分子生物學(xué)英文版教學(xué)課件：Ch 17 Transcriptional regulation in eukaryotes

WelcomeEachofYoutoMyMolecularBiologyClass2MolecularBiologyoftheGene,5/E

Watsonetal.(2004)PartI:ChemistryandGeneticsPartII:MaintenanceoftheGenome

PartIII:ExpressionoftheGenomePartIV:RegulationPartV:Methods2005-5-103PartIVRegulationCh16:TranscriptionalregulationinprokaryotesCh17:TranscriptionalregulationineukaryotesCh18:RegulatoryRNAsCh19:GeneregulationindevelopmentandevolutionCh20:GenomeAnalysisandSystemsBiology4SurfingthecontentsofPartIV--Theheartofthefrontierbiologicaldisciplines

5Chapter17GeneRegulationinEukaryotesMolecularBiologyCourse6TOPIC1ConservedMechanismsofTranscriptionalRegulationfromYeasttoHuman.TOPIC2RecruitmentofProteinComplexestoGenesbyEukaryoticActivators.TOPIC3TranscriptionalRepressorsTOPIC4SignalIntegrationandCombinatorialControl.TOPIC5SignalTransductionandtheControlofTranscriptionalRegulators.TOPIC6GeneSilencingbyModificationofHistonesandDNA.TOPIC7EpigeneticGeneRegulation.

71.GeneExpressionisControlledbyRegulatoryProteins(調(diào)控蛋白)GeneexpressionisveryoftencontrolledbyExtracellularSignals,

whicharecommunicatedtogenesbyregulatoryproteins:PositiveregulatorsoractivatorsINCREASEthetranscriptionNegativeregulatorsorrepressors

DECREASEorELIMINATEthetranscriptionPrinciplesofTranscriptionRegulation8

Similarityofregulationbetweeneukaryotesandprokaryote1.Principlesarethesame:signals(信號(hào)),activatorsandrepressors(激活蛋白和阻遏蛋白)recruitmentandallostery,cooperativebinding(招募，異構(gòu)和協(xié)同結(jié)合)2.Thegeneexpressionstepssubjectedtoregulationaresimilar,andtheinitiationoftranscriptionisthemostpervasivelyregulatedstep.

9DifferenceinregulationbetweeneukaryotesandprokaryotePre-mRNAsplicingaddsanimportantstepforregulation.(mRNA前體的剪接)Theeukaryotictranscriptionalmachineryismoreelaboratethanitsbacterialcounterpart.(真核轉(zhuǎn)錄機(jī)器更復(fù)雜)Nucleosomesandtheirmodifiersinfluenceaccesstogenes.(核小體及其修飾體)Manyeukaryoticgeneshavemoreregulatorybindingsitesandarecontrolledbymoreregulatoryproteinsthanarebacterialgenes.(真核基因有更多結(jié)合位點(diǎn))10AlotmoreregulatorbindingssitesinmulticellularorganismsreflectsthemoreextensivesignalintegrationFig.17-1BacteriaYeastHuman11Enhancer(激活元件):agivensitebindsregulatorresponsibleforactivatingthegene.Alternativeenhancerbindsdifferentgroupsofregulatorsandcontrolexpressionofthesamegeneatdifferenttimesandplacesinresponsibletodifferentsignals.Activationatadistanceismuchmorecommonineukaryotes.

