Gene-X-ppt(Gene10-基因十)--Chapter23

Chapter23CatalyticRNA23.1Introduction•ribozyme–AnRNAthathascatalyticactivity.•RNAediting–AchangeofsequenceatthelevelofRNAfollowingtranscription.23.2GroupIIntronsUndertakeSelf-SplicingbyTransesterification•Theonlyfactorsrequiredforautosplicing(orself-splicing)invitrobygroupIintronsaretwometalionsandaguanosinenucleotide.FIGURE01:SplicingoftheTetrahymena35SrRNAprecursorcanbefollowedbygelelectrophoresis23.2GroupIIntronsUndertakeSelf-SplicingbyTransesterification•Splicingoccursbytwotransesterifications,withoutrequiringinputofenergy.FIGURE02:Self-splicingoccursbytransesterificationreactions23.2GroupIIntronsUndertakeSelf-SplicingbyTransesterification•The3′–OHendoftheguanosinecofactorattacksthe5′endoftheintroninthefirsttransesterification.•The3′–OHendgeneratedattheendofthefirstexonattacksthejunctionbetweentheintronandsecondexoninthesecondtransesterification.•Theintronisreleasedasalinearmoleculethatcircularizeswhenits3′–OHterminusattacksabondatoneoftwointernalpositions.23.2GroupIIntronsUndertakeSelf-SplicingbyTransesterification•InTetrahymena,aninternalbondoftheexcisedintroncanalsobeattackedbyothernucleotidesinatrans-splicingregion.FIGURE03:Theexcisedintronhascatalyticactivities23.3GroupIIntronsFormaCharacteristicSecondaryStructure•GroupIintronsformasecondarystructurewithnineduplexregions.•ThecoresofregionsP3,P4,P6,andP7havecatalyticactivity.FIGURE04:GroupIintronstructureisconserved23.3GroupIIntronsFormaCharacteristicSecondaryStructure•RegionsP4andP7arebothformedbypairingbetweenconservedconsensussequences.•AsequenceadjacenttoP7basepairswiththesequencethatcontainsthereactiveG.23.4RibozymesHaveVariousCatalyticActivities•Bychangingthesubstratebinding-siteofagroupIintron,itispossibletointroducealternativesequencesthatinteractwiththereactiveG.FIGURE06:ExcisionofthegroupIintroninTetrahymenarRNA23.4RibozymesHaveVariousCatalyticActivities•Thereactionsfollowclassicalenzymekineticswithalowcatalyticrate.•Reactionsusing2′–OHbondscouldhavebeenthebasisforevolvingtheoriginalcatalyticactivitiesinRNA.FIGURE09:ReactionscatalyzedbyRNAhavethesamefeaturesasthosecatalyzedbyproteins23.4RibozymesHaveVariousCatalyticActivities•riboswitch–AcatalyticRNAwhoseactivityrespondstoasmallligand.FIGURE10:Aribozymeiscontainedwithinthe5′untranslatedregionReprintedfromCurr.Opin.Struct.Biol.,vol.17,T.E.Edwards,D.J.Klein,andA.R.Ferré-D’Amaré,Riboswitches:small-moleculerecognition...,pp.273-279.Copyright2007,withpermissionfromElsevier[•Mobileintronsareabletoinsertthemselvesintonewsites.•MobilegroupIintronscodeforanendonucleasethatmakesadouble-strandbreakatatargetsite.•Theintrontransposesintothesiteofthedouble-strandbreakbyaDNA-mediatedreplicativemechanism.FIGURE11:Anintroncodesforanendonucleasethatmakesadouble-strandbreakinDNA23.5SomeGroupIIntronsCodeforEndonucleasesThatSponsorMobility•intronhoming–TheabilityofcertainintronstoinsertthemselvesintoatargetDNA.–Thereactionisspecificforasingletargetsequence.23.6GroupIIIntronsMayCodeforMultifunctionProteins•GroupIIintronscanautospliceinvitro,butareusuallyassistedbyproteinactivitiesencodedintheintron.•Asinglecodingframespecifiesaproteinwithreversetranscriptaseactivity,maturaseactivity,aDNA-bindingmotif,andaDNAendonuclease.FIGURE12:Reversetranscriptasecopiestheintronintoanewsite23.6GroupIIIntronsMayCodeforMultifunctionProteins•TheendonucleasecleavestargetDNAtoallowinsertionoftheintronatanewsite.•ThereversetranscriptasegeneratesaDNAcopyoftheinsertedRNAintronsequence.23.7SomeAutosplicingIntronsRequireMaturases•Autosplicingintronsmayrequirematuraseactivitiesencodedwithintheintrontoassistfoldingintotheactivecatalyticstructure.FIGURE13:Self-splicingintronscanacquirecodingsequences23.8TheCatalyticActivityofRNasePIsDuetoRNA•RibonucleaseP(RNaseP)isaribonucleoproteininwhichtheRNAhascatalyticactivity.•RNasePisessentialforbacteria,archaea,andeukaryotes.•RNaseMRPineukaryotesisrelatedtoRNasePandisinvolvedinrRNAprocessinganddegradationofcyclinBmRNA.23.9ViroidsHaveCatalyticActivity•Viroidsandvirusoids(orsatelliteRNA)formahammerheadstructurethathasaself-cleavingactivity.•Similarstructurescanbegeneratedbypairingasubstratestrandthatiscleavedbyanenzymestrand.•Whenanenzymestrandisintroducedintoacell,itcanpairwithasubstratestrandtargetthatisthencleaved.FIGURE14:Self-cleavagesitesofviroidsandvirusoidshaveaconsensussequence23.10RNAEditingOccursatIndividualBases•Apolipoprotein-Bandglutamatereceptorshavesite-specificdeaminationscatalyzedbycytidineandadenosinedeaminasesthatchangethecodingsequence.FIGURE16:Thesequenceoftheapo-Bgeneisthesameinintestineandliver23.11RNAEditingCanBeDirectedbyGuideRNAs•ExtensiveRNAeditingintrypanosomemitochondriaoccursbyinsertionsordeletionsofuridine.FIGURE18:ThemRNAforthetrypanosomecoxIIgenehasaframeshiftrelativetotheDNAFIGURE19:CoxIIImRNAhasextensiveeditingbybothinsertionanddeletionofuridine23.11RNAEditingCanBeDirectedbyGuideRNAs•ThesubstrateRNAbasepairswithaguideRNAonbothsidesoftheregiontobeedited.FIGURE20