交大分子生物学课件,老师-第6章转座子

Sag精英名校考研 /第三遗传信息表达的交通大学生命科学 Sagiv

几个名Cis-actingElementsarethesegmentsofDNAsequences,suchaspromotors,operator,enhancer,silencer,etc.Elementisnotagene,however,thereareoneormoregeneswithinelement,suchastransposableelement,etc.Trans-actingFactorisaregulatoryprotein,suchasrepressor,transcriptivefactors,etc. Sag精英名校考研 /ChapterTransposableGenetic转座遗传 Sag精英名校考研 /ChapterTransposableElements:AnTransposableElementsinCut-and-pasteTransposonin esandTransposableElementsinTheGeneticandEvolutionarySignificanceofTransposableElements Sag精英名校考研 /Maize:AstaplecropwithaculturalOneoftheworld’smostimportantcropplants.

Colorvariationamongkernelsofmaize.Studiesofgeneticbasisofthisvariationledtothediscoveryof Sag精英名校考研 /SectionTransposableElementsAnOverviewTransposableelements—transposons—arefoundinthegenomesofmanykindsoforganisms;theyarestructurallyandfunctionallydiverse. Theworldoftransposableelementsisrichwithdiversity.Manydifferentkindsofelementshavebeenidentifiedinanassortmentoforganisms,includingbacteria,fungi,protists,plantsandanimals.Theseelementsareprominentofcomponentsofgenomes—forexample,morethan40%ofthehumangenome—andtheyclearlyhaverolesinsha thestructureofchromosomesandinmodulatingtheexpressionof Sagiv精英名校考 Weshallexplorethestructuralandbehavioraldiversityofdifferenttypesoftransposableelements,andweshallinvestigatetheirgeneticandevolutionalAlthougheachkindoftransposableelementshasitsownspecialcharacteristics,mostofthemcanbeclassifiedintooneofthreecategoriesbasedonhowtheytranspose. SagivatrizatiQ7543pp599 transpositionmechanism HostI.Cut-andPaterISelements(e.g.,Ac/DsPmarinerhohoTc1II.ReplicativeTn3III.A.likeelements(alsocalledlongterminalrepeat,orLTR,B.F,G,andIomere-specificretropsons(HeT-A,LINEs(e.g.,9SINEs(e.g.,Sag精英名校考研 /Cut-and-pasterTransposonis plished(熟练的byexcisinganelementfromitspositioninachromosomeandinsertingitintoanotherposition.Theexcisionandinsertioneventsarecatalyzedbyanenzymecalledthetransposase(转座酶whichisencodedbytheelementsitself.Geneticistsrefertothismechanismascut-and-pastertransposons(切粘转座子)becausetheelementsisphysicallycutoutofonesiteinachromosomeandpasterintoanewsitewhichmayevenbeonadifferent Sag精英名校考研 /Sagiv精英名校考 ReplicativeSagiv精英名校考 Transpositionis plishedthroughaprocessthatinvolvesreplicationofthetransposableelement’sDNA.Atransposaseencodedbytheelementmediatedaninteractionbetweentheelementandapotentialinsertionsite. Sag精英名校考研 /Duringthisinteraction,theelementisreplicatedandonecopyofitisinsertedatthenewsite;onecopyalsoremainsattheoriginalsite.Becausethereisanetgainofonecopyoftheelement,geneticistsrefertothismechanismasreplcativetransposons. Sagiv精英名校考 Sagiv精英名校考 Transpositionis plishedthroughaprocessthatinvolvestheinsertionofcopiesofanelementthatweresynthesizedfromtheelement’sRNA.Anenzymecalledreversetranscriptaseusestheelement’sRNAasatemplatetosynthesizeDNAmolecules,whicharetheninsertedintonewchromosomalsites. Sag精英名校考研 /Becausethismechanismreversesthedirectioninwhichgeneticinformationflowsincells—thatis,itflowsfromRNAtoDNAinsteadofDNAtoRNA—geneticistsrefertoitasretrotransposons.Someoftheelementsthattransposeinthiswayarerelatedtotheretro ;consequently,theyarecalledretro Otherelementsthatengagedinretrotransposonaresimplecalledretroposons. Sag精英名校考研 /Thecut-and-pastertransposonsarefoundin真核和原核都有Thereplicativetransposonsarefoundonlyin Retrotransposonarefoundonlyin Sag精英名校考研 /KeyAcut-and-pastertransposonisexcisedfromonegenomicpositionandinsertedintoanotherbyanenzyme,thetransposase,whichisusuallyencodedbythetransposonitself.Areplicativetransposoniscopiedduringtheprocessoftransposition.AretrotransposonproducedRNAmoleculesthatarereverse-transcribedintoDNAmolecules;theseDNAmoleculesaresubsequentlyinsertedintonewgenomic Sag精英名校考研 /Section2TransposableElementsinBacterialtransposonsmovewithinandbetweenthebacterialchromosomeandvarioustypesof Sag精英名校考研 /TherearethreemaintypestransposableTheinsertionsequences(插入序列orISelements,aresimplest,containingonlygenesthatencodeproteinsinvolvedinThecompositetransposons(复合转座子)andTn3elementsaremorecomplex,containingsomegenesthatencodeproductsunrelatedtothetranspositionprocess. Sag精英名校考研 /Thesimplestbacterialtransposonsaretheinsertionsequences,orISelements,sonamedbecausetheycaninsertatmanydifferentsitesinbacterialchromosomesandplasmids.ISelementswerefirstdetectedincertainmutationsofE.Thesemutationhadtheunusualpropertyofrevertingtowild-typeatahighrate. Sag精英名校考研 /WhenDNAfromthewide-typerevertants(复体ofthesemutationswascomparedwithfromthemutationthemselves,itwasfoundthattheextraDNAhadbeenlost.Thus,thesegeneticallyunstablemutationswerecausedbyDNAsequencesthathadinsertedintoE.coligenes,andreversiontowide-typewascausedbyexcisionofthesesequences. Sag精英名校考研 /ISelementsarecompactlyTheyconsistoffewerthan2500bpandcontainonlygeneswhoseproductsareinvolvedinpromotingorregulatingtransposition.Thesmallest,IS1,is768bpEachtypeofISelementisdemarcatedbyshortidenticalornearlyidenticalsequencesatitsends(每 Becausetheseterminalsequencesarealwaysininvertedorientationwithrespecttoeachothertheycalledinvertedterminalrepeats(反向末端重复). Sag精英名校考研 /Theirlengthsrangefrom9to40Invertedterminalrepeatsarecharacteristicofmost—butnotall—typesoftransposons.Whennucleotidesintheserepeatsaremutated,thetransposonsusuallylosesitsabilitytomove.Thesemutationsthereforedemonstratethatinvertedterminalrepeatsplayanimportantroleinthetranspositionprocess. Sag精英名校考研 /StructureofaninsertedIS50elementshowingitsterminalinvertedrepeatsandtargetsiteduplication.Theterminalinvertedrepeatsareimperfectbecausethefourthnucleotidepairfromeachendis Sag精英名校考研 /AtleastsomeISelementsencodeaproteinthatneededforThisprotein,calledtransposase,seemstobindatorneartheendoftheelement,whereiscutsbothstrandsoftheDNA.CleavageoftheDNAatthesesitesexcisestheelementfromthechromosomeorplasmid,sothatitcanbeinsertedatanewpositioninthesameordifferentDNAmolecule.ISelementsarethereforecut-and-paster Sag精英名校考研 /WhenISelementsinsertchromosomesorplasmids,theycreateaduplicationofpartoftheDNAsequenceatthesiteoftheinsertion.OnecopyoftheduplicationislocatedoneachsideoftheTheshort(2to13bp),directlyrepeatedsequences,called),arethoughttoariseofthedoublestrandedDNA7

