第5章原核生物基因表达调控

Sagiv精英名校考 Chapter2regulationofGeneExpressioninProkaryotesand 原核生物及 表达调2009-12-

原 表达调 Sagiv精英名校考 ChapterGeneExpressionPositiveandNegativeControlofGeneOperons:Coordina yRegulatedUnitsofGeneExpressionTheLactoseOperoninE.coli:InductionandCataboliteRepression2009-12-

原 表达调 Sagiv精英名校考 ChapterOutlineThetryptophanOperoninE.coli:RepressionandAttenuationBacteriophageLambda:LysogenyorTemporalSequencesofGeneExpressionDuringPhageInfectionPost-TranslationalRegulatory2009-12-

原 表达调 Sagiv精英名校考 D’Hérelle’sDreamofTreatingDysenteryinHumansbyPhageTherapySagiv精英名校考 一个画饼充饥的梦2009-12-

原 表达调 Sagiv精英名校考 MicroorganismexhibitremarkablecapacitiestoadapttodiverseenvironmentalThisadaptabilitydependsinpartontheirabilitytoturnonandturnofftheexpressionofspecificsetsifgenesinresponsetochangesintheenvironment.Theexpressionofparticulargenesisturnedonwhentheproductsofthesegenesareneededforgrowth.Theirexpressionisturnedoffwhenthegenesproductsarenolongerneeded.2009-12-

原 表达调 Sagiv精英名校考 Thesynthesisofgenetranscriptsandtranslationproductsrequirestheexpenditureofconsiderableenergy.Byturningofftheexpressionofgeneswhentheirproductsarenotneeded,anorganismcansaveenergyandcanutilizetheconservedenergytosynthesizeproducts izegrowthrate.What,then,arethemechanismsbywhichmicroorganismsregulategeneexpressioninresponsetochangesintheenvironment?2009-12-

原 表达调 n/severaldifferentlevels:transcription,mRNAprocessing,mRNAturnover,translation,andpost-However,theregulatorymechanismswiththelargesteffectsonphenotypeactatthelevelof录水平.Sagiv精英名校考 Basedonwhatisknownabouttheregulationoftranscription,thevariousregulationmechanismsseemtofitintotwogeneralcategories:Mechanismsthatinvolvetherapidturn-onandturn-offofgeneexpressioninresponsetoenvironmentalchanges.Regulatorymechanismsofthistypeareimportantinmicroorganismsbecauseofthefrequentexposureoftheseorganismstosuddenchangesinenvironment.2009-12-

原 表达调 Sagiv精英名校考 Mechanismsreferredtoasprepro-grammedcircuitsorcascadesofgeneexpression.Inthesecases,someeventtriggerstheexpressionofonesetof 应机制Theproduct(s)ofoneormoreofthesefunctionsbyturningoffthetranscriptionofthefirstsetofgenesorturningonthetranscriptionofasecondsetofgenes.Then,oneormoreoftheproductsofthesecondsetactsbyturningonathirdset,andsoon.2009-12-

原 表达调 Sagiv精英名校考 Inthesescases,thesequentialexpressionofgenesgeneticallypreprogrammed,andthegenescannotusuallybeturnedonoutofsequence.Suchpreprogrammedsequencesofgeneexpressionarewell edinprokaryotesandtheesthatattackthem.Forexample,whenalyticbacteriophageinfectsabacterium,theviralgenesareexpressedinapredeterminedsequence,andthissequenceisdirectlycorrelatedwiththetemporalsequenceofgene-productinvolvementinthereproductionandmorphogenenesisofthe 2009-12-

原 表达调 Sagiv精英名校考 Inmostoftheknownexamplesofgeneexpression,thecircuitryisForexample,duringviralinfections,someeventassociatedwiththepackagingoftheviralDNAorRNAintheproteincoatresetsthegeneticprogramsothatthepropersequenceofgeneexpressionoccursonceagainwhenaprogeny infectsanewhostcell(在 的过程中,有些事件与病 新的宿主细胞后一再发生)2009-12-

原 表达调 Sagiv精英名校考 Section1Constitutive,Inducible,andRepressibleGeneExpression 和抑制 表2009-12-

