Double-stranded RNA (dsRNA) induces sequence-specific post-transcriptionalgene silencing in many organisms by a process known as RNA interference(RNAi). Using a Drosophila in vitro system, we demonstrate that 19-23 nt shortRNA fragments are the sequence-specific mediators of RNAi. The shortinterfering RNAs (siRNAs) are generated by an RNase III-like processingreaction from long dsRNA. Chemically synthesized siRNA duplexes withoverhanging 3' ends mediate efficient target RNA cleavage in the lysate, andthe cleavage site is located near the center of the region spanned by theguiding siRNA. Furthermore, we provide evidence that the direction of dsRNAprocessing determines whether sense or antisense target RNA can be cleaved bythe produced siRNP complex.