Insulators(絕緣子)orboundaryelements(臨界元件)areregulatorysequencesbetweenenhancersandpromoters.Theyblockactivationofalinkedpromoterbyactivatorboundattheenhancer,andthereforeensureactivatorsworkdiscriminately.12Topic1:ConservedMechanismsofTranscriptionalRegulationfromYeast(酵母)toMammals(哺乳動(dòng)物)CHAPTER17GeneRegulationineukaryotes一、真核的轉(zhuǎn)錄激活蛋白的結(jié)構(gòu)特征Thestructurefeaturesoftheeukaryotictranscriptionactivators13Thebasicfeaturesofgeneregulationarethesameinalleukaryotes,becauseofthesimilarityintheirtranscriptionandnucleosomestructure.Yeastisthemostamenabletobothgeneticandbiochemicaldissection,andproducesmuchofknowledgeoftheactionoftheeukaryoticrepressorandactivator.Thetypicaleukaryoticactivatorsworksinamannersimilartothesimplestbacterialcase.Repressorsworkinavarietyofways.141.Eukaryoticactivators(真核激活蛋白)haveseparateDNAbindingandactivatingfunctions【與原核相似】,whichareveryoftenonseparatedomainsoftheprotein.Fig.17-2Gal4boundtoitssiteonDNA15Fig.17-3TheregulatorysequencesoftheYeastGAL1gene.EukaryoticactivatorsExample1:Gal4Gal4isthemoststudiedeukaryoticactivatorGal4activatestranscriptionofthegalactosegenesintheyeastS.cerevisae.Gal4bindstofoursites(UASG)upstreamofGAL1,andactivatestranscription1,000-foldinthepresenceofgalactose16EukaryoticactivatorsExample1:Gal4ExperimentalevidencesshowingthatGal4containsseparateDNAbindingandactivatingdomains.ExpressionoftheN-terminalregion(DNA-bindingdomain)oftheactivatorproducesaproteinboundtotheDNAnormallybutdidnotactivatetranscription.FusionoftheC-terminalregion(activationdomain)oftheactivatortotheDNAbindingdomainofabacterialrepressor,LexAactivatesthetranscriptionofthereportergene.Domainswapexperiment

實(shí)驗(yàn)介紹系列1－Experimentintroductionseries17

Domainswapexperiment

Movingdomainsamongproteins,provingthatdomainscanbedissectedintoseparatepartsoftheproteins.ManysimilarexperimentsshowsthatDNAbindingdomainsandactivatingregionsareseparable.18Box1

ThetwohybridAssay(酵母雙雜交)isusedtoidentifyproteinsinteractingwitheachother.（實(shí)驗(yàn)介紹系列2）FuseproteinAandproteinBgenestotheDNAbindingdomainandactivatingregionofGal4,respectively.Producefusionproteins192.EukaryoticregulatorsusearangeofDNAbindingdomains,butDNArecognitioninvolvesthesameprinciplesasfoundinbacteria.

HomeodomainproteinsZinccontainingDNA-bindingdomain:zincfingerandzincclusterLeucinezippermotifHelix-Loop-Helixproteins:basiczipperandHLHproteins20BacterialregulatoryproteinsMostusethehelix-turn-helixmotiftobindDNAtargetMostbindasdimerstoDNAsequence:eachmonomerinsertsanahelixintothemajorgroove.EukaryoticregulatoryproteinsRecognizetheDNAusingthesimilarprinciples,withsomevariationsindetail.Inadditiontoformhomodimers(同源二聚體),someformheterodimers(異源二聚體)torecognizeDNA,extendingtherangeofDNA-bindingspecificity.21Homeodomainproteins:Thehomeodomain(同源結(jié)構(gòu)域)isaclassofhelix-turn-helixDNA-bindingdomainandrecognizesDNAinessentiallythesamewayasthosebacterialproteins.Figure17-5Whatisthesame?22ZinccontainingDNA-bindingdomains(鋅指結(jié)構(gòu)域):

Zincfingerproteins(TFIIIA)andZincclusterdomain(Gal4)Figure17-623LeucineZipperMotif(亮氨酸拉鏈基序):TheMotifcombinesdimerizationandDNA-bindingsurfaceswithinasinglestructuralunit.Figure17-724Dimerization(二聚化)

ismediatedbyhydrophobicinteractionsbetweentheappropriately-spacedleucine(亮氨酸)toformacoiledcoilstructure2526

Helix-Loop-Helixmotif:similarasinleucinezippermotif.

Figure17-827myogenicfactor:生肌調(diào)節(jié)蛋白是一種轉(zhuǎn)錄因子。28Becausetheregionofthea-helixthatbindsDNAcontainsbaiscaminoacidsresidues,LeucinezipperandHLHproteinsareoftencalledbasiczipperandbasicHLHproteins.Bothoftheseproteinsusehydrophobicaminoacidresiduesfordimerization.293.Activatingregions