原 表达调 Sagiv精英名校考 Genesthatspecifycellularcomponentsthatperformhousekee example,theribosomalRNAsandproteinsinvolvedinproteinsynthesis—areexpressed功能 —比如核糖体RNA和与蛋白合有关的蛋白—它们是 表达的Othergenesoftenareexpressedonlywhentheirproductsarerequiredforgrowth(其它 2009-12-

原 表达调 Sagiv精英名校考 Certaingeneproducts—suchastRNAmolecules,rRNAmolecules,ribosomalproteins,RNApolymerasesubunits,andenzymescatalyzingmetabolicprocessesthatarefrequentlyreferredtoascellular看家基因 gene”functions—essentialcomponentsofalmostalllivingGenethatspecifyproductsofthistypearecontinuallybeingexpressedinmostcells(编 2009-12-

原 表达调 Sagiv精英名校考 Suchgenearesaidtobeexpressedandarereferredtobeasconstitutivegenes(结构基Othergeneproductsareneededforcellgrowthonlyundercertainenvironmentalconditions.Constitutivesynthesisofsuchgeneproductswouldbewasteful,usingenergythatcouldotherwisebeutilizedformorerapidgrowth(这些 合成是很浪费的，所消耗的能量应该用在更快速地生长上2009-12-

原 表达调 Sagiv精英名校考 Theevolutionofregulatorymechanismsprovideforthesynthesisofsuchgeneproductsonlywhenandwheretheyareneededwouldclearlyendowtheorganismsthatprocesstheseregulatorymechanismswithaselectiveadvantageoverorganismsthatlackthem.2009-12-

原 表达调 Sagiv精英名校考 Thisundoubtedlyexplainswhycurrentlyexistingorganisms,includingbacteriaes,exhibithighlyefficientmechanismsforthecontrolofgeneexpression.括细菌和，展现了非常高效的调控基2009-12-

原 表达调 Sagiv精英名校考 Escherichiacoliandmostotherbacteriaarecapableofgrowthusingandoneofseveralcarbohydrates—forexample,glucose,sucrose,galactose,arabinose,andlactose—asanenergysource.Ifglucoseispresentintheenvironment,itwillbepreferentiallymetabolizedbyE.coli先代谢葡萄糖However,intheabsenceofglucose,E.colicellscangrowverywellonothercarbohy-2009-12-

原 表达调 Sagiv精英名校考 Cellsgrowinginmediumcontainingsugarlactose,forexample,asthesolecarbonsourcesynthesizetwoenzymes,-galactosi-daseand–galactosidepermease,whichareuniquelyrequiredforthecatabolismof–galactosidepermeasepumpslactosethecell,where–galactosidasecleavesitintoglucoseandgalactose.2009-12-

原 表达调 Sagiv精英名校考 CatabolismofNeitheroftheseenzymesisofanyusetoE.colicellsifnolactoseisavailabletothem(没有乳2009-12-

原 表达调 Sagiv精英名校考 Thesynthesisofthesetwoenzymesrequiresconsiderableenergy(intheformofATPandGTP).Thus,E.colicellshaveevolvedaregulatorymechanismbywhichthesynthesisoftheselactose-catabolizingenzymesisturnedoninthepresenceoflactoseandturnedoffinits2009-12-

原 表达调 Sagiv精英名校考 Innaturalenvironments(intestinaltractsandsewers),E.colicellsprobablyencounteranabsenceofglucoseandthepresenceoflactoserelativelyinfrequently.Therefore,theE.coligenesencodingtheenzymesinvolvedinlactoseutilizationareprobablyturnedoffmostofthetime.Ifcellsgrowingonacarbohydrateotherthanlactosearetransferredtomediumcontaininglactoseastheonlycarbonsource,theyquicklybegintosynthesizetheenzymesrequiredforlactoseutilization.2009-12-

