Gene-X-ppt(Gene10-基因十)--Chapter20

Chapter20EukaryoticTranscription20.1Introduction•ChromatinmustbeopenedbeforeRNApolymerasecanbindthepromoter.•basaltranscriptionfactors–TranscriptionfactorsrequiredbyRNApolymeraseIItoformtheinitiationcomplexatallRNApolymeraseIIpromoters.–FactorsareidentifiedasTFIIX,whereXisaletter.20.1Introduction•corepromoter–TheshortestsequenceatwhichanRNApolymerasecaninitiatetranscription(typicallyatamuchlowerlevelthanthatdisplayedbyapromotercontainingadditionalelements).–ForRNApolymeraseIIitistheminimalsequenceatwhichthebasaltranscriptionapparatuscanassemble,anditincludesthreesequenceelements:theInr,theTATAbox,andtheDPE.–Itistypically~40bplong.20.1Introduction•enhancer–Acis-actingsequencethatincreasestheutilizationof(most)eukaryoticpromoters,andcanfunctionineitherorientationandinanylocation(upstreamordownstream)relativetothepromoter.•silencer–AshortsequenceofDNAthatcaninactivateexpressionofageneinitsvicinity.FIGURE01:AtypicalgenetranscribedbyRNApolymeraseIIhasapromoterthatextendsupstreamfromthesitewheretranscriptionisinitiated.20.1Introduction•housekeepinggenes–Genesthatare(theoretically)expressedinallcellsbecausetheyprovidebasicfunctionsneededforsustenanceofallcelltypes.•coactivator–FactorsrequiredfortranscriptionthatdonotbindDNA,butarerequiredfor(DNA-binding)activatorstointeractwiththebasaltranscriptionfactors.20.2EukaryoticRNAPolymerasesConsistofManySubunits•RNApolymeraseIsynthesizesrRNAinthenucleolus.•RNApolymeraseIIsynthesizesmRNAinthenucleoplasm.•heterogeneousnuclearRNA(hnRNA)–RNAthatcomprisestranscriptsofnucleargenesmadeprimarilybyRNApolymeraseII;ithasawidesizedistributionandvariablestability.•RNApolymeraseIIIsynthesizessmallRNAsinthenucleoplasm.20.2EukaryoticRNAPolymerasesConsistofManySubunits•AlleukaryoticRNApolymeraseshave~12subunitsandarecomplexesof~500kD.•SomesubunitsarecommontoallthreeRNApolymerases.•ThelargestsubunitinRNApolymeraseIIhasaCTD(carboxy-terminaldomain)consistingofmultiplerepeatsofaheptamer.FIGURE02:RNApolymerasehas10subunits20.3RNAPolymeraseIHasaBipartitePromoter•nontranscribedspacer–Theregionbetweentranscriptionunitsinatandemgenecluster.•TheRNApolymeraseIpromoterconsistsofacorepromoterandanupstreampromoterelement(UPE).•ThefactorUBF1wrapsDNAaroundaproteinstructuretobringthecoreandUPEintoproximity.FIGURE03:PolIpromotershavetwosequencecomponents20.3RNAPolymeraseIHasaBipartitePromoter•SL1includesthefactorTATA-bindingprotein(TBP)thatisinvolvedininitiationbyallthreeRNApolymerases.•RNApolymeraseIbindstotheUBF1-SL1complexatthecorepromoter.20.4RNAPolymeraseIIIUsesBothDownstreamandUpstreamPromoters•RNApolymeraseIIIhastwotypesofpromoters.•Internalpromotershaveshortconsensussequenceslocatedwithinthetranscriptionunitandcauseinitiationtooccuratafixeddistanceupstream.•Upstreampromoterscontainthreeshortconsensussequencesupstreamofthestartpointthatareboundbytranscriptionfactors.FIGURE04:TherearethreetypesofpolIIIpromoters20.4RNAPolymeraseIIIUsesBothDownstreamandUpstreamPromoters•assemblyfactors–Proteinsthatarerequiredforformationofamacromolecularstructurebutarenotthemselvespartofthatstructure.•TFIIIAandTFIIICbindtotheconsensussequencesandenableTFIIIBtobindatthestartpoint.FIGURE05:Type2internalpromotersuseTFIIICFIGURE06:Type1polIIIpromotersuseTFIIIA/C20.4RNAPolymeraseIIIUsesBothDownstreamandUpstreamPromoters•TFIIIBhasTBPasonesubunitandenablesRNApolymerasetobind.•preinitiationcomplex–TheassemblyoftranscriptionfactorsatthepromoterbeforeRNApolymerasebindsineukaryotictranscription.20.5TheStartpointforRNAPolymeraseII•RNApolymeraseIIrequiresgeneraltranscriptionfactors(calledTFIIX)toinitiatetranscription.•RNApolymeraseIIpromotersfrequentlyhaveashortconservedsequencePy2CAPy5(theinitiatorInr)atthestartpoint.•TheTATAboxisacommoncomponentofRNApolymeraseIIpromotersandconsistsofanA-T-richoctamerlocated~25bpupstreamofthestartpoint.FIGURE07:AminimalpolIIpromoterhasonlytwoelements20.5TheStartpointforRNAPolymeraseII•Thedownstreampromoterelement(DPE)isacommoncomponentofRNApolymeraseIIpromotersthatdonotcontainaTATAbox(TATA-lesspromoters).•AcorepromoterforRNApolymeraseIIincludestheInrand,commonly,eitheraTATAboxoraDPE.–Itmayalsocontainotherminorelements.20.6TBPIsaUniversalFactor•TBPisacomponentofthepositioningfactorthatisrequiredforeachtypeofRNApolymerasetobinditspromoter.•ThefactorforRNApolymeraseIIisTFIID,whichconsistsofTBPand~14TAFs,withatotalmass~800kD.FIGURE08:Polymerasesbindviacommitmentfactors20.6TBPIsaUniversalFactor•TBPbindstotheTATAboxintheminorgrooveofDNA.•TBPformsasaddlearoundtheDNAandbendsitby~80°.FIGURE09:TBPbindstothenarrowgrooveofDNA20.7TheBasalApparatusAssemblesatthePromoter•Theupstreamelementsandthefactorsthatbindtothemincreasethefrequencyofinitiation.•BindingofTFIIDtotheTATAboxorInristhefirststepininitiation.FIGURE11:AninitiationcomplexassemblesatpromotersforRNApolymeraseIIAdaptedfromM.E.Maxon,J.A.Goodrich,andR.Tijan,GenesDev.8(1994):515-524.20.7TheBasalApparatusAssemblesatthePromoter•Othertranscriptionfactorsbindtothecomplexinadefinedorder,extendingthelengthoftheprotectedregiononDNA.•WhenRNApolymeraseIIbindstoth