Poly(A) polymerase that creates the 3'-poly(A) tail of specific pre-mRNAs. Localizes to nuclear speckles together with PIP5K1A and mediates polyadenylation of a select set of mRNAs, such as HMOX1. In addition to polyadenylation, it is also required for the 3'-end cleavage of pre-mRNAs: binds to the 3'UTR of targeted pre-mRNAs and promotes the recruitment and assembly of the CPSF complex on the 3'UTR of pre-mRNAs. In addition to adenylyltransferase activity, also has uridylyltransferase activity. However, the ATP ratio is higher than UTP in cells, suggesting that it functions primarily as a poly(A) polymerase. Acts as a specific terminal uridylyltransferase for U6 snRNA in vitro: responsible for a controlled elongation reaction that results in the restoration of the four 3'-terminal UMP-residues found in newly transcribed U6 snRNA. Not involved in replication-dependent histone mRNA degradation. RNA(n) + UTP = diphosphate + RNA(n)-3'-uridine ribonucleotide ATP + RNA(n) = diphosphate + RNA(n)-3'-adenine ribonucleotide Binds 1 divalent cation per subunit. Adenylyltransferase activity is specifically phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2). Associates with the cleavage and polyadenylation specificity factor (CPSF) complex. Interacts with CPSF1 and CPSF3; the interaction is direct. Interacts with PIP5K1A. The zinc-finger domain is required for terminal uridylyltransferase activity. Together with the RRM domain, binds the 5'-area of U6 snRNA. The RRM domain is required for terminal uridylyltransferase activity. Together with the zinc-finger domain, binds the 5'-area of U6 snRNA. The proline-rich region is dispensable for terminal uridylyltransferase activity. Phosphorylated by CK1 in the proline-rich (Pro-rich) region. Belongs to the DNA polymerase type-B-like family.