TheMechanismofRNAInterference(RNAi) Inmammaliancells,introductionoflongdsRNA(>30nt)initiatesapotentantiviralresponse,exemplifiedbynonspecificinhibitionofproteinsynthesisandRNAdegradation.Themammalianantiviralresponsecanbebypassed,however,bytheintroductionorexpressionofsiRNAs.
Longdouble-strandedRNAs(dsRNAs;typically>200nt)canbeusedtosilencetheexpressionoftargetgenesinavarietyoforganismsandcelltypes(e.g.,worms,fruitflies,andplants).Uponintroduction,thelongdsRNAsenteracellularpathwaythatiscommonlyreferredtoastheRNAinterference(RNAi)pathway.First,thedsRNAsgetprocessedinto20-25nucleotide(nt)smallinterferingRNAs(siRNAs)byanRNaseIII-likeenzymecalledDicer(initiationstep).Then,thesiRNAsassembleintoendoribonuclease-containingcomplexesknownasRNA-inducedsilencingcomplexes(RISCs),unwindingintheprocess.ThesiRNAstrandssubsequentlyguidetheRISCstocomplementaryRNAmolecules,wheretheycleaveanddestroythecognateRNA(effecterstep).CleavageofcognateRNAtakesplacenearthemiddleoftheregionboundbythesiRNAstrand.
