RNAi: Double-stranded RNA directs the ATP-dependent cleavage of mRNA at 21to 23 nucleotide intervals

Double-stranded RNA (dsRNA) directs the sequence-specific degradation of mR NA through a process known as RNA interference (RNAi). Using a recently dev eloped Drosophila in vitro system, we examined the molecular mechanism unde rlying RNAi. We find that RNAi is ATP dependent yet uncoupled from mRNA tra nslation. During the RNAi reaction, both strands of the dsRNA are processed to RNA segments 21-23 nucleotides in length. Processing of the dsRNA to th e small RNA fragments does not require the targeted mRNA. The mRNA is cleav ed only within the region of identity with the dsRNA. Cleavage occurs at si tes 21-23 nucleotides apart, the same interval observed for the dsRNA itsel f, suggesting that the 21-23 nucleotide fragments from the dsRNA are guidin g mRNA cleavage.