(激活區(qū)域)

arenotwell-definedstructuresTheactivatingregionsaregroupedonthebasisofaminoacidscontent.Acidicactivationregion(酸性激活區(qū)域):containbothcriticalacidicaminoacidsandhydrophobicacids.yeastGal4Glutamine-rich

region(谷氨酰胺富集區(qū)):mammalianactivatorSP1Proline-richregion(脯氨酸富集區(qū)):mammalianactivatorCTF130Topic2:RecruitmentofProteinComplexestoGenesbyEukaryoticActivatorsCHAPTER17GeneRegulationineukaryotes二、真核轉(zhuǎn)錄激活蛋白的招募調(diào)控方式和遠(yuǎn)距調(diào)控特征Activationoftheeukaryotictranscriptionbyrecruitment&Activationatadistance31Eukaryoticactivators(真核激活蛋白)alsoworkbyrecruiting(招募)asinbacteria,butrecruitpolymeraseindirectlyintwoways:1.Interactingwithpartsofthe

transcriptionmachinery.2.Recruitingnucleosomemodifiersthatalterchromatininthevicinityofagene.321.Activatorsrecruitthetranscriptionmachinerytothegene.33Theeukaryotictranscriptionalmachinerycontainspolymeraseandnumerousproteinsbeingorganizedtoseveralcomplexes,suchastheMediatorandtheTFⅡDcomplex.Activatorsinteractwithoneormoreofthesecomplexesandrecruitthemtothegene.Figure17-934Box2ChromatinImmuno-precipitation(ChIP)(染色質(zhì)免疫共沉淀)

tovisualizewhereagivenprotein(activator)isboundinthegenomeofalivingcell.)（實(shí)驗(yàn)介紹系列3）35

ActivatorBypassExperiment(越過激活子實(shí)驗(yàn))-ActivationoftranscriptionthroughdirecttetheringofmediatortoDNA.（實(shí)驗(yàn)介紹系列4）DirectlyfusethebacterialDNA-bindingproteinLexAproteintoGal11,acomponentofthemediatorcomplextoactivateGAL1expression.Figure17-1036Atmostgenes,thetranscriptionmachineryisnotprebound,andappearatthepromoteronlyuponactivation.Thus,noallostericactivationofthepreboundpolymerasehasbeenevidentineukaryoticregulation。372.ActivatorsalsorecruitmodifiersthathelpthetranscriptionmachinerybindatthepromoterTwotypesofNucleosomemodifiers:Thoseaddchemicalgroupstothetailsofhistones(在組蛋白尾上加化學(xué)基團(tuán)),suchashistoneacetyltransferases(HATs)Thoseremodelthenucleosomes(重塑核小體),suchastheATP-dependentactivityofSWI/SNF.38Howthenucleosomemodificationhelpactivateagene?“Loosen”thechromatinstructurebychromosomeremodeling(Fig.17-11b)andhistonemodificationsuchasacetylation(Fig.17-11a),whichuncoverDNA-bindingsitesthatwouldotherwiseremaininaccessiblewithinthenucleosome.39Fig17-11Localalterationsinchromatindirectedbyactivators（組蛋白乙?；福?/p>

uncoverDNA-bindingsites402.Addingacetylgroupstohistoneshelpsthebindingofthetranscriptionalmachinery.OnecomponentofTFIIDcomplexbearsbromodomainsthatspecificallybindtotheacetylgroups.Therefore,agenebearingacetylatednucleosomesatitspromoterhaveahigheraffinityforthetranscriptionalmachinerythantheonewithunacetylatednucleosomes.41Figure7-39Effectofhistonetailmodification溴區(qū)結(jié)構(gòu)域蛋白OnecomponentofTFIIDcomplexbearsbromodomains.42

Manyenkaryoticactivators－particularlyinhigher

eukaryotes－workfromadistance.

How?Someproteinshelp,forexampleChipproteininDrosophila.Thecompactedchromosomestructurehelp.DNAiswrappedinnucleosomesineukaryotes.Sositesseparatedbymanybasepairsmaynotbeasfarapartinthecellasthought.3.Actionatadistance:loopsandinsulators43Specificcis-actingelementscalledinsulators(絕緣子)controltheactionsofactivators,preventingtheactivatingthenon-specificgenes44InsulatorsblockactivationbyenhancersFigure17-1245

