SnapShot:SmallRNA-MediatedEpigeneticModificationsYanxiaBei,SigalPressman,andRichardCarthewDepartmentofBiochemistry,MolecularBiologyandCellBiology,NorthwesternUniversity,Evanston,IL60208,USA756Cell130,August24,2007©2007ElsevierInc.DOI10.1016/j.cell.2007.08.015Seeonlineversionforlegend,abbreviations,andreferences.(A)HeterochromatinFormationAlthoughheterochromatinformationoccursinthenucleus,itisnotclearinwhichcellularcompartmenttheeventsleadinguptosmallRNA-directedheterochromatinformationoccur.A1:HeterochromatinFormationinSchizosaccharomycespombe.DNArepeatsproducedouble-stranded(ds)RNAsthroughbidirectionaltranscriptionorRNA-dependentRNAsynthesis.dsRNAsarecutintosmall-interfering(si)RNAsthatareloadedintoanRNA-inducedtranscriptionalsilencingcomplex(RITS)thatconsistsofAgo,Tas3,anS.pombespecificprotein,andChp1,achromodomaincontainingprotein.RITSfindstheDNArepeatsthroughsiRNAbasepairingwiththenascenttranscriptandrecruitstheRNA-directedRNApolymerasecomplex(RDRC)andClr4,ahistonemethyltransferasethatmethylateshistoneH3atlysine9(H3K9me).RdRPinRDRCusestheAgo-cutnascentRNAastemplatetosynthesizemoredsRNA,whichinturnwillbecutintosiRNAstoreinforceheterochromatinformation.Chp1intheRITScomplexbindstoH3K9me,resultinginstableinteractionofRITSandheterochromaticDNA.H3K9mealsobindstoanotherchromodomainprotein,Swi6,anHP1homolog,leadingtothespreadingofheterochromatin.A2:HeterochromatinFormationinDrosophila.RepeatassociatedsmallinterferingRNAs(rasiRNAs)areproducedinaDicerindependent,Aub/Piwi-Ago3“ping-pong”mechanism.Aub/PiwiassociateswithantisenserasiRNAswithapreferenceforaUat5′end,whereasAgoassociateswithsense-strandderivedrasiRNAwithapreferencetoanAatnucleotide10.Aub/Piwi-rasiRNAcomplexbindstosense-strandRNAviaa10nucleotide(nt)complementarysequence.Aub/Piwicleavessense-strandRNA,producingsenserasiRNAprecursor.Ayet-to-be-identifiednuclease(?)generatesthesenserasiRNAsthatassociatewithAgo3.Inturn,Ago3-sensesiRNAbindstoantisenseRNAandgeneratesmoreantisenserasiRNAs.Inthisping-pongmodel,th...