原 表达调 Sagiv精英名校考 (a)Inductionofthesynthesisofenzymesrequiredfortheutilizationoflactoseasanenergysource,(b)Repressionofthesynthesisoftheenzymesrequiredforthebiosynthesisoftryptophan,bothinE.coli.Notethatlowlevelsofenzymesynthesisoccurwhetherthemetabolitesarepresentorabsent.2009-12- 原 表达Sagiv精英名校考 Thisprocessofturningontheexpressionofgenesinresponsetoasubstanceintheenvironmentiscalledinduction(诱导).Geneswhoseexpressionisregulatedinthismannerarecalledinduciblegenes(可诱导基因);theirproducts,ifenzymes,arecalledinducibleenzymes(可诱导酶).Enzymesthatareinvolvedincatabolic(degradative)pathways,suchasinlactose,galactose,orarabinoseutilization,arecharacteristicallyinducible.2009-12-

原 表达调 Sagiv精英名校考 2009-12-

原 表达调 Sagiv精英名校考 InductionoccursatthelevelofInductionalterstherateofenzymesynthesis,nottheactivityofexistingenzymemolecules.Inductionshouldnotbeconfusedwith淆),whichoccurswhenthebindingofasmallmoleculetoanenzymeincreasestheactivityoftheenzyme,butdoesnotaffectits2009-12-

原 表达调 Sagiv精英名校考 Bacteriacansynthesizemostoftheorganicmoleculesrequiredforgrowth,suchasaminoacids,purines,pyrimidines,andForexample,theE.coligenomecontainsfivegenesencodingenzymesthatcatalyze2009-12-

原 表达调 Sagiv精英名校考 ThesefivegenesmustbeexpressedinE.colicellsgrowinginanenvironmentdevoidoftryptophaninordertoprovideadequateamountsofthisaminoacidforprotein 时候一定要表达，以提供足够量的这种氨酸以满足蛋白质合成的需求WhenE.colicellsarepresentinanenvironmentcontainingenoughtryptophantosupportoptimalgrowth,thecontinuedsynthesisofthetryptophanbiosyntheticenzymeswouldbeawasteofenergy.2009-12-

原 表达调 Sagiv精英名校考 Thus,aregulatorymechanismSagiv精英名校考 E.colithatturnsoffthesynthesisofthetryptophanbiosyntheticenzymeswhenexternaltryptophanisavailable(在可以利用Agenewhoseexpressionhasbeenturnedoffinthiswayissaidtobe“repressed”;theprocessiscalledrepression(阻遏).Whentheexpressionofthisgeneisturnedon,itissaidtobe“derepressed”;sucharesponseiscalledderepression(去阻遏2009-12-

原 表达调 Sagiv精英名校考 2009-12-

原 表达调 精sthatarecomponentsofanabolic合成谢biosyntheticpathwaysoftenareRepression,likeinduction,occursatthelevelofRepressionshouldnotbeconfusedwithfeedbackinhibition,whichoccurswhentheproductofabiosyntheticpathwaybindstoandinhibitstheactivityofthefirstenzymeinthepathway,butdoesnotaffectthesynthesisoftheenzyme(阻遏不应与反馈抑制相，生物合成途径中的产物结合并抑2009-12-

原 表达调 Sagiv

KeyInprokaryotes,genesthatspecifyhouse- functionssuchasrRNAs,tRNAs,andribosomalproteinsareexpressedconstitutively.OthergenesusuallyareexpressedonlywhentheirproductsareGenesthatencodeenzymesinvolvedincatabolicpathwaysoftenareexpressedonlyinthepresenceofthesubstratesoftheenzymes;theirexpressionisinducible.2009-12-

原 表达调 Sagiv精英名校考 KeyGenesthatencodeenzymesinvolvedinanabolicpathwaysusuallyareturnedoffinthepresenceoftheendproductofthepathway;theirexpressionisrepressible.Althoughgeneexpressioncanberegulatedatmanylevels,transcriptionalregulationisthemostcommon.2009-12-

原 表达调 Sagiv精英名校考 SectionPositiveandNegativeControlofGeneExpression表达的正调控和负调2009-12-

原 表达调 Sagiv精英名校考 Insomecases,theproductofaregulatorygeneisrequiredtoinitiatetheexpressionofoneormoregenes.Inothercases,theproductofaregulatorygeneisrequiredtoturnofftheexpressionofoneormoregenes.2009-12-

原 表达调 Sagiv精英名校考 Theregulationofgeneexpression—induction,orturninggeneson,andrepression,orturninggenesoff—canbeplishedbybothpositivecontrolmechanismsandnegativecontrolBothmechanismsinvolveregulatorgenes—genesencodingproductsthatregulatetheexpressionofothergenes.2009-12-

