Histidine operon

Mechanism of Histidine Operon

1. K. Narayanapura, Kothanur (PO), Bengaluru 560077
Tel[phone] Fax. [phone]
e-mail:[email protected], www.kristujayanti.edu.in
HISTIDINE OPERON
Dr.Manikandan Kathirvel
Assistant Professor,
Department of Life Sciences,
Kristu Jayanti College (Autonomous),
Bengaluru

2. Anoperonisagroupofcloselylinkedgenes(structuralandregulatorygene),which
regulatethemetabolicpathwayinprokaryotes,butnotineukaryotes.
Ineukaryotes,eachgeneismadeupifanindividualmRNAandeachgenehasits
ownpromoter.
Abacteriumcontainsthousandsofgenes,whenallgenesfunctionsatasametime
theusewillbefloodedwithenzymesandproteins.so,thegenesfortherequired
enzymeatthatparticulartimeareswitchedonandtheothergenesareswitchedoff.
ThisONandOFFmechanismisexplainedbytheoperonmodel.
Operon

3. HISTIDINE OPERON
HistidineoperonanditsregulationofactioninthebacteriumSalmonella
typhimurium.
Thebacteriacontrolsitsrateofhistidinebiosynthesisby2ways:
1.Intracellularconcentrationrisesandfeedbackinhibitionshutsdownthe
pathway
Whenhistidineisbeingtransportedintocell,theintracellularconcentrationrises
andfeedbackinhibitionshutsdownthepathway.
Whenexternalhistidineisexhausted,thehistidinepoolfailsuntilfeedback
inhibitionisretrievedandbiosynthesisoftheaminoacidisresumed.
2.Repressioncontrol,Governstheintracellularconcentrationofbiosynthetic
enzymes.
Thismethodisconsiderablyslowerinactionthanfeedbackinhibitionalthoughthe
enzymesconcentrationmaybeginthenewsteadystatelevelisreached.
Presenceorabsenceofexternalhistidineisnottheonlyfactorregulatingtherateof
histidinebiosynthesis.
Thebacteriumalsokeepsitsrateofsynthesisofaminoacidinlinewithitsgrowthrate.
Theabilitytoadjusthistidineproductiontogrowthratecanprovideasignificanteconomy
inmetabolism.

4. Structural organization of Operon
InSalmonellatyphimurium,theenzymesresponsibleforthebiosynthesisofhistidineencodedby9gene
tightlyclusteredinasinglelargeoperon(Hisoperon).
HistidineoperoncontainsaclusterofninestructuralgenehisG,D,C,B,H,A,F,I,Ewhichcodesfor
enzymeofthepathwayoftheSynthesisofhistidinefrom5-phosphoribosyl1-pyrophosphate(PRPP).
TranscriptionproducesalargesinglepolycistronicmRNAabout7300nucleotidelongextendingfroma
primarypromoterHisp1toarhoindependentterminator.
2weakinternalpromotersHisp2andHisp3arelocatedwithinthehisCandhisFgenerespectively.
Thestopcodonofeachcistronoverlapsthetranslationalinitiationcodonofthedownstreamcistron.
TherearefiveregulatorygeneshisR,U,S,TandWassociatedwiththeoperons,butnotcloselylinkedtoit.
ThegenehisR,codesforhistidinetRNA(tRNAhis),whilehisScodesforhistidyl-tRNAsynthetase.
ThehistidineoperonlacksoperatorregionandaCRPsite.

5. Structural organization of Operon

6. Control of Transcription
Initiation and Elongation:
Transcriptionofthehisoperonisaboutfourfoldmoreefficientinbacteriagrowing
inminimalglucosemediumthatwhengrowinginrichmedium.
Thisformofcontrolcalledmetabolicregulationadjusttheexpressionofoperonto
aminoacidsupplyinthecell.
Itismediatedbythe“alarmone”(ppGpp)whichiseffectorofthestringent
response.(ppGpp)regulatetheprimarypromoterHisp1underconditionof
moderateaminoacidstarvation.
Thehistidineoperonisaclusterofa9genewhoseexpressionincreasesin
responsestohistidinestarvation.
Histidinestarvationresultsina10foldincreaseinthetranscriptionofthehistidine
operon.ThishasaninverserelationshipwiththeamountofhistidinetRNApresentin
thecell.
Deprivationofaminoacyl-tRNAdirectlypreventsterminationoftranscriptionin
bothcases,resultingintranscriptionofthestructuralgenes.
Regulation: Metaboliccontrolofhistidineoperon

