On the role of RNA amplification in dsRNA-triggered gene silencing

We have investigated the role of trigger RNA amplification during RNA inter ference (RNAi) in Caenorhabditis elegans. Analysis of small interfering RNA s (siRNAs) produced during RNAi in C. elegans revealed a substantial fracti on that cannot derive directly from input dsRNA. Instead, a population of s iRNAs (termed secondary siRNAs) appeared to derive from the action of a cel lular RNA-directed RNA polymerase (RdRP) on mRNAs. that are being targeted by the RNAi mechanism. The distribution of secondary siRNAs exhibited a dis tinct polarity (5'-->3' on the antisense strand), suggesting a cyclic ampli fication process in which RdRP is primed by existing siRNAs. This amplifica tion mechanism substantially augments the potency of RNAi-based surveillanc e, while ensuring that the RNAi machinery will focus on expressed mRNAs.