RNA interference is mediated by 21-and 22-nucleotide RNAs

Double-stranded RNA (dsRNA) induces sequence-specific posttranscriptional g ene silencing in many organisms by a process known as RNA interference (RNA i). Using a Drosophila in vitro system, we demonstrate that 21- and 22-nt R NA fragments are the sequence-specific mediators of RNAi. The short interfe ring RNAs (siRNAs) are generated by an RNase III-like processing reaction f rom long dsRNA. Chemically synthesized siRNA duplexes with overhanging 3' e nds mediate efficient target RNA cleavage in the lysate, and the cleavage s ite is located near the center of the region spanned by the guiding siRNA. Furthermore, we provide evidence that the direction of dsRNA processing det ermines whether sense or antisense target RNA can be cleaved by the siRNA-p rotein complex.