The Arabidopsis Nucleotidyl Transferase HESO1 Uridylates Unmethylated Small RNAs to Trigger Their Degradation

Summary 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.