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李希分子医学教育部重点实验室lixi@TranscriptionandPost-transcriptionModification销售信
CentraldogmaWHICHCAMEFIRST,THEchickenortheegg?Thebiologicalsilenceshaveavariation:whichcamefirst,DNAorprotein?Yousee,amongthemanytasksperformedbyproteinsisassemblingDNAmolecules.ButDNAcontainstheinformationneededtomakeproteins.Sowhichcamefirst?RNAandRNAworldWalterGilbert1980NobelPrize
Origin-of-Life
TheoriesRNAhastheabilitytoactasbothgenesandenzymes
ThesynthesisofRNAmoleculesusingDNAstrandsasthetemplatessothatthegeneticinformationcanbetransferredfromDNAtoRNA.Fourstages:Initiation,Elongation,Termination,Post-transcriptionalmodification
Transcription
Onlythetemplatestrandisusedforthetranscription,butthecodingstrandisnot.Bothstrandscanbeusedasthetemplates.Thetranscriptiondirectionondifferentstrandsisopposite.Thisfeatureisreferredtoastheasymmetrictranscription.AsymmetrictranscriptionTemplateThetemplatestrandisthestrandfromwhichtheRNAisactuallytranscribed.Itisalsotermedasantisensestrand.Thecodingstrandisthestrandwhosebasesequencespecifiestheaminoacidsequenceoftheencodedprotein.Therefore,itisalsocalledassensestrand.BothprocessesuseDNAasthetemplate.Phosphodiesterbondsareformedinbothcases.Bothsynthesisdirectionsarefrom5´to3´.Similaritybetween
replicationandtranscription
ReplicationTranscriptionTemplateDoublestrandsSinglestrandSubstratedNTPNTPPrimeryesnoEnzymeDNApolymeraseRNApolymeraseProductdsDNAssRNABasepairA-T,G-CA-U,T-A,G-CDifferencesbetween
replicationandtranscription
ThewholegenomeofDNAneedstobereplicated,butonlysmallportionofgenomeistranscribedinresponsetothedevelopmentrequirement,physiologicalneedandenvironmentalchanges.DNAregionsthatcanbetranscribedintoRNAarecalledstructuralgenes.WhatdothemostDNAdoindeed?
GeneralconceptsofTranscriptionProcessThreephases:initiation,elongation,andtermination.TheprokaryoticRNA-polcanbindtotheDNAtemplatedirectlyinthetranscriptionprocess.TheeukaryoticRNA-polrequiresco-factorstobindtotheDNAtemplatetogetherinthetranscriptionprocess.
TranscriptionbubbleTranscriptioninprokaryotesTranscriptionUnit
OptimalPromoterThe–35sequenceisusedforinitialrecognition,andthe–10sequenceisusedforthemeltingreactionthatconvertsaclosedcomplextoanopencomplex.
Duringtranscription,thebubbleismaintainedwithinbacterialRNApolymerase,whichunwindsandrewindsDNA,maintainstheconditionsofthepartnerandtemplateDNAstrands,andsynthesizesRNA.
BacterialRNAPolymerasesAsingletypeofRNApolymeraseisresponsibleforalmostallsynthesisofmRNA,rRNAandtRNAinaeubacterium.About7,000
RNApolymerasemoleculesarepresentinanE.colicell.Probably2,000~5,000enzymesaresynthesizingRNAatanyonetime,thenumberdependingonthegrowthconditions.HowdoesRNApolymerasework?RNApol
bsubunitisthetargetofrifamycinHowmany
sigmafactorsexistinE.coli?
Howdoestranscriptioninitiate?FourstagesofTranscriptionTermination
Theterminatorisinthetranscript,nottheDNAFormsahairpinSelf-complementaryThehairpinstructureisthesignalforterminationRho(ρ)-dependentvs.ρ-independentIntrinsicterminatorsρ-independentAninvertedrepeatthatallowsahairpintoformattheendofthetranscriptsAstringofT’sinthenontemplatestrandthatresultsinastringofweakrU-dAbasepairsholdingthetranscripttothetemplatestrandRhofactorpursuesRNApolymerasealongtheRNAandcancauseterminationwhenitcatchestheenzymepausingatarho-dependentterminator.Terminationofρ-dependentTranscriptioninEukaryotesRNApolymerasesinEukaryotesRNApolymeraseItranscribesrRNA
RNApolymeraseIItranscribeshnRNARNApolymeraseIIItranscribestRNAandothersmallRNAs.Amanitaphalloides
(thedeathcap)Structureofα-amanitinAnimalRNAPolymerasesAnimalDNA-dependentRNAPolymerasesClassα-amanitinsensitivityMajorProducts
IInsensitiverRNA
IILowConc.(1-10nM)hnRNA
IIIHighconc.tRNA,5SRNAandsmallRNAs
Allhaveincommon2largesubunitsandanumberofsmallersubunits,aswellasbeingzincmetalloenzymes.
EukaryoticTranscriptionInitiationTranscriptioninitiationneedspromoterandupstreamregulatoryregions.The
cis-actingelements
arethespecificsequencesontheDNAtemplatethatregulatethetranscriptionofoneormoregenes.
