Types of Gene Regulation

Constitutive genes are always expressed · Tend to be vital for basic cell functions (often called housekeeping genes) · Regulated genes can be inducible or ... TypesofGeneRegulation Generegulationcanoccuratvarioussteps.Theamountof productdependson: rateofmRNAsynthesis(transcription), mRNAdegradation, proteinsynthesis(translation)etc. Prokaryotescommonlycontroltranscription Constitutivegenesarealwaysexpressed Tendtobevitalforbasiccellfunctions(oftencalled housekeepinggenes) Regulatedgenescanbeinducibleor repressible Induciblegenesarenormallyoff,butcanbeturnedon whensubstrateispresent Commonforcatabolicgenes(i.e.fortheutilization ofparticularresources)' Thelacoperonisinducible. Repressiblegenesarenormallyon,butcanbeturnedoff whentheendproductisabundant Commonforbiosynthesisgenes MoreTerminology RepressorsandActivatorsareproteinsthatbind toDNAandcontroltranscription.Thosegenesaresaidtobe repressibleorinducible. InhibitorsandInducersaresmall "effector"moleculesthatbindtorepressorsor activators. Thoughthereislotsofterminology,theconceptsare prettysimple:variousfactorsinteracttoturnthingsonoroff. Forexample,repressorssimplyactansanobstacletoblock transcription.Ifanothermolecule(theinducer)interfereswith therepressor,theydon'tbindDNAandtranscriptionproceeds normally. Operons InProkaryotes,functionallyrelatedgenesareregulatedasa unit,calledanoperon. Operonsconsistof: Severalstructuralgenes ONEpromoterandoneterminator Acontrolsite(operator) Aseparateregulatorgene(codesforproteinthatbinds tooperator) OverviewofthelacOperon Geneisnormallyoff Thereisnotranscriptionbecausearepressorbindstothe controlsite Whenlactoseispresent,itinactivatestherepressor, allowingtranscriptiontobegin.ThatproducesbothlacZ(to breakdownlactose)andlacY(toletitintothe cell) Whenlactoseisusedup,therepressorisagainfreetobind toDNA,andhalttranscription. Glucosemustbeabsent.Ifglucoseispresent,transcription doesn'tstart. Cellsrespondquicklytoavailablesugars: B-galactosidaseactivityincreasesalmostimmediatelyupon transfertolactoseasasolecarbonsource,andstopsrapidly aftertheadditionofglucose. SeeandstudyFig.16.5and16.7toseehowthe normallacoperonrespondstodifferenttypesof sugars. Rememberthatitisprobablybettertolearnthe conceptsof"repressor"and"induciblegene" and"operon"andthenuselogictoapplythattolac, ratherthanjustmemorizingthegenes. Polarmutations InpolycistronicmRNA,anythingthatprematurelystops translationwillalsoaffectallothergenesdownstreaminthe operon Usinggeneticstofigureoutthelac operon JacobandMonodidentifiedseveralmutationsingenesofthe lacoperon lacZ-,lacOc, etc. usedF'strainsthathadvariouscombinationsofmutant andwildtypegenes ThoseF'strainswere"partialdiploids"for thelacoperon. Firstlookatsinglemutations: WhateffectsdoyoupredictformutationsinB-galactosidase, lacZ?Asanapproach,askthesekindsofquestions: WhatdoeslacZdo? Whatwillbetheconsequenceifitis non-functional? Willthemutationbedominantorrecessiveinpartial diploids? Inthiscase,amutantlacZwillnotbeabletometabolize lactose,whetherornotthegeneisinduced.Butitwillbe recessive:ifthereisanotherfunctiona;copyontheplasmid, B-gal.shouldbeproducednormally. Whatabouttherepressorlocus,lacI? Whateffectsdoyoupredictformutationsintheregulator genelacI-? WhatdoeslacIdo? Whatwillbetheconsequenceifitis non-functional? Willthemutationbedominantorrecessive? SeeFig.16.9whichshowstheeffectsofa lacI-mutation MutationsintheregulatorygenelacIaffectthegeneson bothDNAmolecules,soitistrans-dominant.Thatis typicalofbindingproteinsandotherdiffusiblesubstances. Testofthemodel JacobandMonodpredictedtherewouldbeathirdtypeof mutation:mutationsintheoperator,lacO Whateffectsshouldthosehave? Whatdoestheoperatordo? Whatwillbetheconsequenceifitis non-functional? Willthemutationbedominantorrecessive? Fig.16.8showseffectsofoperator mutations. Mutationsintheoperatoronlyaffectthegeneslinkedtoiton thesameDNAmolecule,sotheyaresaidtobecis-dominant. Thisistypicaloftranscriptioncontrolelementslikepromoters, etc. AnotherlacImutation:super-repressors (lacIs) Inthosemutants,therepressorisalwaysactive,whetherornot theinducer(allolactose)ispresent. Remember,repressorshavetwosites:onetobindDNAandthe othertobindallolactose.Whichsitedoyouthinkismutatedin lacIs Whataboutglucose? Ifglucoseispresent,lacgenesremainsilent Apositiveactivatorisneeded CAP-cAMP,helpsRNApolymerasebindtothe promoter Ifglucoseislow,cAMPbuildsupincellsandactivates CAP TheCAP-cAMPsystemisaseparateregulatoroflac, whichoperatesbyacompletelydifferentmechanism.Ratherthan blockingtranscription,itisanactivatorthatisrequiredfor efficientbindingofthepromotoertotheDNA.