转录水平的调控.ppt

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1、二、转录水平的调控（一）原核生物的操纵子模型1.乳糖操纵子1.乳糖操纵子大肠杆菌生长需要碳源,常见的是糖类,最方便利用的是葡萄糖,但有些条件下培养基中并无葡萄糖，仅有半乳糖等糖类，这时分解它们的酶类必须合成，才能利用半乳糖。当葡萄糖与乳糖共存，情况比较复杂，分解它们的酶类必须不合成。降解物基因活化蛋白CAP（catabolicgeneactivateprotein,CAP),与cAMP形成复合物结合于启动子部位，引起DNA构象的变化，促进RNA聚合酶与启动子结合，使转录的起始更加频繁，是一种正调控。当有葡萄糖存在时，其分解代

2、谢产物可抑制腺苷酸环化酶活性，激活磷酸二酯酶活性，cAMP含量下降，使CAP失活。cAMP-CAP-DNAEversincethefunctionofCRPasanactivatoroftranscriptionwasestablished,agreatdealofworkhasbeendonetotryandunderstandexactlyhowthisoccurs.DoesCRPactivatetranscriptiondirectly?Inotherwords,doesitassistRNApolymerasetor

3、ecognizethepromoterbymeansofdirectprotein-proteincontactsbetweenCRPandRNApolymerase?ORDoesCRPactivatetranscriptionindirectly?SinceCRPbendsandthusdistortstheDNAdoublehelixwhenitbinds,isitpossiblethatincreasedRNApolymerasebindingisaresultofstructuralchangesintheDNA?A

4、thirdpossibilityisthatsomecombinationofbothdirectandindirecteffectsmightberesponsibleforactivationatindividualpromoters.Wecannowdistinguish3classesofCRP-activatedpromoter:CLASSIClassICRP-activatedpromotersrequireonlyCRPforactivationandtheCRPbindingsiteislocatedupst

5、reamofthepromoter.TheprototypeCLASSICRP-activatedpromoteristhelacoperonpromoter(lacP1).ThisCRP-bindingsiteiscentred61.5bpupstreamofthestartpointoftranscription.CLASSIICLASSIICRP-activatedpromotersrequireonlyCRPforactivationandtheCRPbindingsiteoverlapsthepromoter.In

6、thesepromoters,theCRPbindingsiteappearstoreplacetheusual-35regionofthepromoter.TheprototypeCLASSIICRP-activatedpromoteristhegaloperonpromoter(galP1).ThisCRP-bindingsiteiscentred41.5bpupstreamofthestartpointoftranscription.CLASSIIICLASSIIICRP-activatedpromotersrequi

7、readditionalregulatorproteinsaswellasCRPforactivation.ThelocationoftheCRPbindingsitecanbequitevariablethoughitistypicallymorethan90bpupstreamofthestartpointoftranscription.ThereisnoprototypeCLASSIIICRP-activatedpromotersincetheyallhavedifferentrequirementsbywayofad

8、ditionalregulatorproteins.ExamplesofCLASSIIICRP-activatedpromotersarethearaBADpromoterandthemalKpromoter2.色氨酸操纵子大肠肝菌培养在只含无机盐及单一碳源的培养基中，大肠杆菌细胞内可以测