gene regulation-30-4-19.ppt

gene regulation

1. Gene regulation in
prokaryotes and eukaryotes

2. INTRODUCTION
•Geneisthebasicunitofgeneticinformation.
•GeneismadeupofDNA,doublehelixoftwointer
woundpolynucleotide.
•Thebiologicalinformationcarriedbyageneiscontained
initsnucleotidesequence.
•Thisinformationisinessenceasetofinstructionforthe
synthesisofanRNAmoleculesthatmaysubsequently
directthesynthesisofanenzymeorotherprotein
molecule.
•Thissumofthetotalprocessiscalled“GeneExpression”

3. •Theexpressionofgenemaybeassayedintermof
RNAproduction,protein/enzymeactivityorthe
specificphenotypeproduced.
•Eg.prokaryotesproduceonlythoseenzymesthatare
neededbythematparticulartime.
•Nocellproducesalltheproteinsthatiscapableof;it
producesonlythatgroupofproteinsthatarerequired
foritsefficientfunction.

4. Gene Expression
•Geneexpressionistheactivationofagenethat
resultsinaprotein
GENEREGULATION
•Themechanismbywhichtheexpressionof
differentgenesiscontrolledindifferenttissues
and/oratdifferenttimesiscalledGene
Regulation,RegulationofGeneExpressionor
RegulationofGeneAction.

5. Levels of Regulation of Gene Action
The expression of a gene in Prokaryotes may be
subjected to regulation at one or more of the
following levels:
Gene Amplification, Destruction or Distribution
Transcription
Post-Transcription
Translation
Post-Translation

6. i.GeneAmplification,DestructionorDistribution:Regulationatthislevel
determineswhetherageneispresentinacell,andifpresentthenumberofcopiesinwhich
itispresent.Ingeneral,thosegeneswhoseproductsarerequiredinlargequantitiesare
presentinmultiplecopiesinthegenome.Inaddition,additionalcopiesofsomegenesare
alsoproducedbyreplication.Theseremaininextrachromosomalstate;thecopiesare
usuallyproducedatspecifictimes,areusedfortranscription,andaresubsequently
digested,thisphenomenoniscalledAmplification.
ii.Transcription:Thismechanismofgeneregulationdetermineswhetheragivengenewill
betranscribedornotinagivencelloratagiventime.Thismodeofreplicationis
universal,somuchsothatordinarilythetermgeneregulationimpliesregulationof
transcription.
iii.Post-transcriptionalRegulation:Controlsaftertranscriptiondeterminesifthe
mRNAproducedbyageneisavailablefortranslation.
iv.TranslationalRegulation:Translationalregulationofgeneactioncontrolsifan
mRNAthatissuitablefortranslationwillbetranslatedornot.Thismodeofregulationis
basedonribosome,tRNA,mRNA,regulatoryproteinsandregulatoryRNA.
v.Post-TranslationalRegulation:Itgovernsactivityoftheproteinproductsofgenes;it
mainlyinvolvesproteinmodification,proteindegradationandfeed-backinhibition.

7. Cont…
•Genesthatencodeaproductrequiredinthemaintenanceofbasic
cellularprocessesorcellarchitecturearecalledhousekeepinggenes
orconstitutivegenes.
•Aconstructivegeneisanunregulatedgene,whoseexpressionis
uninterrupted,incontrasttotheregulatedexpressionofagene.
•Thestudiesofbacterialgeneticsindicatethatallgenesnotonly
specifythestructureofanenzymebutsomeofthemalsoregulate
theexpressionofothergenes.
•Thesegenesarecalledregulatorgenes.
•ThisconceptofgeneregulationhasbeenstudiedbyF.JacobandJ.
Monodin1961inE.coli,whoproposedtheoperonconcept.
•TheywereawardedNobelprizeforthisdiscoveryin1965.The
operonmodelwasdevelopedworkingwithlactoseregion(lac
region)ofthehumanintestinebacteriaE.coli.
•Thegeneregulationwasstudiedfordegradationofthesugar
lactose.