7. Inadditiontothisgeneralmetaboliccontrol,Hisoperontranscriptionisspecifically
regulatedbyattenuationoftranscriptionamechanisminwhicharegulatoryelement,
locatedupstreamofthefirststructuralgeneoftheclustermedulatesthelevelof
expressionofhistidinebiosynheticenzymesinresponsetotheintracellularlevelsof
chargedhistidyltransferRNA.
TheHisspecificregulatoryelementistranscribedina180nucleotideleaderwhich
exhibits2promotionalfeatures:
1.16residuecodingsequenceincluding7consecutivecodonspecifyinghistidine.
2.overlappingregionofdyadsymmetrycapableoffoldingintomutuallyexclusive
alternativesecondarythatsignaleithertranscriptionterminationorantitermination.
Regulation: Attenuationcontrolofhistidineoperon

8. Attenuationcontrolofhistidineoperon
Attenuationisthemodificationofgeneexpressionbytheeventsthatinfluenceaspects
oftranscriptionandtranslationotherthaninitiationoftranscription.
Theattenuationmechanismoftryptophanandhistidineoperonsaresimilar.
TheattenuatorislocatedintheleaderregionoftheRNAtranscriptthatliesbetweenthe
promoterandthefirststructuralgene.Itcomprisesasegmentcodingfortheleader
peptidefollowedbyaterminatorsequence.
TheleaderpeptidecodingregionoftheRNAtranscriptcontains14codonsthatcode
fora14aminoacidresidueleaderpeptidehavingthefollowingaminoacidsequence:
Met-Thr-Agr-Val-Gln-Phe-Lys-His-His-His-His-His-His-His-Pro-Asp-
Theleaderpeptidecontainsasequenceofsevenregulatoryhistidineresiduesinarow.
Inhistidineoperonthe
concentrationofhistidine,the
biosyntheticendproduct,
regulatestranslationofthe
leaderpeptide.

9. Negative regulation: RNATermination
TranslationalcontrolofHisoperontranscriptionisdeterminedbyribosomesoccupancyof
leaderRNAwhichinturndependsgiventhepeculiarcompositionofHisleaderpeptideon
theavailabilityofHistRNA.
Whenhistidineispresentinsufficientconcentration,thesupplyofhistidyltRNAisplentiful.
ThispermitsthetranslatingribosometopassacrosstheleadersequenceofRNAwithoutpausing.
Therelevantcodonsandadjacentnucleotidesarefreetobasepair,resultingintheformationof
theRNAterminatorthispreventsthetranslationofthehisoperonstructuralgenes.
6RNAsegmentareinvolvedin
basepairingandthestemloop
structureformedbyEandF
RNAregionandadjacentrun
ofuridylatedresidues
constiutestheattenuationa
strongRhoindependent
transcriptionterminator.
AthighlevelofHistRNA,allows
rapidmovement ofribosomes
uptotheBsegmentinthiscase
formationofC:DandE:Fstem
loopstructurewillresultin
premature transcription
termination.TheaminoacyltRNAsthereforefunctionsascorepressor

10. IncaseofseverelimitationofintracellularpHofallchargedtRNAs
translationofleaderpeptidefailstoinitiateunderthesecondition
theA:B,C:D,E:Fstemloopstructuresfromsequentiallyproducinga
strongtranscriptiontermination.
Negative regulation: RNATermination
RNApolymerasepausesaftersynthesisofthefirstRNAhairpin(A:B).Thispausingisbelievedtosynchronize
transcriptionandtranslationofleaderregionbyhaltingtheelongationRNApolymeraseuntilaribosomestarts
translationofleaderpeptide.

11. Positive regulation: Anti-termination
Whenhistidineisscared(low),thesupplyofhistidyltRNAislimited,thisresult
intheribosomepausingorstallingasitpassesacrosstheleadersequenceand
maskingthecodons,thispermitsthebasedownstreamtobasepairandfromthe
antiterminationstructure,whichinturnpreventstheformationoftheterminator.
Translationofthehisoperonstructuralgenesthereforetakesplace.
InlowlevelofchargedtRNA-
His,ribosomedelayedatthe
consecutivehistidinecodonsof
theleaderpeptideandpreventthe
A:BpathwaybymakingA
segment.
BasepairingbetweenB&Cand
betweenD&Eregionsprevents
formationoftheattenuatorand
determinetheantitermination
conformation.

12. RNApolymerasepausesaftersynthesisofthefirstRNAhairpin(A:B).Thispausingisbelievedtosynchronize
transcriptionandtranslationofleaderregionbyhaltingtheelongationRNApolymeraseuntilaribosomestarts
translationofleaderpeptide.

13. Thank You