The Arabidopsis nucleotidyl transferase HESO1 uridylates unmethylated small RNAs to trigger their degradation.

MicroRNAs (miRNAs), small interfering RNAs (siRNAs), and piwi-interacting RNAs (piRNAs) impact numerous biological processes in eukaryotes. In addition to biogenesis, turnover contributes to the steady-state levels of small RNAs. One major factor that stabilizes miRNAs and siRNAs in plants as well as siRNAs and piRNAs in animals is 2'-O-methylation on the 3' terminal ribose by the methyltransferase HUA ENHANCER1 (HEN1) [1-6]. Genetic studies with Arabidopsis, Drosophila, and zebrafish hen1 mutants show that 2'-O-methylation protects small RNAs from 3'-to-5' truncation and 3' uridylation, the addition of nontemplated nucleotides, predominantly uridine [2, 7, 8]. Uridylation is a widespread phenomenon that is not restricted to small RNAs in hen1 mutants and is often associated with their reduced accumulation ([7, 9, 10]; reviewed in [11]). The enzymes responsible for 3' uridylation of small RNAs when they lack methylation in plants or animals have remained elusive. Here, we identify the Arabidopsis HEN1 SUPPRESSOR1 (HESO1) gene as responsible for small RNA uridylation in hen1 mutants. HESO1 exhibits terminal nucleotidyl transferase activity, prefers uridine as the substrate nucleotide, and is completely inhibited by 2'-O-methylation. We show that uridylation leads to miRNA degradation, and the degradation is most likely through an enzyme that is distinct from that causing the 3' truncation in hen1 mutants.