原 表达调 Sagiv精英名校考 Inpositivecontrolmechanisms,theproductoftheregulatorgeneisrequiredtoturnontheexpressionofoneormorestructuralgenes,whereasinnegativecontrolmechanisms,theproductofregulatorgeneisnecessarytoshutofftheexpressionofstructuralgenes.2009-12-

原 表达调 Sagiv精英名校考 2009-12-

原 表达调 Sagiv精英名校考 RecallthatageneisexpressedwhenRNApolymerasebindstoitspromoterandsynthesizesanRNAtranscriptthatcontainsthecodingregionofthegene.Theproductoftheregulatorygeneactsbybindingtothesitecalledtheregulatorproteinbindingsite(RPBS)adjacenttothepromoterofthestructuralgene(s).WhentheproductoftheregulatorgeneisboundattheRPBS,transcriptionofthestructuralgene(s)isturnedoffinanegativecontrolsystem,orturnedoninapositivecontrolsystem.2009-12-

原 表达调 Sagiv精英名校考 Theregulatorygeneproductsarecalledactivators(活化物)—becausetheyactivatethegeneexpression—inpositivecontroltheyrepressgeneexpression—innegativecontrolWhetherornotaregulatorproteincanbindtotheRPBSdependsonthepresenceorabsenceofeffectormolecules(效应器分子)inthe2009-12-

原 表达调 Sagiv精英名校考 Theeffectorsareusuallysmallmoleculessuchasaminoacids,sugar,andsimilarmetabolites.Theeffectormoleculesinvolvedininductionofgeneexpressionarecalledinducers(诱导物thoseinrepressionofgeneexpressionarecalledco-Theeffectormolecules(inducersandco-repressors)bindtoregulatorgeneproducts(activatorsandrepressors)andcausechangesinthethree-dimensionalstructuresoftheseproteins.2009-12-

原 表达调 Sagiv精英名校考 Conformationalchangesinproteinresultingfromthebindingofsmallmoleculesarecalledallosterictransitions(别构转换).Conformationalchangesinproteinsfrequentlyresultinalterationsintheiractivity.2009-12-

原 表达调 Sagiv精英名校考 Inthecaseofactivatorsandrepressors,allosterictransitionscausedbythebindingofeffctormoleculesusuallyaltertheirabilitytobindtoregulatorproteinbindingsitesadjacenttothepromotersofthestructuralgenestheycontrol(无论是活化物 2009-12-

原 表达调 Sagiv精英名校考 Inanegative,induciblecontrolmechanism,thefreerepressorbindstotheRPBSandpreventsthetranscriptionofthestructuralgene(s)intheabsenceofinducer在负诱导2009-12-

原 表达调 Sagiv精英名校考 Wheninducerispresent,itisboundbytherepressor,andtherepressor/inducercomplexcannotbindtotheRPBS.WithnorepressorboundtotheRPBS,RNApolymerasebindstothepromoterandtranscribesthestructuralgene(s). 2009-12-

原 表达调 theactivatorcannotbindtotheRPBSunlessinducerispresent,andRNApolymerasecannottranscribethestructuralgene(s)unlesstheactivator/inducercomplexisboundtotheRPBS(在正诱导调控机制中调控者 物，活化者，必须结合到RPBS上，RNA酶才能结合到启动子上，结 才能转录Thustranscriptionofthestructuralgenesisturnedononlyinthepresenceofinducer(结构的转录仅在诱导物存在的情况下进2009-12-

原 表达调 Sagiv精英名校考 Inanegative,repressiblemechanism,transcriptionofthestructuralgene(s)occursintheabsenceoftheco-repressor,butnotinitspresence.Whentherepressor/co-repressorcomplexisboundtotheRPBS,itpreventsRNApolymerasefromtranscribingthestructuralInaabsenceofco-repressor,freerepressorcannotbindtothepromoterandtranscribethestructuralgenes.2009-12-