TranscriptionalSilencing(轉(zhuǎn)錄沉默)TranscriptionalSilencingisaspecializedformofrepressionthatcanspreadalongchromatin,switchingoffmultiplegeneswithouttheneedforeachtobearbindingsitesforspecificrepressor.Insulatorelements(絕緣子元件)canblockthisspreading,soinsulatorsprotectgenesfrombothindiscriminateactivationandrepression.[絕緣子的概念到此才介紹完畢]Application：Ageneinsertedatrandomintothemammaliangenomeisoften“silenced”,andplacinginsulatorsupstreamanddownstreamofthatgenecanprotectthegenefromsilencing.464Appropriateregulationofsomegroupsofgenesrequireslocuscontrolregion(LCR).HumanandmouseglobingenesareclusteredingenomeanddifferentlyexpressedatdifferentstagesofdevelopmentAgroupofregulatoryelementscollectivelycalledthelocuscontrolregion(LCR),isfound30-50kbupstreamoftheclusterofglobingenes.Itbindsregulatoryproteinsthatcausethechromatinstructureto“openup”,allowingaccesstothearrayofregulatorsthatcontrolexpressionoftheindividualgenesinadefinedorder.47Figure17-13PleasecompareLCRwiththeLacoperoncontrolledgeneexpressioninbacteria48

AnothergroupofmousegeneswhoseexpressionisregulatedinatemporarilyandspatiallyorderedsequencearecalledHoxDgenes.TheyarecontrolledbyanelementcalledtheGCR(globalcontrolregion)inamannerverylikethatofLCR.49Topic3:TranscriptionalRepressor&itsregulationCHAPTER17GeneRegulationineukaryotesIneukaryotes,mostrepressorsdonotrepresstranscriptionbybindingtositesthatoverlapwiththepromoterandthusblockbindingofpolymerase.(Bacteriaoftendoso)三、真核轉(zhuǎn)錄阻遏蛋白（或抑制蛋白）及其調(diào)控50Commonly,eukaryoticrepressorsrecruitnucleosomemodifiersthatcompactthenucleosomeorremovethegroupsrecognizedbythetranscriptionalmachinery[contrasttotheactivatorrecruitednucleosomemodifers,histonedeacetylases(組蛋白去乙酰化酶)

removingtheacetylgroups].Somemodifieraddsmethylgroupstothehistonetails,whichfrequentlyrepressthetranscription.Thismodificationcausestranscriptionalsilencing.51Threeotherwaysinwhichaneukaryoticrepressorworksinclude:Competeswiththeactivatorforanoverlappedbindingsite.Bindstoasitedifferentfromthatoftheactivator,butphysicallyinteractswithanactivatorandthusblockitsactivatingregion.Bindstoasiteupstreamofthepromoter,physicallyinteractswiththetranscriptionmachineryatthepromotertoinhibittranscriptioninitiation.52Figure17-19：WaysinwhicheukaryoticrepressorworkCompetesfortheactivatorbindingInhibitsthefunctionoftheactivator.53BindstothetranscriptionmachineryRecruitsnucleosomemodifiers(mostcommon)54Inthepresenceofglucose,Mig1bindstoasitebetweentheUSAGandtheGAL1promoter,andrecruitstheTup1repressingcomplex.Tup1recruitshistonedeacetylases,andalsodirectlyinteractswiththetranscriptionmachinerytorepresstranscription.Aspecificexample:RepressionoftheGAL1geneinyeast55Topic4:SignalIntegrationandCombinatorialControlCHAPTER17GeneRegulationineukaryotes四、真核轉(zhuǎn)錄調(diào)控的特色：信號(hào)整合和組合調(diào)控Featuresoftheeukaryotictranscriptionalregulation:signalintegrationandcombinatorialcontrol561.Activatorsworktogethersynergistically(協(xié)作地)tointegratesignals.激活蛋白一起協(xié)作來整合多種信號(hào)57ReviewtheLacoperoncontrolinbacteria.TwosignalsareintegratedtocontrolLacexpressionGlucoseLactoseFigure16-658Inmulticellularorganisms,signalintegration(信號(hào)整合)isusedextensively.Insomecases,numeroussignalsarerequiredtoswitchageneon.However,eachsignalistransmittedtothegenebyaseparateregulator,andtherefore,multipleactivatorsoftenworktogether,andtheydososynergistically(twoactivatorsworkingtogetherisgreaterthanthesumofeachofthemworkingalone.)59Threestrategiesofthesynergy

