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