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). Mediates the polyadenylation of RNAs that are associated with polynucleotide phosphorylase (e.g. PNP1) (PubMed:10872823). 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 CPSF100, CPSF30, FIPS5 and PABN2 (PubMed:18479511). Mostly expressed in flowers (highly in the style, receptacle and pedicel, but weakly in the vasculature of sepals) and hypocotyls, and, to a lower extent, in roots and stems. Barely detected in leaves (petioles and vascular system) (PubMed:15297145, PubMed:19956626). Lethal. Belongs to the poly(A) polymerase family.