(協(xié)作的三種方式):S1:Multipleactivatorsrecruitasinglecomponentofthetranscriptionalmachinery.Forexample,bytouchingthedifferentpartofthemediatorcomplex(中介體復(fù)合物).Thecombinedbindingenergyhasanexponentialeffectonrecruitment.S2:Multipleactivatorseachrecruitadifferentcomponentofthetranscriptionalmachinery.ThesecomponentsbindstothepromoterDNAinefficientlywithouthelp.60S3:Multipleactivatorshelpeachotherbindtotheirsitesupstreamofthegenetheycontrol.(Figure17-14)61Figure17-14:Cooperativebindingofactivatorsa.“Classical”cooperativebinding.b.Bothproteinsinteractingwithathirdprotein.c.Thefirstproteinrecruitanucleosomeremodellerwhoseactionrevealabindingsiteforthesecondprotein.d.BindingaproteinunwindstheDNAfromnucleosomealittle,revealingthebindingsiteforanotherprotein.622.Signalintegration:theHOgeneiscontrolledbytworegulators;onerecruitsnucleosomemodifiersandtheotherrecruitsmediator.[S3策略的example]TheHOgeneisonlyexpressedinmothercellsandonlyacertainpointinthecellcycle,resultinginthebuddingdivisionfeatureofyeastS.cerevisiae(啤酒酵母).Themothercellandcellcycleconditions

(signals)arecommunicatedtotheHOgene(target)bytwoactivators:SWI5andSBF(communicators).63SWI5:actsonlyinthemothercellandbindstomultiplesitessomedistancefromthegeneunaided,whichrecruitenzymestoopentheSBFbindingsites.SBF:onlyactiveatthecorrectstagesofthecellcycle,andcannotbindthesitesunaided.AlterthenucleosomeFigure17-15(組蛋白乙?；?64Figure17-14:Cooperativebindingofactivators.(c)Thefirstproteinrecruitanucleosomeremodellerwhoseactionrevealabindingsiteforthesecondprotein.653.Signalintegration:Cooperativebindingofactivatorsatthehumanb-interferongene.[S3策略的example]

Thehumanb-interferongene

(targetgene)isactivatedincellsuponviralinfection(signal).Infectiontriggersthreeactivators

(communicator):NFkB,IRF,andJun/ATF.Activatorsbindcooperativelytositesadjacenttooneanotherwithinanenhancerlocatedabout1kbupstreamofthepromoter,whichformsastructurecalledenhanceosome.66Figure17-16增強(qiáng)體(人的b干擾素基因)增強(qiáng)子ActivatorsinteractwitheachotherHMG-Ibindswithintheenhancerandaidsthebindingoftheactivators(bendstheDNAtopromotetheactivatorinteraction)67HMG-Iisconstitutivelyactiveinthecells,andplayanarchitecturalroleintheINF-bgeneactivationprocess.68Figure17-14:Cooperativebindingofactivators.(a)“Classical”cooperativebindingthroughdirectinteractionbetweenthetwoproteins.694.Combinatorycontrol(組合調(diào)控)liesattheheartofthecomplexityanddiversityofeukaryotes,inwhichBothactivatorsandrepressorsworktogether.組合調(diào)控是真核生物復(fù)雜性和多樣性的核心。它屬于協(xié)作調(diào)控的一種具體方式。在組合調(diào)控中，激活蛋白和阻遏蛋白共同起作用。70Aregulator(CAP)workstogetherwithdifferentrepressorsatdifferentgenes,thisisanexampleofCombinatorialControl.Infact,CAPactsatmorethan100genesinE.coli,workingwithanarrayofpartners.7:CombinatorialControl(組合調(diào)控):CAPcontrolsothergenesaswellPage499&Slide47inCh16Review71Incomplexmulticellularorganisms,combinatorialcontrolinvolvesmanymoreregulatorsandgenesthanshownabove.Bothactivatorsandrepressorscanbeinvolved.FoursignalsThreesignalsFigure17-18Thereisextensivecombinatorialcontrolineukaryotes.Agenericpicture725.Anexample:combinatorycontrolofthemating-typegenesfromS.cerevisiae(啤酒酵母的交配型的組合調(diào)控)73 TheyeastS.cerevisiaeexistsinthreeforms:twohaploidcells(單倍體)ofdifferentmatingtypes－

a

anda.thediploidcells(雙倍體)(a/a)formedwhenana

andanacellmateandfuse.

