We are interested in understanding the mechanism by which RNA interference (RNAi) silences gene expression. Studies in a number of different systems have led to the following model: Introduction of double-stranded RNA initiates the RNAi response by being cleaved into small interfering RNAs (siRNAs) by the RNase III-like enzyme Dicer (DCR-1), aided by the product of the
rde-4 gene. These important mediators of RNAi then direct various factors, including RDE-1 and, potentially, parts of a 'RISC' complex, to target mRNAs which are consequently degraded. A major point of divergence between the RNAi mechanisms of certain species concerns the mode of signal amplification; worms, plants, and fungi achieve amplification of their RNAi response, in part, through the actions of the RNA-dependent RNA polymerases (RdRPs) whereas humans and flies apparently lack such enzymes. The C. elegans genome encodes four RdRP-like factors: RRF-1, RRF-2, RRF-3, and EGO-1. Of these, RRF-1 appears to propagate RNAi in the soma whereas EGO-1 does so in the germ line. Eleanor Maine and colleagues have also shown that
ego-1 activity is needed for fertility in C. elegans . We have initiated a study of the role of RdRPs and their products in RNAi and in germ line function. Our initial work on this has combined structural approaches (experiments designed to determine structures of RdRP templates and products) with functional approaches (working to understand the role of RdRP catalysis in RNAi). (1) Sijen, T., Fleenor, J., Simmer, F., Thijssen, K.L., Parrish, S., Timmons, L., Plasterk, R.H.A., and Fire, A. 2001. On the role of RNA amplification in dsRNA-triggered gene silencing. Cell 107:465-476 (2) Smardon, A., Spoerke, J.M., Stacey, S.C., Klein, M.E., Mackin, N., and Maine, E.M. 2000. EGO-1 is related to RNA-directed RNA polymerase and functions in germ-line development and RNA interference in C. elegans . Curr Biol 10:169-178