Uridylates small RNAs to trigger their degradation (PubMed:22464191, PubMed:22464194, PubMed:25928341). Catalyzes the uridylation of 5' fragments produced by AGO1-mediated cleavage of miRNA target RNAs (PubMed:24733911). Acts synergistically with URT1 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). Able to act on AGO1-bound miRNAs and the uridylated species stay associated with AGO1 (PubMed:24733911, PubMed:25928405). RNA(n) + UTP = diphosphate + RNA(n)-3'-uridine ribonucleotide Completely inhibited by 2'-O-methylation on the substrate RNA. No effect on miRNA accumulation in the wild-type background, but increased abundance of the heterogeneous miRNA species in a hen1 background. Reduced 3' uridylation of miRNAs without affecting the 3' truncation. Belongs to the DNA polymerase type-B-like family.