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乳糖操縱子

LactoseOperon

forgraduatestudents牛登科生命科學(xué)學(xué)院2010年10月LactoseOperonStructuralgeneslacz,lacy,&lacaPromoterPolycistronicmRNARegulatorygeneRepressorOperatorOperonInducer-lactoseiOperonRegulatoryGenepozyaDNAm-RNAβ-GalactosidasePermeaseTransacetylaseProteinLactoseOperonInducer--lactoseAbsenceActiverepressorNoexpressionipozyaNolacmRNAAbsenceoflactoseActiveipozyaΒ-GalactosidasePermeaseTransacetylasePresenceoflactoseInactivePresenceInactivationofrepressorExpressionNegativecontrolIsthestorycomplete?ThesetwopagesarecopiedfromthePPTofanAmericanteacher.Hasyourtextbooktoldyousomethingmore?Ifno,areyousatisfiedwiththistheory?Nolactose,nogenestranscribed,noproteinsproduced,andthennothingtotransportlactoseintothecell.AdeadcycleDoyoustillhaveanyquestion?Whyoperon?Operonscontaingenesbelongingtothesamefunctionalpathwayinordertoguaranteethemasimilarexpressionlevel.Abouthalfoftheprotein-codinggenesareorganizedinoperons.However,prokaryoticgenomesareratherunstableandonly5–25%ofgenesbelongtostringssharedbyatleasttwodistantlyrelatedspecies.Thebiologicalsignificanceandmechanism(s)ofoperonformationarestillunderdebate.Atleastsevendifferentmodelshavebeenproposedforoperonformation.Fordetails,seeRes.Microbiol.160:502-512,2009.Morequestions?Whynegativecontrol?Thatis,isthereanytroubleifthelactoseoperongenesareconstitutivelyexpressed?WasteofaminoacidsinuselessproteinReally?Consideraworkshop,e.g.frozensucker,popsicle.Iftheproductscouldnotbesoldout,whatisthecost?Wasteofwater?Thecostisassociatedwiththeprocessofmakingthelacgeneproducts.SeeGenetics178:1653–1660,2008.ThecostofwastefulproteinexpressionCellsgrowinginrichmediahavehigherribosomecontentthancellsgrowinginpoormedia.Thus,whencellsareshiftedfromrichtopoormediatheyhaveanexcessofribosomesandcanabsorbthecostofexcesstranslation.Onshiftfrompoortorichmedia,cellshaveinsufficientribosomesandexhibitthecost.Sincethecostsaremanifestonlyinsometransitions,itisnotchangealonebutadeficitofsomespecificcellularcomponent(ribosomesarethelikelycandidate)thatexacerbatescosts.ShachraiI,ZaslaverA,AlonU,DekelE:CostofUnneededProteinsinE.coliIsReducedafterSeveralGenerationsinExponentialGrowth.MolCell2010,38(5):758-767.StoebelDM,DykhuizenDE:Wasteandyetwantnot.MolCell2010,38(5):625-626.WhyoperoninC.eleganrevealedWefindthattranscriptionalresourcesareinitiallylimitingduringrecoveryandthatrecoveringanimalsarehighlysensitivetoanyadditionaldecreaseintranscriptionalresources.Weprovideevidencethatoperonsbecomeadvantageousbecause,byclusteringgrowthgenesintooperons,fewerpromoterscompeteforthelimitedtranscriptionalmachinery,effectivelyincreasingtheconcentrationoftranscriptionalresourcesandacceleratingrecovery.FromZaslaverA,BaughLR,SternbergPW:Metazoanoperonsacceleraterecoveryfromgrowth-arrestedstates.Cell2011,145(6):981-992.Andsomethingelse?LeftforyoutodiscoverAndEamesM,KortemmeT:Cost-BenefitTradeoffsinEngineeredlacOperon

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