prokaryotic gene expression · prokaryotic gene expression chapter 18 in text. concept 18.1:...

ProkaryoticGeneExpressionCHAPTER18INTEXT

Concept18.1:BacteriaoftenrespondtoenvironmentalchangebyregulatingtranscriptionNaturalselectionhasfavoredbacteriathatproduceonlytheproductsneededbythatcell

AcellcanregulatetheproductionofenzymesbyfeedbackinhibitionorbygeneregulationGeneexpressioninbacteriaiscontrolledbytheoperonmodel

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ProkaryoticGeneRegulationBacteriaandotherprokaryotesdonotneedtotranscribealloftheirgenesatthesametime◦ Savestimeandresources◦ Whymakeaproteinthatispresentintheenvironment?

◦ RegulatewithDNAbindingproteins◦ Turngenes“on” or“off.”

Operons:TheBasicConceptAclusterofrelatedgenescanbeundercoordinatedcontrol byasingle“on-offswitch”

The“switch”isasegmentofDNAcalledanoperatorAnoperonistheentirestretchofDNAthatincludestheoperator,thepromoter,andthegenesthattheycontrol

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RepressorsTheoperoncanbeswitchedoffbyaproteinrepressor

TherepressorpreventsgenetranscriptionbybindingtotheoperatorandblockingRNApolymerase

Therepressoristheproductofaseparateregulatorygene

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CorepressorsTherepressorcanbeinanactiveorinactiveform,dependingonthepresenceofothermolecules

Acorepressor isamoleculethatcooperateswitharepressorproteintoswitchanoperonoffForexample,E.colicansynthesizetheaminoacidtryptophan

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Figure 18.3a

Promoter

DNA

Regulatory genemRNA

trpR

5¢

3¢

Protein Inactive repressor

RNApolymerase

Promoter

trp operon

Genes of operon

Operator

mRNA 5¢

Start codon Stop codon

trpE trpD trpC trpB trpA

E D C B A

Polypeptide subunits that make upenzymes for tryptophan synthesis

(a) Tryptophan absent, repressor inactive, operon on

Figure 18.3b-1

(b) Tryptophan present, repressor active, operon off

DNA

mRNA

Protein

Tryptophan (corepressor)

Activerepressor

Figure 18.3b-2

(b) Tryptophan present, repressor active, operon off

DNA

mRNA

Protein

Tryptophan (corepressor)

Activerepressor

No RNAmade

Trp OperonBydefaultthetrpoperonisonandthegenesfortryptophansynthesisaretranscribed

Whentryptophanispresent,itbindstothetrprepressorprotein,whichturnstheoperonoffTherepressorisactiveonlyinthepresenceofitscorepressortryptophan;thusthetrpoperonisturnedoff(repressed)iftryptophanlevelsarehigh

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RepressibleandInducibleOperons:TwoTypesofNegativeGeneRegulationArepressibleoperonisonethatisusuallyon;bindingofarepressortotheoperatorshutsofftranscription

Thetrp operonisarepressibleoperonAninducibleoperonisonethatisusuallyoff;amoleculecalledaninducerinactivatestherepressorandturnsontranscription

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LacOperonThelac operonisaninducibleoperonandcontainsgenesthatcodeforenzymesusedinthehydrolysisandmetabolismoflactose(milksugar)

Byitself,thelacrepressorisactiveandswitchesthelacoperonoff

Amoleculecalledaninducerinactivatestherepressortoturnthelacoperonon

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Figure 18.4a

(a) Lactose absent, repressor active, operon off

Regulatorygene

PromoterOperator

DNA lacZlacIDNA

mRNA5¢

3¢

NoRNAmade

RNApolymerase

ActiverepressorProtein

Figure 18.4b

(b) Lactose present, repressor inactive, operon on

lacI

lac operon

lacZ lacY lacADNA

mRNA5¢

3¢

Protein

mRNA 5¢

Inactiverepressor

RNA polymerase

Allolactose(inducer)

b-Galactosidase Permease Transacetylase