Viral RNA pUGylation Promotes Antiviral Immunity in C. elegans.

RNA interference (RNAi) is a component of the innate immune systems of many eukaryotes, including C. elegans. During RNAi in C. elegans, the nucleotidyltransferase RDE-3 modifies the 3' termini of mRNAs with polyUG (pUG) tails, which recruit RNA-dependent RNA Polymerase (RdRP) enzymes that drive gene silencing by synthesizing antisense small interfering (si)RNAs. During normal growth and development, RDE-3 pUGylates transposon RNAs to silence transposons and protect genomic integrity. How C. elegans identifies specific RNAs for pUGylation and whether the pUGylation system is used for other biological purposes is not yet known. Here we show that pUGylation contributes to antiviral immunity in C. elegans: During infection of C. elegans with Orsay virus, RDE-3 adds pUG tails to viral RNAs, which converts these RNAs into templates for RdRP-based antiviral siRNA production, thereby limiting viral replication. We present evidence that MUT-15 is critical for viral pUGylation because it interacts with RDE-3 and the NYN domain-containing endonuclease RDE-8, thus bridging the enzymes that cleave and pUGylate viral RNA, ensuring efficient antiviral immunity. We conclude that pUGylation promotes antiviral immunity in C. elegans and we provide molecular insights into how C. elegans identifies and neutralizes its internal and external parasitic threats.