Posttranscriptional Gene Silencing.pdf

SnapShot:Posttranscriptional Gene SilencingSigal Pressman,Yanxia Bei,and Richard CarthewDepartment of Biochemistry,Molecular Biology and Cell Biology,Northwestern University,Evanston,IL 60208,USA570 Cell130,August10,20072007ElsevierInc.DOI10.1016/j.cell.2007.07.042Seeonlineversionforlegend,abbreviations,andreferences.(A)siRNA-Mediated Posttranscriptional Gene SilencingDouble-strand(ds)RNAsthataretakenupexogenously,formedduringviralreplication,orgeneratedfromaberrantcellularRNAsarecutinto2123nucleotide(nt)siRNAsbyDicerinassociationwithdsRNA-binding(dsRBD)protein.ThesiRNAsarethenloadedintoRISC,acomplexcontainingArgonaute(Ago)protein.Afterelimi-nationofthesiRNApassengerstrand,RISCpairswithcognateRNAviathesiRNAguidestrandandcutsitstargetusingthesliceractivityofAgo.InmammalsandDrosophilathecleavedtargetisthendegraded.Inplants,C.elegans,andfungiwhosegenomesencodeRNA-dependentRNApolymerase(s)(RdRPorRDR)thecleavedtargetRNAservesastemplateforRdRPtogeneratemoredsRNAsthatwillbedicedintosecondarysiRNAs.ThesesecondarysiRNAs,whichinC.elegansarerecognizedbySAGO(syntheticsecondary-siRNAdefectiveAgo),furtherdownregulatetargetRNAs.(B)miRNA-Mediated Posttranscriptional Gene SilencingB1:Animal miRNA biogenesis.RNApolymeraseIItranscribesmicroRNAgenesasprimarymiRNAs(pri-miRNAs)thatfoldbacktoformastem-loopstructure.Animalpri-miRNAsarecleavedinthenucleusbytheRNaseIIIenzymeDroshaanditsdsRBDproteinpartnerinto5080ntprecursormiRNAs(pre-miRNAs).Pre-miRNAsarethenactivelyexportedbyExportin5tothecytoplasm,wheretheyundergoasecondcleavagebytheDiceranditsdsRBDproteinpartner.Theresulting21ntduplex,containingthematuremiRNAstrandandthepassengermiRNA*strand,isunwoundandthematuremiRNAstrandisincorporatedintoamicroribonucleoprotein(miRNP)complex,containinganAgoprotein.Recently,asecondpathwayforgeneratingmiRNAswasidentifiedinC.elegansandDrosophila,whereshortintronsaresplicedandde-branched,yieldingpre-miRNA-likehairpinstermedmirtrons.BypassingDroshacleavage,themirtronpathwayisthoughttomergewiththecanonicalmiRNApathwayduringhairpinexport.B2:Plant miRNA biogenesis.PlantmiRNAgenesaretranscribedbyRNApolymeraseII.IncontrasttoanimalmiRNAprocessing,thetwocleavagestepsthatgeneratethemiRNA-miRNA*duplex,occurinthenucleusbythesameenzyme,DCL1,togetherwiththedsRBDproteinHYL1.Next,HEN1methylatesthe3endsoftheduplex,anditistransportedoutsideofthenucleus,presumablybytheplantExportin5ortholog,Hasty.ThemiRNAduplexisloadedontoAgo1proteinwithinamiRNPcomplex,andafterunwinding,thematuremiRNAguidesmRNAcleavageinmostcasesorrepressestranslationinafewcases.ItisstillnotclearwhetherAgo1loadingoccursonlyinthecytoplasmorwithinthenucleusaswell.B3:miRNA-mediated posttranscriptional gene regulation.AfewmechanismsformiRNAposttranscriptionalgeneregulationhavebeensuggestedalthoughmanydetailsarestillmissing.Partialbase-pairingbetweenanmiRNAandthe3UTRofanmRNAtargetleadstotranslationrepressionofthemRNAattheinitiationstep,atapost-initiationstep,orbydecayofthemRNAfollowingmiRNA-mediateddeadenylation.Prefectbase-pairingbetweenanmiRNAandanmRNAcanresultinendo-nucleolyticcleavageanddegradationofthecleavageproducts.Anothermechanismthathasbeensuggested,buthasnotbeenprovenexperimentally,isproteolysisofnascentpolypeptidechains.