Each cell needs long double-stranded RNA for silencing by feeding RNAi in C. elegans

Long double-stranded RNA (dsRNA) can silence genes of matching sequence upon ingestion in many invertebrates and is therefore being developed as a pesticide. Such feeding RNA interference (RNAi) is best understood in the worm C. elegans, where it is thought that derivatives of ingested dsRNA, including short dsRNAs, move between cells and cause systemic silencing. Movement of short dsRNAs has been inferred using tissue-specific rescue of the long dsRNA-binding protein RDE-4 by expressing it from repetitive transgenes. We found that the use of repetitive transgenes for the tissue-specific rescue of a gene could inhibit RNAi within that tissue and could result in misexpression of the gene in other tissues. Both inhibition and misexpression were not detectable when a single-copy transgene was used for tissue-specific rescue. In animals with single-copy rescue of RDE-4, RNAi was restricted to the tissue with RDE-4 expression. Thus, unlike previous observations using repetitive transgenes, these results suggest that binding of long dsRNA by RDE-4 in each silenced cell is required for systemic RNAi. Taken together with the requirement for long dsRNA to trigger RNAi in insects, these results suggest that the entry of long dsRNA is a necessary first step for feeding RNAi in animal cells.