Acts as a specific terminal uridylyltransferase for U6 snRNA. Responsible for the addition of UTP at the 3' end of U6 snRNA which stabilizes U6 snRNA (PubMed:25753661). Does not have activity towards modified uridine containing 3'-monophosphorylation or 2'-O-methylation (PubMed:25753661). RNA(n) + UTP = diphosphate + RNA(n)-3'-uridine ribonucleotide Binds 1 divalent cation per subunit. Forms a complex composed of sart-3, terminal uridylyltransferase usip-1 and U6 snRNA; complex formation is mediated by usip-1 and sart-3 binding to U6 snRNA. Ubiquitously expressed. Belongs to the DNA polymerase type-B-like family.