Gene-X-ppt(Gene10-基因十)--Chapter24

Chapter24Translation24.1IntroductionFIGURE01:AribosomebindsmRNAandtRNAs.FIGURE02:Ribosomesareribonucleoproteinparticles.24.2TranslationOccursbyInitiation,Elongation,andTermination•TheribosomehasthreetRNA-bindingsites.•Anaminoacyl-tRNAenterstheAsite.•Peptidyl-tRNAisboundinthePsite.•DeacylatedtRNAexitsviatheEsite.FIGURE03:AminoacylatedtRNAsoccupythePandAsites.24.2TranslationOccursbyInitiation,Elongation,andTermination•Anaminoacidisaddedtothepolypeptidechainbytransferringthepolypeptidefrompeptidyl-tRNAinthePsitetoaminoacyl-tRNAintheAsite(translocation).FIGURE05:Aminoacyl-tRNAenterstheAsite24.2TranslationOccursbyInitiation,Elongation,andTermination•initiation–Thestagesoftranslationuptosynthesisofthefirstpeptidebondofthepolypeptide.•elongation–Thestageoftranslationinwhichthepolypeptidechainisextendedbytheadditionofindividualsubunits.•termination–Aseparatereactionthatendstranslationbystoppingtheadditionofsubunitsand(typically)causingdisassemblyofthesyntheticapparatus.FIGURE07:Translationhasthreestages24.3SpecialMechanismsControltheAccuracyofTranslation•Theaccuracyoftranslationiscontrolledbyspecificmechanismsateachstage.FIGURE08:Errorratesdifferateachstageofgeneexpression24.4InitiationinBacteriaNeeds30SSubunitsandAccessoryFactors•ribosome-bindingsite–AsequenceonbacterialmRNAthatincludesaninitiationcodonthatisboundbya30Ssubunitintheinitiationphaseofpolypeptidetranslation.FIGURE09:Ribosomesubunitsrecycle24.4InitiationinBacteriaNeeds30SSubunitsandAccessoryFactors•Shine–Dalgarnosequence–ThepolypurinesequenceAGGAGGcenteredabout10bpbeforetheAUGinitiationcodononbacterialmRNA.–Itiscomplementarytothesequenceatthe3′endof16SrRNA.24.4InitiationinBacteriaNeeds30SSubunitsandAccessoryFactors•Initiationoftranslationrequiresseparate30Sand50Sribosomesubunits.•Initiationfactors(IF-1,IF-2,andIF-3),whichbindto30Ssubunits,arealsorequired.FIGURE10:Initiationrequiresfactorsandfreesubunits24.4InitiationinBacteriaNeeds30SSubunitsandAccessoryFactors•A30SsubunitcarryinginitiationfactorsbindstoaninitiationsiteonmRNAtoformaninitiationcomplex.•IF-3mustbereleasedtoallow50Ssubunitstojointhe30S-mRNAcomplex.FIGURE11:IF3controlstheribosome-subunitequilibrium24.5InitiationInvolvesBasePairingbetweenmRNAandrRNA•AninitiationsiteonbacterialmRNAconsistsoftheAUGinitiationcodonprecededwithagapof~10basesbytheShine–Dalgarnopolypurinehexamer.•TherRNAofthe30SbacterialribosomalsubunithasacomplementarysequencethatbasepairswiththeShine–Dalgarnosequenceduringinitiation.FIGURE12:TheAUGisprecededbyaShine-Dalgarnosequence.24.6ASpecialInitiatortRNAStartsthePolypeptideChain•TranslationstartswithamethionineaminoacidusuallycodedbyAUG.•DifferentmethioninetRNAsareinvolvedininitiationandelongation.24.6ASpecialInitiatortRNAStartsthePolypeptideChain•N-formyl-methionyl-tRNA(tRNAfMet)–Theaminoacyl-tRNAthatinitiatesbacterialpolypeptidetranslation.–Theaminogroupofthemethionineisformylated.FIGURE14:InitiatorMet-tRNAisformylated.FIGURE15:InitiatortRNAhasdistinctfeatures.24.6ASpecialInitiatortRNAStartsthePolypeptideChain•tRNAmMet–ThebacterialtRNAthatinsertsmethionineatinternalAUGcodons.•TheinitiatortRNAhasuniquestructuralfeaturesthatdistinguishitfromallothertRNAs.•TheNH2groupofthemethionineboundtobacterialinitiatortRNAisformylated.24.7UseoffMet-tRNAfIsControlledbyIF-2andtheRibosome•context–Thefactthatneighboringsequencesmaychangetheefficiencywithwhichacodonisrecognizedbyitsaminoacyl-tRNAorisusedtoterminatepolypeptidetranslation.FIGURE16:30Ssubunitsinitiate;ribosomeselongate24.7UseoffMet-tRNAfIsControlledbyIF-2andtheRibosome•IF-2bindstheinitiatorfMet-tRNAfandallowsittoenterthepartialPsiteonthe30Ssubunit.FIGURE17:Initiationiscontrolledbythreefactors24.8SmallSubunitsScanforInitiationSitesonEukaryoticmRNA•Eukaryotic40Sribosomalsubunitsbindtothe5′endofmRNAandscanthemRNAuntiltheyreachaninitiationsite.•Aeukaryoticinitiationsiteconsistsofaten-nucleotidesequencethatincludesanAUGcodon.•60Sribosomalsubunitsjointhecomplexattheinitiationsite.FIGURE18:mRNAhastwofeaturesrecognizedbyribosomes24.8SmallSubunitsScanforInitiationSitesonEukaryoticmRNA•IRES(internalribosomeentrysite)–AeukaryoticmessengerRNAsequencethatallowsaribosometoinitiatepolypeptidetranslationwithoutmigratingfromthe5′end.24.9EukaryotesUseaComplexofManyInitiationFactors•Initiationfactorsarerequiredforallstagesofinitiation,includingbindingtheinitiatortRNA,40SsubunitattachmenttomRNA,movementalongthemRNA,andjoiningofthe60Ssubunit.•EukaryoticinitiatortRNAisaMet-tRNAthatisdifferentfromtheMet-tRNAusedinelongation,butthemethionineisnotformylated.FIGURE19:Eukaryoticinitiationusesseveralcomplexes24.9EukaryotesUseaComplexofManyInitiationFactors•eIF2bindstheinitiatorMet-tRNAiandGTP,formingaternarycomplexthatbindstothe40SsubunitbeforeitassociateswithmRNA.•Acap-bindingcomplexbindstothe5′endofmRNApriortoassociationofthemRNAwiththe40Ssubunit.FIGURE21:43Scomplex=40Ssubunit+factors+tRNA24.10ElongationFactorTuLoadsAminoacyl-tRNAintotheASite•EF-Tu(anelongationfactor)isamonomericGproteinwhoseactiveform(boundtoGTP)bindstoaminoacyl-tRNA.•TheEF-Tu-GTP-amino