Identification of an RNA-dependent RNA polymerase in Drosophila establishes a common theme in RNA silencing.

In lower eukaryotes, such as A. thaliana, C. elegans, S. pombe and N. crassa, RNA-dependent RNA polymerase (RdRP) is a required component of the RNA silencing pathway. Remarkably, even though robust RNA silencing occurs in Drosophila in response to exogenous dsRNA and siRNAs, no RdRP homolog has been identified in the Drosophila genome or in any other higher eukaryote characteristic of the known cellular RdRPs. We showed recently that the largest subunit of the Drosophila RNA polymerase II core elongator complex, called D-elp1, has RdRP activity capable of using unprimed or primed synthesis mechanisms to convert single stranded RNA templates into double stranded RNA (dsRNA) that can be cleaved by Dcr-2. Loss of D-elp1 inhibits both siRNA and dsRNA directed RNAi in S2 cells but does not affect micro RNA targeting. Transposon RNA levels also increase with the loss of D-elp1 while the corresponding endo siRNAs, critical for transposon suppression, are dramatically reduced and this is correlated with a reduction in transposon antisense RNA levels. D-elp1 associates tightly with Dicer-2, similar to the Dicer-RdRP interaction observed in lower eukaryotes. With the exception of S. cerevisiae, which lacks the RNAi machinery altogether, RdRP activity is conserved in the elp1 homologs from S. pombe to human. This commentary focuses on the importance and universality of RdRP in RNA silencing.