8. Cont…
Accordingtotheoperonconcept,generegulation
inprokaryotesandbacteriophagesinvolves:
•Structuralgenes
•Theoperator
•Thepromoter
•Theregulatorgenes
•Repressorproteins
•Corepressor
•Inducer

9. Lac Operon of E. coli
•Ageneticunitthatconsistsofoneormore“structural
genes”(cistronsthatcodeforpolypeptides)andan
adjacent“operator–promoter”regionthatcontrols
thetranscriptionalactivityiscalledoperon.
•Operatorandpromoterareupstreamtothestructural
genes.
•Thusanoperonreferstoagroupofcloselylinked
geneswhichacttogetherandcodeforvarious
enzymesrequiredforaparticularbiochemical
pathway.

10. Cont…
•Lacoperonconsistsofseveralcomponents:
1.Structuralgenes
•ThelactoseoperonofE.coliiscomposedofthreestructuralgenes
z,yandathe‘z’genecodesforanenzymeß-galactosidase,
whichconvertslactoseintoglucoseandgalactose.
•The‘y’genecodesforanenzymepermease,whichfacilitatesthe
entryoflactoseintothecell.The‘a’genespecifiestheenzyme
thiogalactosidetransacetylase,whichtransfersanacetylgroup
fromacetylco-Atoß-galactoside.
•Henceallthethreegeneproductsinlacoperonarerequiredforthe
metabolismoflactose.
•Suchgenes,whicharesequentialandtranscribedasasinglem-RNA
fromasinglepromoterarecalledstructuralgenes.
•Them-RNAsynthesizedisthepolycistronicmRNA.
•Onlythelastcistronhasthesignalsfortheterminationof
transcription.

11. • LAC OPERON
I
P O Lac Z lacY Lac A
Repressor genePromoter-operatorΒ-galactose genePermease geneTransacetylase gene
Regulatory region Structural gene
Lac operon

12. Cont…
2.Theoperatorregion
•Operatorliesimmediatelyupstreamtothe
structuralgenesbetweenthepromoterand
structuralgenes.
•Operatoristhetargetsitefortheattachmentof
repressorproteinproducedbytheregulatorgene.
•Bindingofrepressorwithoperatorprevents
initiationoftranscriptionbyRNApolymerase.
•Whenoperatorisfree,theRNApolymerasecan
bindtothepromotertoinitiatethemRNA
synthesis.

13. Cont…
3.Thepromoterregion
•Theactualsiteoftranscriptioninitiationisknown
aspromoterregion.
•Italsoliesupstreamtothestructuralgenesnextto
theoperatorregion.
•mRNAtranscriptionbythestructuralgeneis
catalysedbyanenzymeRNApolymerase.
•Thisenzymefirstbindstothepromoterregion
andthenmovesalongtheoperatorregionand
structuralgenes.

14. Cont…
4.Regulatorgene
•Regulatorgene(i)specifiesarepressorprotein,which
intheabsenceoftheinducer(lactose),boundtothe
operator(o),therebyinactivatingtheoperatorand
preventingtranscriptionofthethreestructuralgenes
byRNApolymerase.
•Inthepresenceofaninducer(lactose),therepressor
isinactivatedbyinteractionwiththeinducer.
•ThisallowstheRNApolymerasetobindtothe
promoterallowingthetranscriptionoftheadjacent
structuralgenes.

15. Cont…
5.Repressor
•Repressorisaproteinmoleculespecifiedby
theregulatorgene.
•Repressormaybeinactiveformorinactive
form.
•Intheactiveform,repressorbindstothe
operatorregionandpreventstranscription.
•Whentherepressorisininactivateform,the
transcriptiontakesplace.

16. Cont…
6.Corepressor
•Corepressorisperhapsaproductofoneofthe
enzymessynthesizedbystructuralgenes.
•Thecorepressormakestheinactiverepressor
activeinarepressiblesystemaftercombining
withthesame.
•Therepressor–corepressorcomplexcan
blocktheoperatorgeneandstopprotein
synthesisbystructuralgenes.

