Functional anatomy of a dsRNA trigger: Differential requirement for the two trigger strands in RNA interference

In RNA-mediated interference (RNAi), externally provided mixtures of sense and antisense RNA trigger concerted degradation of homologous cellular RNAs . We show that RNAi requires duplex formation between the two trigger stran ds, that the duplex must include a region of identity between trigger and t arget RNAs, and that duplexes as short as 26 bp can trigger RNAi. Consisten t with in vitro observations, a fraction of input dsRNA is converted in viv o to short segments of similar to 25 nt. Interference assays with modified dsRNAs indicate precise chemical requirements for both bases and backbone o f the RNA trigger. Strikingly, certain modifications are well tolerated on the sense, but not the antisense, strand, indicating that the two trigger s trands have distinct roles in the interference process.