted to the stigma and the pollen in mature anthers) (PubMed:15297145, PubMed:19956626). Active in the primary and secondary root systems (PubMed:19956626). Lethal. Belongs to the poly(A) polymerase family.