原 表达调 Sagiv精英名校考 2009-12-

原 表达调 Sagiv精英名校考 Inapositive,repressiblecontrolmechanism,theproductoftheregulatorgene,theactivator,mustbeboundtotheRPBSinorderforRNApolymerasetobindtothepromoterandtranscribethestructuralWhenco-repressorispresent,itformsacomplexwiththeactivatorprotein,andthisactivator/co-repressorcomplexisunabletobindtotheRPBS;consequently,RNApolymerasecannotbindtothepromoterandtranscribethestructuralgene(s).2009-12-

原 表达调 i thesefourmechanismsofregulation,focusonthekeydifferencesbetween①Theregulatorgeneproduct,theactivator,participatesinturningongeneexpressioninapositivecontrolmechanism,whereastheregulatorgeneproduct,therepressor,isinvolvedinturningoffgeneexpressioninanegativecontrolmechanism(在正调控机制中，调控 产物，活化物参与开放了 2009-12-

原 表达调 Sagiv精英名校考 ②Withbothpositiveandnegativecontrolmechanisms,whethergeneexpressionisinducibleorrepressibledependsonwhetherthefreeregulatorproteinortheregulatorprotein/effectormoleculecomplexbindstotheregulatorproteinbindingsite(无论是正调控还是负调控机制 表达都是可诱2009-12-

原 表达调 Sagiv

KeyGeneexpressioniscontrolledbybothpositiveandnegativeregulatorymechanisms.Inpositivemechanisms,theproductofaregulatorgene,anactivator,isrequiredtoturnontheexpressionofthestructuralgene(s).Innegativecontrolmechanisms,theproductofaregulatorgene,arepressor,isnecessarytoturnofftheexpressionofthestructuralgene(s).2009-12-

原 表达调 Sagiv精英名校考 KeyActivatorsandrepressorsregulategeneexpressionbybindingtositesadjacenttothepromotersofstructuralgenes.Whetherornottheregulatorproteinscanbindtotheirbindingsitesdependsonthepresenceorabsenceofsmallfreeeffectormoleculesthatformcomplexwiththeregulatorproteins.Theeffectormoleculesarecalledinducersininduciblesystemsandco-repressorsinrepressible2009-12-

原 表达调 Sagiv精英名校考 Section3Operon: RegulatedUnitsofGene子表达的协调2009-12-

原 表达调 Sagiv精英名校考 Inprokaryotes,geneswithrelatedfunctionsoftenarepresentincoordina yregulatedgeneticunitscalledoperons. 常常存在于 2009-12-

原 表达调 Sagiv精英名校考 The 子model,anegativemechanism,wasdevelopedin1961byandMonodtoexplaintheregulationofgenesrequiredforlactoseutilizationinE.coli.JacobandMonodproposedthatthetranscriptionofasetofcontiguousstructuralgenesisregulatedbytwocontrollingelements.Oneoftheelements,therepressorgene,encodesarepressor(阻遏物whichundertheappropriateconditions)bindtothesecondelement,theoperator(操作子).2009-12-

原 表达调 Sagiv精英名校考 Transcriptionisinitiatedatpromoterlocatedjustupstream(5’)fromthecodingregionsofthestructuralgenesintheoperon Operatorregionsarecontiguouswithpromoterregions(操作子与启动子相邻);sometimesoperatorsandpromotersevenoverlap,sharingashortDNAsequence(甚 2009-12-

原 表达调 Sagiv精英名校考 Operatorregionsareoftenlocatedbetweenthepromotersandstructuralgenesthattheyregulate(操作子常常位于启动子和它们调节的结 之间Thecompletecontiguousunitincludingthestructuralgenes,theoperator,andthepromoter,iscalledanoperon( 2009-12-

原 表达调 Sagiv精英名校考 Componentsofanoperon:oneormorestructuralgenes(three,SG1,SG2andSG3,areshown)andtheadjoiningoperator(O)andpromoter(P)sequences.2009-12-

原 表达调 Sagiv精英名校考 2009-12-

原 表达调 Sagiv精英名校考 Whethertherepressorwillbindtotheoperatorandturnoffthetranscriptionofthestructuralgenesinanoperonisdeterminedbythepresenceorabsenceofeffectormolecules.阻遏物是否能够结合到子的操作子上并关闭结构的表达是有是否存在着效应器2009-12-

