Gene-X-ppt(Gene10-基因十)--Chapter22

Chapter22mRNAStabilityandLocalization22.1Introduction•3′UTR(3′untranslatedregion)–TheuntranslatedsequencedownstreamfromthecodingregionofanmRNA.•5′UTR(5′untranslatedregion)–TheuntranslatedsequenceupstreamfromthecodingregionofanmRNA.FIGURE01:FeaturesofprokaryoticandeukaryoticmRNAs.22.1Introduction•stem-loop–AsecondarystructurethatappearsinRNAsconsistingofabase-pairedregion(stem)andaterminalloopofsingle-strandedRNA.–Botharevariableinsize.22.2MessengerRNAsAreUnstableMolecules•mRNAinstabilityisduetotheactionofribonucleases.•Ribonucleasesdifferintheirsubstratepreferenceandmodeofattack.•endoribonuclease–AribonucleasethatcleavesanRNAataninternalsite(s).•exoribonuclease–AribonucleasethatremovesterminalribonucleotidesfromRNA.FIGURE02:Typesofribonucleases.22.2MessengerRNAsAreUnstableMolecules•processive(nuclease)–Anenzymethatremainsassociatedwiththesubstratewhilecatalyzingthesequentialremovalofnucleotides.•distributive(nuclease)–Anenzymethatcatalyzestheremovalofonlyoneorafewnucleotidesbeforedissociatingfromthesubstrate.•mRNAsexhibitawiderangeofhalf-lives.FIGURE03:MethodfordeterminingmRNAhalf-lives.22.2MessengerRNAsAreUnstableMolecules•mRNAdecay–mRNAdegradation,assumingthatthedegradationprocessisstochastic.•DifferentialmRNAstabilityisanimportantcontributortomRNAabundanceandthereforethespectrumofproteinsmadeinacell.•steadystate(molecularconcentration)–Theconcentrationofpopulationofmoleculeswhentheratesofsynthesisanddegradationareconstant.22.3EukaryoticmRNAsExistintheFormofmRNPsfromTheirBirthtoTheirDeath•mRNAassociateswithachangingpopulationofproteinsduringitsnuclearmaturationandcytoplasmiclife.•Somenuclear-acquiredmRNPproteinshaverolesinthecytoplasm.•AverylargenumberofRNA-bindingproteins(RBPs)exist,mostofwhichremainuncharacterized.•DifferentmRNAsareassociatedwithdistinct,butoverlapping,setsofregulatoryproteins,creatingRNAregulons.FIGURE04:TheconceptofanRNAregulon.22.4ProkaryoticmRNADegradationInvolvesMultipleEnzymes•polyribosome(orpolysome)–AnmRNAthatissimultaneouslybeingtranslatedbymultipleribosomes.•monocistronicmRNA–mRNAthatcodesforonepolypeptide.22.4ProkaryoticmRNADegradationInvolvesMultipleEnzymes•DegradationofbacterialmRNAsisinitiatedbyremovalofapyrophosphatefromthe5′terminus.•MonophosphorylatedmRNAsaredegradedduringtranslationinatwo-stepcycleinvolvingendonucleolyticcleavages,followedby3′to5′digestionoftheresultingfragments.FIGURE05:DegradationofbacterialmRNAs22.4ProkaryoticmRNADegradationInvolvesMultipleEnzymes•3′polyadenylationcanfacilitatethedegradationofmRNAfragmentscontainingsecondarystructure.•poly(A)polymerase(PAP)–Theenzymethataddsthestretchofpolyadenylicacidtothe3′endofeukaryoticmRNA.–Itdoesnotuseatemplate.22.4ProkaryoticmRNADegradationInvolvesMultipleEnzymes•poly(A)–Astretchofadenylicacidthatisaddedtothe3′endofmRNAfollowingitssynthesis.•Themaindegradationenzymesworkasacomplexcalledthedegradosome.22.5MostEukaryoticmRNAisDegradedviaTwoDeadenylation-DependentPathways•ThemodificationsatbothendsofmRNAprotectitagainstdegradationbyexonucleases.•poly(A)bindingprotein(PABP)–Theproteinthatbindstothe3′stretchofpoly(A)onaeukaryoticmRNA.22.5MostEukaryoticmRNAisDegradedviaTwoDeadenylation-DependentPathways•ThetwomajormRNAdecaypathwaysareinitiatedbydeadenylationcatalyzedbypoly(A)nucleases.•Deadenylationmaybefollowedeitherbydecappingand5′to3′exonucleasedigestion,orby3′to5′exonucleasedigestion.FIGURE06:Themajordeadenylation-dependentdecaypathwaysineukaryotes22.5MostEukaryoticmRNAisDegradedviaTwoDeadenylation-DependentPathways•Thedecappingenzymecompeteswiththetranslationinitiationcomplexfor5′capbinding.•cytoplasmiccap-bindingprotein–Acomponentoftheeukaryoticinitiationfactor4F(eIF4F)thatbindsthe7-methylguanosinecapatthe5′endofeukaryoticmRNA.•Theexosome,whichcatalyzes3′to5′mRNAdigestion,isalarge,evolutionarilyconservedcomplex.22.5MostEukaryoticmRNAisDegradedviaTwoDeadenylation-DependentPathways•Degradationmayoccurwithindiscretecytoplasmicparticlescalledprocessingbodies(PBs).•AvarietyofparticlescontainingtranslationallyrepressedmRNAsexistindifferentcelltypes.•maternalmRNAgranules–OocyteparticlescontainingtranslationallyrepressedmRNAsawaitingactivationlaterindevelopment.22.5MostEukaryoticmRNAisDegradedviaTwoDeadenylation-DependentPathways•neuronalgranules–ParticlescontainingtranslationallyrepressedmRNAsintransittofinalcelldestinations.•stressgranules–Cytoplasmicparticles,containingtranslationallyinactivemRNAs,thatforminresponsetoageneralinhibitionoftranslationinitiation.22.6OtherDegradationPathwaysTargetSpecificmRNAs•FouradditionaldegradationpathwaysinvolveregulateddegradationofspecificmRNAs.•Deadenylation-independentdecappingproceedsinthepresenceofalongpoly(A)tail.FIGURE08:Otherdecaypathwaysineukaryoticcells22.6OtherDegradationPathwaysTargetSpecificmRNAs•ThedegradationofthenonpolyadenylatedhistonemRNAsisinitiatedby3′additionofapoly(U)tail.•DegradationofsomemRNAsmaybeinitiatedbysequence-specificorstructure-specificendonucleolyticcleavage.•AnunknownnumberofmRNAsaretargetsfordegradationortranslationalrepressionbymicroRNAs.22.6O