UTP:RNA uridylyltransferase with a marked preference for uridine polymerization and a distributive activity for the first added nucleotides (PubMed:23748567, PubMed:25928405). Uridylates oligo(A)-tailed mRNAs to prevent 3' to 5' ribonucleotytic attacks (PubMed:23748567). Reduces the accumulation of oligo(A)-tailed mRNAs (PubMed:33637717). Prevents the accumulation of excessively deadenylated mRNAs to avoid siRNA biogenesis (PubMed:26972004, PubMed:33637717). Uridylation repairs deadenylated extremities to restore the size distribution observed for non-uridylated oligo(A) tails (PubMed:26972004). Can prevent the 3' trimming of mRNAs still engaged on polysomes (PubMed:23748567). Acts synergistically with HESO1 in unmethylated miRNA uridylation, leading to their degradation (PubMed:25928341). URT1 and HESO1 prefer substrates with different 3' end nucleotides and act cooperatively to tail different forms of the same miRNAs (PubMed:25928405). URT1 and HESO1 act sequentially, with URT1 mono-uridylating the miRNAs followed by their further uridylation by HESO1 (PubMed:25928405). URT1 and HESO1 are involved in the uridylation and clearance of RISC-generated 5' mRNA fragments (PubMed:30364210). Has no effect on uridylation of heterochromatic siRNAs (PubMed:25928341). Able to act on AGO1-bound miRNAs and the uridylated species stay associated with AGO1 (PubMed:25928405). Acts as post-transcriptional gene silencing (PTGS) suppressor (PubMed:31076735). RNA(n) + UTP = diphosphate + RNA(n)-3'-uridine ribonucleotide Completely inhibited by 2'-O-methylation on the substrate RNA. Interacts with AGO1. The P618L single amino acid substitutiontarget URT1 to the nucleoplasm. Belongs to the DNA polymerase type-B-like family.