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 are 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.IMPORTANCEViruses are a threat to all organisms. Therefore, organisms have evolved numerous systems to recognize and neutralize viruses. Many of these systems, which are referred to as innate immune systems, function by recognizing unique molecular characteristics of the viral genetic material. One such innate immune system is RNA interference (RNAi). RNA interference uses double-stranded RNA, which is an obligatory byproduct of replication for many viruses, as a weapon to fight viruses. In this work, we provide molecular insights into how the nematode C. elegans uses RNA interference and viral double-stranded RNAs to defend itself against viral invaders.