原 表达调 Sagiv精英名校考 Inducibleoperonsandrepressibleoperonscanbedistinguishedfromoneanotherbydeterminingwhetherthenakedrepressorortherepressor/effectorcomplexisactiveinbindingtotheoperator 露的阻遏物或阻遏物/效应分子复合物否可以结合到操作子2009-12-

原 表达调 Sagiv精英名校考 Inthecaseofaninducibleoperon,thefreerepressorbindstotheoperator,turningofftranscription.Inthecaseofarepressibleoperon,thesituationisreversed.ThefreerepressorcannotbindtotheOnlytherepressor/effectormolecule(co-repressor)complexisactiveinbindingtotheoperator.2009-12-

原 表达调 Sagiv精英名校考 2009-12-

原 表达调 Sagiv精英名校考 ThedifferencebetweenSagiv精英名校考 Thus,aninducibleoperonisturnedoffintheabsenceoftheeffecor(inducer)molecule,andarepressibleoperonisturnedonintheabsenceoftheeffector(co-repressor)molecule.2009-12-

原 表达调 Sagiv精英名校考 AsinglemRNAtranscriptcarriesthecodinginformationofanentireoperon.Thus,themRNAsofopronsconsistingofmorethanonestructuralgenearemultigenic Forexample,thetryptophanoperonmRNAofE.colicontainsthecodingsequencesoffivedifferentgenes.Becausetheyareco-transcribed(由于它们是共转录的),allstructuralgenesinanoperonarecoordina yexpressed( 2009-12-

原 表达调 Sagiv精英名校考 Althoughthe tiesofdifferentgeneproductsneednottobethesame(becauseofdifferentefficienciesofinitiationoftranslationtherelativeamountofthedifferentpolypeptidesspecifiedbygenesinanoperonusuallyremainthesame,regardlessofthestateofinductionor子中 编码的不同多肽的相对量 2009-12-

原 表达调 Sagiv精英名校考 Insomecases,thedifferentialuseoftranscriptionterminationsignalscanaltertheamountsofgeneproductssynthesized. 还有其它的调控方法吗2009-12-

原 表达调 Sagiv精英名校考 KeyInbacteria,geneswithrelatedfunctionsfrequentlyoccurincoordina yregulatedunitscalledoperons.Eachoperoncontainsasetofcontiguousstructuralgenes,apromoter(thebindingsiteforRNApolymerase),andanoperator(thebindingsiteforaregulatoryproteincalledrepressor).Whenarepressorisboundtotheoperator,RNApolymerasecannottranscribethestructuralgenesintheoperon.Whentheoperatorisfreeofrepressor,RNApolymerasecantranscribetheoperon.2009-12-

原 表达调 Sagiv精英名校考 SectionTheLactoseOperoninE.coli:InductionandCataboliteThestructuralgenesinlacoperonaretranscribedonlywhenlactoseispresentandglucoseisabsent. 2009-12-

原 表达调 Sagiv精英名校考 JacobandMonadproposedtheoperonmodelontheirstudiesofthelactose(lac)operoninE.Thelacoperoncontainsapromoter(P),andoperator(O),andthreestructuralgenes,lacZ,lacY,andlacA,encodingtheenzymes-galactosidase,-galactosidepermease,and-galactosidetransacetylase,respectly.-galactosidepumpslactoseintothecell,where-galactosidasecleavesitintoglucoseandgalactose.Thebiologicalroleofthetransacetylasein2009-12-

原 表达调 Sagiv精英名校考 ThelacoperonofE.2009-12-

原 表达调 Sagiv精英名校考 oflactose2009-12-

原 表达调 Sagiv精英名校考 InJacobandMonad’smodel,thelacoperoncontainedasingleoperator(nowdesignatedHowever,twoadditionaloperators(O2O3)weresubsequentlyInitially,O2andO3werethoughttoplayveryminorroles.Then,Müller-Hillandcoworkersdemonstratedthatthedeletionofboth“minor”operatorshadalargeeffectontheleveloftranscriptionoftheoperon.2009-12-

原 表达调 Sagiv精英名校考 Morerecentstudieshaveshownthatefficientrepressionofthelacoperonrequiresthemajoroperator(O1)andatleastoneoftheminoroperators(O2orO3).Nevertheless,wewillfirstdevelopJacobandMonad’smodelofthelacoperonwithasingleoperator.Then,wewillextendthemodelandexaminethefunctionsofallthreeoperators. 2009-12-