Cellsofthetwomatingtypes(a

anda)differbecausetheyexpressdifferentsetsofgenes:a

specificgenesandaspecificgenes.74acellsmaketheregulatoryproteina1,a

cellsmaketheproteina1anda2.BothcelltypesexpressthefourthregulatorproteinMcm1thatisalsoinvolvedinregulatorythemating-typespecificgenes.Howdotheseregulatorsworktogethertokeepacellinitsowntype?[Figure17-19]75Figure17-19:Controlofcell-typespecificgenesinyeastCooperativebindingCooperativebinding76Topic5:SignalTransduction(信號(hào)傳導(dǎo))andtheControlofTranscriptionalRegulatorsCHAPTER17GeneRegulationineukaryotes五、基于真核轉(zhuǎn)錄調(diào)控的前沿學(xué)科：信號(hào)傳導(dǎo)SignaltransductionAlifesciencefrontiercenteredontheeukaryotictranscriptionalregulation.771.SignalsareoftencommunicatedtotranscriptionalregulatorsthroughsignaltransductionpathwayTopic4:SignalTransductionandtheControlofTranscriptionalRegulators信號(hào)經(jīng)常通過信號(hào)傳導(dǎo)途徑被運(yùn)輸?shù)睫D(zhuǎn)錄調(diào)節(jié)蛋白78EnvironmentalSignals/Information(信號(hào))1.Smallmoleculessuchassugar,histamine(組胺).2.Proteinsreleasedbyonecellandreceivedbyanother.Ineukaryoticcells,mostsignalsarecommunicatedtogenesthroughsignaltransductionpathway(indirect),inwhichtheinitiatingligandisdetectedbyaspecificcellsurfacereceptor.Whataboutinbacteria?793*.Thesignalisthenrelayed(分程傳遞)

totherelevanttranscriptionalregulator.Signaltransductionpathway1.Theinitialligand(“signal”)bindstoanextracellulardomainofaspecificcellsurfacereceptor2.Thesignalisthuscommunicatedtotheintracellulardomainofreceptor(viaanallostericchangeordimerization)4.Thetranscriptionalregulatorcontrolthetargetgeneexpression(topic2-4).80a.TheSTATpathwayb.TheMAPkinasepathwayFigure17-22：Signaltransductionpathway812.SignalscontroltheactivitiesofeukaryotictranscriptionalregulatorsinavarietyofwaysThemechanismsofsignaltransduction.Topic4:SignalTransductionandtheControlofTranscriptionalRegulators信號(hào)傳導(dǎo)的機(jī)制：信號(hào)經(jīng)常通過不同方式控制轉(zhuǎn)錄調(diào)節(jié)蛋白的活性Ineukaryotes,asignalcanbecommunicated,directlyorindirectly,toatranscriptionalregulator.82Mechanism1:unmaskinganactivatingregion(Topic2&3):Aconformationalchangetorevealthepreviouslyburiedactivatingregion.Releasingofthepreviouslyboundmaskingprotein.Example:theactivatorGal4iscontrolledbythemaskingGal80)(Fig.17-23).Somemaskingproteinsnotonlyblocktheactivatingregionofanactivatorbutalsorecruitadeacetylaseenzymetorepressthetargetgenes.Example:RbrepressesthefunctionofthemammaliantranscriptionactivatorE2Finthisway.PhosphorylationofRbreleasesE2Ftoactivatethetargetgeneexpression.83ActivatorGal4isregulatedbyamaskingproteinGal80Gal4Figure17-2384Mechanism2:Transportintoandoutofthenucleus(Fig.17-21):Whennotactive,manyactivatorsandrepressorsareheldinthecytoplasm.Thesignalingligandcausesthemtomoveintothenucleuswheretheyactivatetranscription(Fig.19-4b).85OtherMechanisms#1:Acascadeofkinasesthatultimatelycausethephosphorylationofregulatorinnucleus