17. Cont…
7.Inducer
•Theinducerbindstotherepressormakingit
inactivesuchthatitcannotbindtotheoperator.
•RNApolymerasepathwayisclearedallowingthe
expressionofstructuralgenes.
•Afewmoleculesoflactosepresentinthe
cytoplasmofE.coliaremetabolizedinto
allolacatose,whichisanisomeroflactose.
•Suchmoleculesthatinducetheexpressionofany
operonbybindingtotherepressorarecalled
inducersandsuchoperonsareinducibleoperons.

18. Regulation of Lac Operon
•InanuninducedE.coli,repressorproteinbindstotheoperator.
•Hence,expressionofstructuralgenesisnotinduced.E.coliinitially
containsafewmoleculesofß-galactosidaseenzyme.
•Afewmoleculesoflactoseslowlydiffuseintocytoplasm.
•ß-galactosidasepresentincytoplasmmetaboliseslactoseinto
allolactosewhichactsasaninducer.
•InaninducedE.coli,allolactosebindstorepressorprotein.
•Therepressorproteinisdetachedfromtheoperator.
•RNApolymeraseallowsthetranscriptionofstructuralgenesto
synthesizeapolycistronicmRNA.
•PermeasesynthesizedfrommRNAallowstherapiduptakeof
lactose.
•Largenumberofß-galactosidasemoleculesinthecytoplasm
metaboliselactoseintogalactoseandglucose.

19. In the "repressed or uninduced" state, the repressor bound to
the operator

20. In the "induced" state, the lac repressor can not bound to the
operator site

21. Mechanism of Gene Regulation
•Themechanismofgeneregulationisoftwotypes,viz,
(1)Negativeregulation
(2)Positiveregulation
•NegativeControl
•Inthenegativeregulation,absenceofaproductenhancesthe
synthesisofenzymeandpresenceoftheproductdecreasesthe
synthesisofenzyme.
•InthelacoperonofE.coli.Thesynthesisofproteindepends
whethertheoperatorgeneisblockedorfree.
•Whentheoperatorgeneisfree,proteinsynthesisbystructuralgenes
willtakeplace.
•Ontheotherhand,whentheoperatorgeneisblocked,theprotein
synthesisisprevented.
•Thus,theon-offofproteinsynthesisisgovernedbythefreeand
occupiedpositionoftheoperatorgene.

22. Cont…
•Innegativecontrol,regulatorproteinactsasainhibitorandprevents
proteinsynthesis.
•InlacoperonofE.coli,thereisnegativecontrolofgeneregulation.
•Inthenegativecontrol,theregulatorproteinistherepressorwhich
inhibitsproteinsynthesis.
•Intheinduciblesystem,theeffectormoleculeistheinducer.
•Theinducerbindswithrepressorandinactivatesitsothatitcannot
bindwithoperator.
•Thus,inducerpermitsproteinsynthesisbyinactivatingtherepressor.
•Intherepressiblesystem,theeffectormoleculeisthecorepressor.
•Thecorepressoronbindingwithin-activerepressormakesitactive
andinhibitsproteinsynthesis,becausewhenrepressorbecomes
activeitwillbindwithoperatorandstoptranscription.

23. Cont…
•PositiveControl
•Inpositiveregulation,presenceofaproduct
willenhancethesynthesisofenzyme.
•Inotherwords,inpositivecontroltheregulator
proteinactsasanactivatorandenhancesthe
proteinsynthesis.
•ThearabinoseoperonofE.coliisanexample
ofpositivegeneregulation.

24. Positive and negative types of control can be of
two types :
•Inducible
•Repressible

25. Negative control
1.Inducible operon
•The regulator genes of such operons produce active
repressors that bind to the operator on their own.
•When these repressors interact with molecules called
inducer (effector) they become inactive.
•The inactive repressors unable to bind operator .
•Transcription of the operon begins.

