Figure 7.

Proposed model for RNAi. RNAi is predicted to begin with processing of dsRNA (sense strand in black, antisense strand in red) to predominantly 21- and 22-nt short interfering RNAs (siRNAs). Short overhanging 3′ nucleotides, if present on the dsRNA, may be beneficial for processing of short dsRNAs. The dsRNA-processing proteins, which remain to be characterized, are represented as green and blue ovals and assemble on the dsRNA in asymmetric fashion. In our model, this is illustrated by binding of a hypothetical blue protein or protein domain with the siRNA strand in 3′ to 5′ direction while the hypothetical green protein or protein domain is always bound to the opposing siRNA strand. These proteins or a subset remain associated with the siRNA duplex and preserve its orientation, as determined by the direction of the dsRNA processing reaction. Only the siRNA sequence associated with the blue protein is able to guide target RNA cleavage. The endonuclease complex is referred to as small interfering ribonucleoprotein complex or siRNP. It is presumed here that the endonuclease that cleaves the dsRNA may also cleave the target RNA, probably by temporarily displacing the passive siRNA strand not used for target recognition. The target RNA is then cleaved in the center of the region recognized by the sequence-complementary guide siRNA. Because the cleavage site is displaced by 10–12 nt relative to the dsRNA processing site, a conformational rearrangement or a change in the composition of an siRNP must occur before target RNA cleavage.