RNAi as random degradative PCR: siRNA primers convert mRNA into dsRNAs that are degraded to generate new siRNAs

In posttranscriptional gene silencing (PTGS), "quelling," and RNA interfere nce (RNAi), 21-25 nucleotide RNA fragments are produced from the initiating dsRNA. These short interfering RNAs (siRNAs) mediate RNAi by an unknown me chanism. Here, we show that GFP and Pp-Luc siRNAs, isolated from a protein complex in Drosophila embryo extract, target mRNA degradation in vitro. Mos t importantly, these siRNAs, as well as a synthetic 21-nucleotide duplex GF P siRNA, serve as primers to transform the target mRNA into dsRNA. The nasc ent dsRNA is degraded to eliminate the incorporated target mRNA while gener ating new siRNAs in a cycle of dsRNA synthesis and degradation. Evidence is presented that mRNA-dependent siRNA incorporation to form dsRNA is carried out by an RNA-dependent RNA polymerase activity (RdRP).