原 表达调 Sagiv精英名校考 Thelacoperonisanegativelycontrolledinducibleoperon;thelacZ,lacY,andlacAgenesareexpressedonlyinthepresenceofThelacregulatorgene,designatedtheIgene,encodesarepressorthatis360aminoacidslong.However,theactiveformofthelacrepressorisatetramercontainingfourcopiesoftheIgeneproduct.2009-12-

原 表达调 英Q/ tothelacoperator,whichinturnpreventsRNApolymerasefromcatalyzingthetranscriptionofthethreestructuralgens.AfewmolecularoflacZ,lacY,andlacAgeneproductsaresynthesizedintheuninducedstate,provingalowbackgroundlevelofenzymeactivity.Thisbackgroundactivityisessentialforinductionoflacoperonbecausetheinduceroftheoperon,allolactose(别乳糖isderivedfromlactoseinareactioncatalyzedby-2009-12-

原 表达调 Sagiv精英名校考 Onceformed,allolactoseisboundbyrepressor,causingthereleaseoftherepressorfromtheoperator(别乳糖一旦形成，就会将阻遏物从操作子上解离下来Inthisway,allolactoseinducesthetranscriptionofthelacZ,lacY,andlacAstructuralgenes.ThelacIgene,thelacoperator,andthelacpromoterwereallinitiallyidentifiedgeneticallybytheisolationofmutantstrainsthatexhibitedalteredexpressionofthelacoperongenes.2009-12-

原 表达调 Sagiv精英名校考 MutationintheIgeneandtheoperatorfrequentlyresultinconstitutivesynthesisofthelacgeneproducts(I 致 产物的 表达2009-12-

原 表达调 Sagiv精英名校考 ThesemutationaredesignatedI-andOc,TheI-andOcconstitutivemutationcanbedistinguishednotonlybymapposition,butalsobytheirbehaviorinpartialdiploidsinwhichtheyarelocatedincisandtransconfigurationsrelatedtomutationsinlacstructuralgenes.I-和Oc构 2009-12-

原 表达调 Sagiv精英名校考 Activitylevelsinwild-typebacterialhavebeensetat100unitsforboth-galactosidase(theproductofgeneZ)and-galactosidepermease(theproductofgeneY).TheAgeneanditsproduct-galactosidetransacetylasearenotshownforthesakeofbrevity.2009-12-

原 表达调 Sagiv精英名校考 Likemonoploidwild-type(I+P+O+Z+Y+A+)partialdiploids(alsocalledmerozygotes,部分合子)ofgenotypeF’I+P+O+Z+Y+A+/I+P+O+Z-Y-A-or F’I+P+O+Z-Y-A-/I+P+O+Z+Y+A+areinduciblefortheutilizationoflactoseasacarbonThewild-typealleles(Z+,Y+,andA+)ofthethreestructuralgenesaredominancetotheirmutantalleles(Z-,Y-,andA-).Thisdominanceisexpectedbecausethewild-typeallelesproducefunctionalenzymes,whereasthemutantallelesproducenoenzymesordefective(inactive)enzymes.2009-12-

原 表达调 Sagiv精英名校考 PartialdiploidsofgenotypeI+P+O+Z+Y+A+/P+O+Z+Y+A+(I+/I-)arealsoinducibleforthesynthesisofthethreeenzymesspecifiedbythelacoperon.Thus,I+isdominanttoI-asexpected,becauseI+encodesafunctionalrepressormoleculeanditsI-allelespecifiesaninactiverepressor.ThedominanceofI+overI-alsoindicatesthattherepressorisdiffusible,becausetherepressorproducedbythelacI+alleleonthechromosomecanturnoffthelacstructuralgenesonbothoperonsinthecell.2009-12-

原 表达调 Sagiv精英名校考 Likewild-typecells,partialdiploidsofgenotypeF’I+P+O+Z+Y+A+/I+P+O+Z-Y-A-orgenotypeF’I+P+O+Z-Y-A-/I+P+O+Z+Y+A+areinduciblefor-galactosidase,-galactosidepermease,and-galactosidetransacetylase.Theinducibilityofthesegenotypesdemonstratesthatlacrepressor(I+geneproduct)controlstheexpressionofstructuralgeneslocatedeithercisortranstothelac+allele.2009-12-

