12.1: The lac Operon - Biology LibreTexts

In addition to the three protein-coding genes, the lac operon contains short DNA sequences that do not encode proteins, but are instead binding ... Skiptomaincontent BasiclacOperonstructurecis-andtransRegulatorslacIisanallostericallyregulatedrepressorCAPisanallostericactivatorofthelacoperon EarlyinsightsintomechanismsoftranscriptionalregulationcamefromstudiesofE.colibyresearchersFrancoisJacob&JacquesMonod.InE.coli,andmanyotherbacteria,genesencodingseveraldifferentproteinsmaybelocatedonasingletranscriptionunitcalledanoperon.Thegenesinanoperonsharethesametranscriptionalregulation,butaretranslatedindividually.Eukaryotesgenerallydonotgroupgenestogetherasoperons(exceptionisC.elegansandafewotherspecies). BasiclacOperonstructure E.coliencountersmanydifferentsugarsinitsenvironment.Thesesugars,suchaslactoseandglucose,requiredifferentenzymesfortheirmetabolism.Threeoftheenzymesforlactosemetabolismaregroupedinthelacoperon:lacZ,lacY,andlacA(Figure\(\PageIndex{1}\)).LacZencodesanenzymecalledβ-galactosidase,whichdigestslactoseintoitstwoconstituentsugars:glucoseandgalactose.lacYisapermeasethathelpstotransferlactoseintothecell.Finally,lacAisatrans-acetylase;therelevanceofwhichinlactosemetabolismisnotentirelyclear.Transcriptionofthelacoperonnormallyoccursonlywhenlactoseisavailableforittodigest.Presumably,thisavoidswastingenergyinthesynthesisofenzymesforwhichnosubstrateispresent.AsinglemRNAtranscriptincludesallthreeenzyme-codingsequencesandiscalledpolycistronic.Acistronisequivalenttoagene. Figure\(\PageIndex{1}\):DiagramofasegmentofanE.colichromosomecontainingthelacoperon,aswellasthelacIcodingregion.Thevariousgenesandcis-elementsarenotdrawntoscale.(Originall-Deyholos-CC:AN) cis-andtransRegulators Inadditiontothethreeprotein-codinggenes,thelacoperoncontainsshortDNAsequencesthatdonotencodeproteins,butareinsteadbindingsitesforproteinsinvolvedintranscriptionalregulationoftheoperon.Inthelacoperon,thesesequencesarecalledP(promoter),O(operator),andCBS(CAP-bindingsite).Collectively,sequenceelementssuchasthesearecalledcis-elementsbecausetheymustbelocatedonthesamepieceofDNAasthegenestheyregulate.Ontheotherhand,theproteinsthatbindtothesecis-elementsarecalledtrans-regulatorsbecause(asdiffusiblemolecules)theydonotnecessarilyneedtobeencodedonthesamepieceofDNAasthegenestheyregulate. lacIisanallostericallyregulatedrepressor Oneofthemajortrans-regulatorsofthelacoperonisencodedbylacI.FouridenticalmoleculesoflacIproteinsassembletogethertoformahomotetramercalledarepressor(Figure\(\PageIndex{2}\)).Thisrepressorbindstotwooperatorsequencesadjacenttothepromoterofthelacoperon.BindingoftherepressorpreventsRNApolymerasefrombindingtothepromoter(Figure\(\PageIndex{3}\)).Therefore,theoperonwillnotbetranscribedwhentheoperatorisoccupiedbyarepressor. Figure\(\PageIndex{2}\):StructureoflacIhomotetramerboundtoDNA(Origianl-Deyholos-CC:AN) BesidesitsabilitytobindtospecificDNAsequencesattheoperator,anotherimportantpropertyofthelacIproteinisitsabilitytobindtolactose.WhenlactoseisboundtolacI,theshapeoftheproteinchangesinawaythatpreventsitfrombindingtotheoperator.Therefore,inthepresenceoflactose,RNApolymeraseisabletobindtothepromoterandtranscribethelacoperon,leadingtoamoderatelevelofexpressionofthelacZ,lacY,andlacAgenes.ProteinssuchaslacIthatchangetheirshapeandfunctionalpropertiesafterbindingtoaligandaresaidtoberegulatedthroughanallostericmechanism.TheroleoflacIinregulatingthelacoperonissummarizedinFigure\(\PageIndex{4}\). Figure\(\PageIndex{3}\):Whentheconcentrationoflactose[Lac]islow,lacItetramersbindtooperatorsequences(O),therebyblockingbindingofRNApol(green)tothepromoter(P).Alternatively,when[Lac]ishigh,lactosebindstolacI,preventingtherepressorfrombindingtoO,andallowingtranscriptionbyRNApol.(Origianl-Deyholos-CC:AN) CAPisanallostericactivatorofthelacoperon Asecondaspectoflacoperonregulationisconferredbyatrans-factorcalledcAMPbindingprotein(CAP,Figure\(\PageIndex{4}\)).CAPisanotherexampleofanallostericallyregulatedtrans-factor.OnlywhentheCAPproteinisboundtocAMPcananotherpartoftheproteinbindtoaspecificcis-elementwithinthelacpromotercalledtheCAPbindingsequence(CBS).CBSislocatedveryclosetothepromoter(P).WhenCAPisboundtoatCBS,RNApolymeraseisbetterabletobindtothepromoterandinitiatetranscription.Thus,thepresenceofcAMPultimatelyleadstoafurtherincreaseinlacoperontranscription. Figure\(\PageIndex{4}\):CAP,whenboundtocAMP,helpsRNApoltobindtothelacoperon.cAMPisproducedonlywhenglucose[Glc]islow.(Origianl-Deyholos-CC:AN) ThephysiologicalsignificanceofregulationbycAMPbecomesmoreobviousinthecontextofthefollowinginformation.TheconcentrationofcAMPisinverselyproportionaltotheabundanceofglucose:whenglucoseconcentrationsarelow,anenzymecalledadenylatecyclaseisabletoproducecAMPfromATP.Evidently,E.coliprefersglucoseoverlactose,andsoexpressesthelacoperonathighlevelsonlywhenglucoseisabsentandlactoseispresent.Thisprovidesanotherlayeroflogicalcontroloflacoperonexpression:onlyinthepresenceoflactose,andintheabsenceofglucoseistheoperonexpressedatitshighestlevels. Figure\(\PageIndex{6}\):Whenglucose[Glc]andlactose[Lac]arebothhigh,thelacoperonistranscribedatamoderatelevel,becauseCAP(intheabsenceofcAMP)isunabletobindtoitscorrespondingcis-element(yellow)andthereforecannothelptostabilizebindingofRNApolatthepromoter.Alternatively,when[Glc]islow,and[Lac]ishigh,CAPandcAMPcanbindnearthepromoterandincreasefurtherthetranscriptionofthelacoperon.(Origianl-Deyholos-CC:AN)