(C)trans-Acting-siRNAs in PlantsInplants,somemiRNAsguidethecleavageofRNAtranscriptsderivedfromatrans-actingsiRNA(ta-siRNA)gene.Thecleavageproductsareprotectedfromdeg-radationbySGS3,andRdR6convertseitherthe5orthe3fragmentintodsRNA.DCL4(andpossiblyDCL1)cleavesthedsRNAinto21ntsiRNAsthatfunctionasta-siRNAs,guidingthecleavageofatargetmRNAdifferentfromthetranscriptfromwhichtheywereproduced.AbbreviationsAgo,Argonaute;DCL,Dicerlike;DCR,Dicer;dsRBD,double-strandedRNA-bindingdomain-containingprotein;HEN1,huaenhancer1;HST,Hasty;HYL1,HYPONAS-TICLEAVES1;miRNA,microRNA;miRNP,microribonucleoprotein;PolII,RNApolymeraseII;Pol,RNApolymerase;pre-miRNA,precursormiRNA;pri-miRNA,primarymiRNA;RDI,R2D2-DCR2initiatorcomplex;RdRP/RDR,RNA-dependentRNApolymerase;RISC,RNA-inducedsilencingcomplex;SAGO,syntheticsecondary-siRNAdefectiveAGO;siRNA,smallinterferingRNA;ta-siRNAs,trans-actingsiRNAs.RefeReNCeSBonnet,E.,VandePeer,Y.,andRouze,P.(2006).ThesmallRNAworldofplants.NewPhytol.171,451468.Carthew,R.W.(2006).GeneregulationbymicroRNAs.Curr.Opin.Genet.Dev.16,203208.Engels,B.M.,andHutvagner,G.(2006).PrinciplesandeffectsofmicroRNA-mediatedpost-transcriptionalgeneregulation.Oncogene25,61636169.Hutvagner,G.(2005).SmallRNAasymmetryinRNAi:functioninRISCassemblyandgeneregulation.FEBSLett.579,58505857.Jackson,R.J.,andStandart,N.(2007).HowdomicroRNAsregulategeneexpression?Sci.STKE2007,re1.Kim,K.,Lee,Y.S.,andCarthew,R.W.(2007).Conversionofpre-RISCtoholo-RISCbyAgo2duringassemblyofRNAicomplexes.RNA13,2229.Okamura,K.,Hagen,J.W.,Duan,H.,Tyler,D.M.,andLai,E.C.TheMirtronpathwaygeneratesmicroRNA-classregulatoryRNAsinDrosophila.Cell130,89100.Pillai,R.S.,Bhattacharyya,S.N.,andFilipowicz,W.(2007).RepressionofproteinsynthesisbymiRNAs:howmanymechanisms?TrendsCellBiol.17,118126.Ruby,J.G.,Jan,C.H.,andBartel,D.P.(2007).IntronicmicroRNAprecursorsthatbypassDroshaprocessing.Nature448,8386.Ulvila,J.,Parikka,M.,Kleino,A.,Sormunen,R.,Ezekowitz,R.A.,Kocks,C.,andRamet,M.(2006).Double-strandedRNAisinternalizedbyscavengerreceptor-medi-atedendocytosisinDrosophilaS2cells.J.Biol.Chem.281,1437014375.Yigit,E.,Batista,P.J.,Bei,Y.,Pang,K.M.,Chen,C.C.,Tolia,N.H.,Joshua-Tor,L.,Mitani,S.,Simard,M.J.,andMello,C.C.(2006).AnalysisoftheC.elegansArgonautefamilyrevealsthatdistinctArgonautesactsequentiallyduringRNAi.Cell127,747757.Zeng,Y.(2006).Principlesofmicro-RNAproductionandmaturation.Oncogene25,61566162.SnapShot:Posttranscriptional Gene Silencing Sigal Pressman,Yanxia Bei,and Richard CarthewDepartment of Biochemistry,Molecular Biology and Cell Biology,Northwestern University,Evanston,IL 60208,USA570.e1 Cell130,August10,20072007ElsevierInc.DOI10.1016/j.cell.2007.07.042