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Distinct Populations of Primary and Secondary Effectors During RNAi in C. elegans
Julia Pak and
Andrew Fire*
RNA interference (RNAi) is a phylogenetically widespread gene-silencingprocess triggered by double-stranded RNA. In plants and Caenorhabditiselegans, two distinct populations of small RNAs have been proposedto participate in RNAi: "Primary siRNAs" (derived from DICERnuclease-mediated cleavage of the original trigger) and "secondarysiRNAs" [additional small RNAs whose synthesis requires an RNA-directedRNA polymerase (RdRP)]. Analyzing small RNAs associated withongoing RNAi in C. elegans, we found that secondary siRNAs constitutethe vast majority. The bulk of secondary siRNAs exhibited structureand sequence indicative of a biosynthetic mode whereby eachmolecule derives from an independent de novo initiation by RdRP.Analysis of endogenous small RNAs indicated that a fractionderive from a biosynthetic mechanism that is similar to thatof secondary siRNAs formed during RNAi, suggesting that smallantisense transcripts derived from cellular messenger RNAs byRdRP activity may have key roles in cellular regulation.
Departments of Pathology and Genetics, Stanford University School of Medicine, Stanford, CA 943055324, USA.
* To whom correspondence should be addressed. E-mail: afire{at}stanford.edu
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