原 表达调 Sagiv精英名校考 Thelac+geneisdominanttolac-(a)Controlslacoperatorlocatedeithercis(b)ortrans(c)to表达调 Sagiv精英名校考 Theoperatorconstitutive(Oc)mutationsactonlyincis;thatis,OcmutationaffecttheexpressionofonlythosestructuralgeneslocatedonthesameThecis-actingnatureofOmutationsislogicalgiventhefunctionoftheoperator.Omutationsshouldnotactintransiftheoperatoristhebindingsitefortherepressor;assuch,theoperatordoesnotencodeanyproduct,diffusibleorotherwise.Aregulatorgeneshouldactintransonlyifitspecifiesadiffusibleproduct.2009-12-

原 表达调 Sagiv精英名校考 Therefore,apartialdiploidofgenotypeI+P+O+Z-Y-A-/I+P+O+Z+Y+A+isinducibleforthethreeenzymesspecifiedbythestructuralgenesofthelacoperon,whereasapartialdiploidofgenotypeF’I+P+O+Z+Y+A+/I+P+O+Z-Y-A-synthesizestheseenzymesconstitutively(因此,当 是F’I+P+O+Z-Y-A-/I+P+O+Z+Y+A+的时候， 子上的三个结 被诱导表达 的时候，这些酶 合成2009-12-

原 表达调 Sagiv精英名校考 Activitylevelsinwild-typebacterialhavebeensetat100unitsforboth-galactosidase(theproductofgeneZ)and-galactosidepermease(theproductofgeneY).TheAgeneanditsproduct-galactosidetransacetylasearenotshownforthesakeofbrevity.2009-12-

原 表达调 Sagiv精英名校考 Theoperonactsonlyinthecis2009-12-

原 表达调 Sagiv精英名校考 2009-12-

原 表达调 Sagiv精英名校考 Sagiv精英名校考 SomeoftheIgenemutations,designatedI-d,aredominanttothewild-typeallele(I+).I Thisdominanceresultsfromtheinabilityofheteromultimeresthatcontainbothwild-typeandmutantpolypeptidestobindtothe2009-12-

原 表达调 Sagiv精英名校考 OtherIgenemutations,thosedesignatedIs,causethelacoperontobeuninducible.InstrainscarryingtheseIsmutations,thelacstructuralgenescanusuallybeinducedtosomedegreewithhighconcentrationsofinducer,buttheyarenotinducedatnormalconcentrationsofinducer.2009-12-

原 表达调 Sagiv精英名校考 Whenstudiedinvitro,themutantIspolypeptidesformstetramersthatbindtolacoperatorDNA.However,theyeitherdonotbindinducerorexhibitalowaffinityforinducer.Thus,theIsmutationsaltertheinducerbindingsiteofthelacrepressor.2009-12-

原 表达调 Sagiv精英名校考 PromotermutationsdonotchangeinducibilityofthelacInstead,theymodifythelevelsofgeneexpressionintheinducedanduninducedstatebychangingthefrequencyofinitiationoflacoperontranscription—thatis,theefficiencyofRNApolymerasebinding. 2009-12-

原 表达调 Sagiv精英名校考 Thelacpromoteractuallycontainstwoseparatecomponents:①TheRNApolymerasebinding②AbindingsiteforanotherproteincalledcataboliteactivatorproteinCAP，代谢活化者)thatpreventsthelacoperonfrombeinginducedinthepresenceofglucose. 2009-12-

原 表达调 Sagiv精英名校考 CataboliteRepression(代谢抑制Thepresenceofglucosehaslongbeenknowntopreventtheinductionofthelacoperon,aswellasotheroperonscontrollingenzymesinvolvedincarbohydratecatabolism.Thisphenomenoncalledcataboliterepressionortheglucoseeffect,代谢抑制或葡萄糖效应),assuresthatglucoseismetabolizedwhenpresent,inpreferencetoother,lessefficient,energy较低的能源而能够被代谢2009-12-

原 表达调 Sagiv精英名校考 Thecataboliterepressionthelacoperonandseveralother