(new)(Fig.19-4a).86OtherMechanisms#2:Theactivatedreceptoriscleavedbycellularproteases(蛋白酶),andthec-terminalportionofthereceptorentersthenucleaseandactivatestheregulator

(new):(Fig.19-4c).87Topic6:Gene“Silencing”byModificationofHistonesandDNACHAPTER17GeneRegulationineukaryotesTranscriptionalsilencingisapositioneffect.(1)Ageneissilencedbecauseofwhereitislocated,notinresponsetoaspecificenvironmentalsignal.(2)SilencingcanspreadoverlargestretchesofDNA,switchingoffmultiplegenes,eventhosequitedistantfromtheinitiatingevent.Themostcommonformofsilencingisassociatedwithadenseformofchromatincalled“heterochromatin”.Heterochromatinisfrequentlyassociatedwithparticularregionsofthechromosome,notablythetelomeres,andthecentromeres.Inmammaliancells,about50%ofthegenomeisestimatedtobeinsomeformofheterochromatin.TranscriptionalsilencingisassociatedwithModificationofnucleosomesthatalterstheaccessibilityofagenetothetranscriptionalmachineryandotherregulatoryproteins.Themodificationenzymesforsilencingincludedeacetylases,DNAmethylases.916-1.SilencinginyeastismediatedbydeacetylationandmethylationofthehistonesTopic6:Gene“Silencing”byModificationofHistonesandDNA在酵母中的沉默是通過對組蛋白的去乙?；图谆閷?dǎo)的Thetelomeres,thesilentmating-typelocus,andtherDNAgenesareall“silent”regionsinS.cerevisiae.Threegenesencodingregulatorsofsilencing,SIR2,3,and4havebeenfound(SIR

standsforSilentInformationRegulator).Fig.17-24.SilencingattheyeasttelomereRap1recruitsSircomplextothetemomere.Sir2deacetylatesnearbynucleosome.93SilencingspecificityisdeterminedbyRap1,thetelomereDNA-bindingprotein.ItcanalsobedeterminedbyRNAmoleculesusingRNAimachinery(Chapter18).Thespreadingofsilencingisrestricted/controlledbyinsulatorsandotherkindofhistonemodificationsthatblockbindingoftheSir2proteins.TranscriptioncanalsobesilencedbymethylationofDNAbyhistonemethyltransferase(H3andH4,Chapter7).Thisenzymehavebeenrecentlyfoundinyeast,butiscommoninmammaliancells.Itsfunctionisbetterunderstoodinhighereukaryotes.Inhighereukaryotes,silencingistypicallyassociatedwithchromatincontaininghistonesthatbothdeacetylatedandmethylated.956-2.InDrosophila,HP1recognizesMethylatedHistonesandCondenseChromatin.Topic6:Gene“Silencing”byModificationofHistonesandDNA在果蠅中，HP1蛋白識(shí)別甲基化的組蛋白和濃縮的染色質(zhì)。HP1proteinisacomponentofheterochromatininDrosophilathatbindstomethylatedresidueinhistoneH3.SuchahistonemodificationisproducedbySu(Var)3-9.Differenttypesofmodificationatthehistonescanbeinvolvedindistinctgeeneregulation.Whatwillhappenwhenmultipleformsofmodificationoccur?A“HistoneCode”hypothesis.

Box17-4Istherea

histonecode?Accordingtothisidea,differentpatternsofmodificationonhistonetailscanbe“read”tomeandifferentthings.The“meaning”wouldbetheresultofthedirecteffectsofthesemodificationsonchromatindensityandform.Butinaddition,theparticularpatternofmodificationsatanygivenlocationwouldrecruitspecificproteins.986-3.DNAMethylationIsAssociatedwithSilencedGenesinMammliancells.Topic6:Gene“Silencing”byModificationofHistonesandDNADNA甲基化與哺乳動(dòng)物細(xì)胞的沉默基因相關(guān)聯(lián)。【注DNA甲基化不是組蛋白甲基化】SomemammaliangenesarekeptsilentbymethylationofnearbyDNAsequences.LargeregionsofmammaliangenomearemarkedbymethylationofDNAsequences,whichisoftenseeninheterochromaticregions.[Why?]ThemethylatedDNAsequencesareoftenrecognizedbyDNA-bindingproteins(suchasMeCP2)thatrecruit