26. lacoperonis a cluster of three genes coding for enzymes involved in the conversion of
disaccharide lactose to monosaccharides, glucose and galactose.
These genes do not express all the time, but only in presence of lactose
First Operon to
be discovered
was lacOperon
Jacob & Monod

27. lacoperon model for gene regulation
In presence of lactose, E. coli produces -
galactosidaseto break down and utilize lactose
In the absence of lactose in the medium, E. coli
shuts of the expression of this enzyme
lactose
-galactosidase
galactose
glucose

28. Lactose binding changes repressor property
1. When lactose, the inducer binds to the lacrepressor protein
2.The binding of lactose changes the configuration of the lacrepressor protein,
inactivates.
3.The inactivated repressor protein is unable to bind to the operator region.

29. RNA polymerase binds to promoter
4. Since lactose bound repressor cannot bind to the operator region, RNA polymerase can
now bind to the promoter region and transcribes the genes

30. Transcription occurs in presence of lactose
5. RNA polymerase transcribes the three genes (Z, Y and A) of the lacoperoninto RNA

31. Negative control
Repressible operon
•The repressors encoded by the regulator gene is inactive and
unable to bind operator.
•The operonis normally functional or depressed, when the
repressor interacts with the effector(co repressor), it
becomes active and binds operator DNA .
•Transcription of the operonstopped.

32. Eg. Tryptophan operon
•Consist of 5 structural genes -TrpE, TrpD, TrpB, TrpC, TrpA
•When repressor encoded by gene R is inactive; it can bind the
operator sequence as a result, Trpoperontranscribed.
•When Trpaccumulates in cell above threshold level it interacts
with inactive repressor , transcription is prevented .

33. Positive control
1.Inducible positive control
•The activator is in an inactive state, and can not bind
DNA.
•When an inducer molecule interacts with the activator, it
becomes active and binds DNA.
•Transcription takes place.

34. Repressible positive control
•The activators is by itself is active .
•Binds to the promoter and allows transcription
•The activators become inactive when it interacts with co
repressors
•Transcription does not takes place

35. REGULATION OF GENE
ACTIVITY IN EUKARYOTES

36. •Eukaryotes have involved a more complex system of gene regulation.
EUKARYOTIC GENE REGULATION : DIFFERENT FROM REGULATION
IN PROKARYOTES
•Eukaryoticcellcontainamuchgreateramountofgeneticinformation
thenprokaryoticcells,andthisDNAcomplexedwithhistonesandother
proteinstoformchromatin.
•Geneticinformationineukaryotesiscarriedonmanychromosomes,
andthesechromosomesareenclosedwithinthedoublemembrane
boundnucleus.
•Sincethegeneticinformationineukaryotesissegregatedfromthe
cytoplasm,transcriptionisspeciallyandtemporallyseparatedfrom
translation-transcriptionoccurinthenucleusandtranslationoccurlater
inthecytoplast.Becauseofthis,attenuationcontrol,aregulatory
mechanisminprokaryotes,isnotpossible.
•Thetranscriptsofeukaryotegenesareprocessedbeforetransportto
thecytoplasm.

37. •EukaryotesmRNAhasamuchlongerhalflifethandoesthe
prokaryoticmRNA.Whenprokaryoteswanttostopmakinga
protein,theyturnofftranscriptionandthemRNAdecayswithin
minutes.
•BecausemRNAismuchmorestable,eukaryoteshaveaseriesof
translationcontrol.
•Moreeukaryotesaremulticellularwithdifferentiatedcelltypes.
• Generegulationineukaryotes
•Mechanismgeneregulationisnotwellunderstoodanditismore
complex
•Generalmodeofgeneregulationsimilarinbothi.eprokaryotes
andeukaryotes
•Donothaveoperon

38. In Eukaryotes, gene action may be regulated at the level of :
Activation of gene structure
Transcription
Translation
Gene Replication
After Transcription
After Translation

40. Regulation of transcription
•Transcriptioninitiationbeginsonlyafterbindingoftranscriptionfactors
topromoterDNA.
•WhichenablesRNApolymerasetobindthepromoter.
•Itisapositiveregulation
•Examples
•GRE(gluco-corticoidresponseelement)
•BLE(basallevelelement)
•MRE(metalresponseelement)
•TRE(TPAresponseelement)
Herethebindingofregulatorytranscriptionfactorstoanyoneofthe
responseelementisabletoactivatetranscriptioninitiation.