Cis-actingelementRNA-poldoesnotbindthepromoterdirectly.RNA-polIIassociateswithsixtranscriptionfactors,TFIIA-TFIIH.Thetrans-actingfactorsaretheproteinsthatrecognizeandbinddirectlyorindirectlycis-actingelementsandregulateitsactivity.Transcriptionfactors
InitiationofRNApolymeraseIITATAboxisaseptamer(TATAAAA)at-25andisinvolvedinpositioningtheenzymeforcorrectinitiation.CAATbox(CCAATCT)isat–75andisrecognizedbyalargegroupoftranscriptionfactorsandplaysastrongroleindeterminingtheefficiencyofthepromoter.GCboxisat-90containsthesequenceGGGCGGandisrecognizedbythefactorSP1.ElementscombinationintypeIIPromoters
Enhancer;Dehancer;Silencer;UpstreamActivatingSequences(UAS)EnhancerEnhancersWorkUpstream,DownstreamorintheMiddleofaGeneTheyalsoworkforwardsorbackwardsPossiblewaysofworkingDifferenttranscriptionfactorsOrderofbinding(differingconcentrations)AffinityoftranscriptionfactorsTFIIAactivatesTBPbyrelievingtherepressionthatiscausedbytheTAFs
TFIIBbindsadjacenttoTBPandTATAboxTFIIDisacomplexproteincontainingaTATA-boxbindingproteinand8-10TBP-associatedfactors(TAFs)TBP:
TATA-bindingprotein
TAFs:
TBP-associatedfactorsTFIIFconsistsoftwosubunits.ThelargersubunithasanATP-dependentDNAhelicaseactivityandthesmallonecontactsthecorepolymerase.
TFIIEandTFIIHarerequiredforpromoterclearancetoallowRNApolymerasetocommencemovementawayfromthepromoter.ClassIItranscriptionfactorspolⅡTFⅡHTAFTFⅡFTAFTAFTFⅡATFⅡBTBPRNApolⅡwithtranscriptionfactorsformtranscriptioninitiationcomplex.TFIIDistheonlyfactorwhichcanrecognizespecificsites.TATADNATATAPOL-ⅡTFⅡFⅡAⅡBPreinitiationcomplexPOL-ⅡTFⅡFⅡHⅡETBPTAFTFⅡD-ⅡA-ⅡB-DNAcomplexⅡAⅡBTATAⅡHⅡECTD-PCTDtailofRNApolIIisphosphorylatedbyTFⅡHTBPTAFTBPTAFTBPTAFTBPCTD(CarboxylTerminalDomain)isrepeatedsequenceofTyr-Ser-Pro-Thr-Ser-Pro-Ser
TFIIHhasseveralactivities,includinganATPase,ahelicase,andakinaseactivitythatcanphosphorylatetheCTDtailofRNApolymeraseII;itisalsoinvolvedinrepairofdamagetoDNA.MostoftheTFIIfactorsarereleasedbeforeRNApolymeraseIIleavesthepromoter.PhosphorylationoftheCTDbythekinaseactivityofTFIIHmaybeneededtoreleaseRNApolymerasetostarttranscription.EndofInitiation
TranscriptionunitforRNApolymeraseIPromotersintypeIIIgeneupstreampromoter(type3)andinternalpromoter(type1,2)upstreampromoter:U6snRNAInternalpromoter:5SRNAandtRNAInitiationintypeIIIgenewithpolymeraseIIItRNA5SRNA
InitiationRNApolIRNApolIIIRNApolII__________________________________________________ATPrequirementnonoyes__________________________________________________AandBorTATAboxcoreconsensussq.coreelementCboxInr
__________________________________________________CAATboxupstreamelementUCEGCboxetc__________________________________________________generalTFsSL1TFIIIABCvariousTFIIs___________________________________________________upstreamfactorsUBFvariousup-streamfactors_____________________________________________________TBPisauniversalfactorTranscriptionalelongationCTDphosphorylationstatusofRNApolIIStepsleadingtotranscriptionalactivationPromoterescape/clearanceTransitiontoelongationphaseWhathappensduringtranscriptionalelongation?
Originalcontactswithinpre-initiationcomplexabolished
Formationofnewcontactswithelongationfactors
PhosphorylationofCTD
ChangeofRNApolIItoaternarycomplex=highstabilityModelofnucleosomedynamicsduringtranscription
PhosphorylationoftheCTDdefinesthestageoftranscription
CTDconsistsofheptadrepeatsoftheconsensussequence:YSPTSPS
#ofrepeatsdifferinorganisms
Promoterclearance:Ser#5getsphosphorylatedCTD:Notphosphorylated
Transitiontoelongation:Ser#2getsphosphorylatedCTD:phosphorylatedExperimentalevidenceforelongationfactors
ComparisonofRNAPIIelongationrate
invitro:100-300nt/min,frequentpauses,andsometimesfullarrest
invivo:1200-2000nt/minWhythediscrepancy?
Useofpharmacologicalagents
DRB(5,6-dichloro-1-ß-D-ribofuranosylbenzimidazole
DRB,nucleotide-analogue,causeinhibitionofhnRNAtranscriptionbyarrestingRNApolIIinvivo,butnotpurifiedRNApolII.Possibletarget?TheseevidencesuggestexistenceoffactorsthatfacilitatetranscriptionalelongationRNApolymeraseIIoftenencounterspauses&
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