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1、TranscriptionalRegulationTheregulationoftranscriptionisoneofthemostimportantwaysinwhichgenesareregulated.ProkaryotesandEukaryotesDiffernucleosomeProkaryotesandEukaryotesDifferThereisafundamentallydifferentlogicofgeneregulationinbacteriaandeukaryotes.Inbacteria,thereisaactivegroun
2、dstateandgenesareoftenregulatedbyrepressionProkaryotesandEukaryotesDifferWhileeukaryoteshaveaninactivegroundstateandgenesaretypicallyregulatedbyactivation.AllostericRegulationSmallmoleculeeffectors(效應物)canaltertheactivityoftranscriptionfactors.ThelacOperonThelacoperonwasoneofthef
3、irstgeneticregulatorysystemstobeunderstoodindetail.Thelacoperonisresponsibleforlactoseutilization,anditisonlyactivewhenlactose(orrelated?galactosides)arepresent.Lactoseandb-Galactosidease半乳糖葡萄糖b-GalactosideaseLacZgeneproduct乳糖β-半乳糖苷酶透性酶半乳糖苷轉乙酰酶異丙基-β-D-硫代半乳糖苷甘油葡萄糖Mutationsinthelac
4、OperatorlacOcmutantshaveanalteredsequencethatdoesnotallowtherepressortobind.Thisresultsinacis-actingconstitutivephenotype.MutationsinthelacRepressorIncontrast,lacImutants,whichlackrepressoractivity,canbecomplementedinmerozygotes.Thus,lacImutantsaretrans-acting.MutationsinthelacRe
5、pressorlacIsmutants,inwhichtherepressorcannotbindtotheinducer,areuninduciblemutants.TheselacImutantsarealsotrans-acting.TheCorePromoterThecorepromotersequenceswesawinourpreviousdiscussionofprokaryotictranscriptioncanbeshowntohaveanimpactontranscriptionalactivity.GlucoseRepression
6、Thelacoperonisalsoregulatedbyglucoserepression-itisonlyactivewhenlactoseispresentandglucoseisabsent.Thisisbecauseglucoseisapreferredcarbonsource.Glucoseregulationisindirect,sinceitinvolvesthesecondmessengercAMP(cyclicAMP).ThecAMPsignaltellsthecellthatglucoseisabsent.降解物激活蛋白CAPPro
7、teinBendsDNAWhentheCAP-cAMPcomplexbindsDNAitcausestheDNAtobend.ThisbendingappearstobethecauseofthetranscriptionalactivationbytheCAP-cAMPcomplex.ThetrpOperonThetrpoperonisresponsiblefortryptophanbiosynthesis.Whenwouldweexpecttoseetrpoperonactivity?分支酸鄰胺苯甲酸磷酸核糖吲哚ThetrpOperonThetrpo
8、peronisactivewhentryptophanisabsent.Tryp
