Poly(A) polymerase that creates the 3'-poly(A) tail of specific pre-mRNAs (PubMed:18288197, PubMed:21102410). Localizes to nuclear speckles together with PIP5K1A and mediates polyadenylation of a select set of mRNAs, such as HMOX1 (PubMed:18288197). 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 (PubMed:21102410). In addition to adenylyltransferase activity, also has uridylyltransferase activity (PubMed:16790842, PubMed:18288197, PubMed:28589955). However, the ATP ratio is higher than UTP in cells, suggesting that it functions primarily as a poly(A) polymerase (PubMed:18288197). 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 (PubMed:16790842, PubMed:18288197, PubMed:28589955). 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). kcat is 0.059 sec(-1) with UTP as substrate. kcat is 0.002 sec(-1) with ATP as substrate. kcat is 0.061 sec(-1) with U6 snRNA-u4 as substrate. Associates with the cleavage and polyadenylation specificity factor (CPSF) complex (PubMed:21102410). Interacts with CPSF1 and CPSF3; the interaction is direct (PubMed:21102410). Interacts with PIP5K1A (PubMed:18288197). Widely expressed. The zinc-finger domain is required for terminal uridylyltransferase activity (PubMed:28589955). Together with the RRM domain, binds the 5'-area of U6 snRNA (PubMed:28589955). The RRM domain is required for terminal uridylyltransferase activity (PubMed:28589955). Together with the zinc-finger domain, binds the 5'-area of U6 snRNA (PubMed:28589955). 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. Truncated N-terminus.