The function of TUT1 (terminal uridylyl transferase 1, U6 snRNA-specific, Ensembl gene identifier ENSG00000149016) is as follows. 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. {ECO:0000269|PubMed:16790842, ECO:0000269|PubMed:18288197, ECO:0000269|PubMed:21102410, ECO:0000269|PubMed:28589955}.