Figure 1.

RNA-dependent RNA polymerases (RdRPs) synthesize RNA on RNA templates. During RNAi in C. elegans this process is used to amplify the initial RNAi response, and without it, no effect of RNAi is observed. The precise RNA molecules that serve as templates for RdRPs have not been identified, but it is assumed that they will at least contain the 5' end of the mRNA that is targeted by RNAi, and hence will have a 5'-cap structure. De novo RNA synthesis on this RNA template would result in a 5' end having a triphosphate group (indicated with three stars). PIR-1 may function in trimming such ends to monophosphate groups, allowing Dicer to cleave the dsRNA. The RNA species accumulating when PIR-1 is absent is indicated in green.