41. Negative regulation
•Eg. Gene encoding histoneH2B in sea urchin; expressed only during
spermatogenesis
•Promoter has two CAAT boxes
•The CAAT binding factor must bind these two boxes for transcription to
be initiated
•But in tissues other than testiseg. embryonic tissue. These are occupied
by CAAT displacement factor
•CAAT binding factor unable to bind the CAAT boxes
•Transcription does not take place

42. The Britten-Davidson Model of Regulation
•In1969,RoyBrittenandEricDavidsonproposedatheorytoexplain
generegulationinthecellsofhigherorganisms.
•Itsummarizesmanyoftheobservationsandassumptionsmadeabout
regulationinhigherorganisms.
•Forexample,ascellsundergodifferentiation,itisapparentthat
previouslyinactivesetsofgenebecomeactivated.
•Suchactivationissometimesassociatedwithexternalsignalssuchas
hormonalactionorembryonicinductiveevents.
•Theessenceofthemodelisthesimultaneousregulationofbatteriesof
genesduringdevelopment.
•BrittenandDavidsonproposedthatrepetitivesequencesserveasmajor
controlunits.

43. The basic components of Britten-Davidson model
•Aseriesofbatteriesofgenesisactivatedbythepresenceofsomesignal
molecule,suchashormone.
•Thehormoneinteractswithasensorgene.
•Thiseventactivatesacontiguousintegratorgene,whichproducesan
activatorRNAmolecule.
•Itisthismoleculewhichactivatesgenestoproducematerialsessential
tothecell.
•ActivatorRNAinteractswithreceptorgenes(comparabletooperator
regionsinbacteria)toactivatetranscription.
•Thereceptorgenescontrolthetranscriptionofadjacentproducergenes,
whicharecomparabletothestructuralgenesinbacteria.

44. THE COMPONENTS OF THE BRITTEN-DAVIDSON
MODEL OF GENE REGULATION IN EUKARYOTES

45. •Themodelproposesthateachsetofproducergenesina
givenbatterycontainsacommonnucleotidesequencein
itsadjacentreceptorgenesite.
•Thusasingleactivatormoleculemayactivatenumerous
noncontiguousproducergenes,calledabattery.
•Thebasicmodelmaybeexpandedintomorecomplex
interrelationships.
•Infigure,threedifferentsensor/integratorgenesetsare
showninrelationshiptosixreceptor/structuralgenesets.

46. THE BRITTEN-DAVIDSON MODEL OF EUKARYOTIC
GENE REGULATION AND THE COMPLEX INTERACTIONS
PROPOSED IN THE MODEL

47. •BrittenandDavidsonrevisedtheirtheoreticalmodelin1979
•Inthemodifiedversion,repetitivesequencesstillserveasthefocalpoint
ofgeneticregulationineukaryotes,butplayquitedifferentroleinthe
process
Thenewermodelbasedonfollowingobservationsandassumptions:
•ThemRNAnucleotidesequencesfoundinthecytoplasm,differin
variouscellandtissuetypes.
•Thatis,transcriptionalproductsofstructuralgenesfoundinthe
cytoplasmareuniquewhendifferentcelltypesarecompared.
•However,ifRNAfoundinthenucleiofthesevariouscelltypesis
examined,theuniquenessdisappears.
•Thus,itmaybethatgeneticregulationdoesnotoccuratthe
transcriptionallevel.
.

48. •ThesecondimportantobservationconcernstheRNA
transcriptsofrepetitiveDNAsequences,asfoundinthe
nucleiofdifferentcelltypes.
•ItisproposedthatthisRNA,alsopartofthetotalhnRNA,
variesquantitativelyandqualitativelybetweenspecialized
celltypes.
•Thefinalportionoftherevisedmodelincorporatedtherecent
discoveryofinterveningsequences.
•ThesesequencesfoundwithinDNAandtheoriginalRNA
transcriptofstructuralgenesaresubsequentlyexcisedduring
maturationofmRNA.

49. THANK YOU