ProductionoftargetsiteduplicationsbytheinsertionofanISSag精英名校考研 /AbacterialchromosomemaycontaincopiesofaparticulartypeofISE.coli:6~10copiesofFplasmidhasatleast2differentISelements,IS2andIS3.WhenaparticularISelementresidesinbothaplasmidandachromosome,itcreatestheopportunityforhomologous betweendifferentDNAmolecules. binationappearstoberesponsiblefortheintegrationoftheFplasmidintotheE.coli Sag精英名校考研 /BoththeE.colichromosomeandtheplasmidarecircularDNAWhentheplasmidandthechromosomebineinaregionofhomology—e.g.,inanISelementcommontobothofthem—thesmallestplasmidisintegratedintothelargerchromosome,creatingasinglecircularmolecule.SuchintegrationeventsproduceHfrstrainscapableoftransferringtheirchromosomesduringconjugation. Sag精英名校考研 /ThesestrainsvaryintheintegrationsiteoftheFplasmidbecausetheISelementsthatmediatebinationoccupydifferentchromosomepositionsindifferentE.colistrains—aresultoftheirabilitytotranspose(这些株由于介导重组的IS元件在不同的E.coli株中占据了不同的 BecauseoftheirroleinformingHfrstrains,ISelementspotentiatetheexchangeofchromosomalgenesbetweendifferentstrainsofbacteria.Thisexchangecreatesgeneticvariability,which,alongwithmutation,isthebasisforevolutionary200-7 Sag精英名校考研 /CompositeCompositetransposons(复合转座子),whicharebacterialcut-and-pastertransposonsdenotedbythesymbolTn,arecreatedwhentwoISelementsinsertneareachother.Theregionbetweenthemcanthenbetransposedbythejointactionoftheflankingelements.Ineffect,twoISelements“captureaDNAsequencethatisotherwiseimmobileandendowitwiththeabilitytomove(实际上是两个IS元件捕获 Sag精英名校考研 / Tn9consistsoftwoelementsflankingagene)Tn5consistsoftwoelementsflankinggenesforkanamycin(卡那霉素), )Tn10consistsoftwoIS10elementsflankingagenefortetracycline ) Geneticorganizationof Sag精英名校考研 /InTn9,theflankingISelementsareinthesameorientationwithrespecttoeachother,whereasinTn5andTn10,theorientationisTheregionbetweentheISelementsineachofthesetransposonscontainsgenesthathavenothingtodowithtransposition.Infact,inallthreetransposons,thegenesbetweentheflankingISelementsconferto Sagiv精英名校考 TheflankingISelementsSagiv精英名校考 InTn5,theelementsontheright,calledIS50R,iscapableofproducingatransposasetostimulatetransposition,buttheelementontheleft.CalledTn50L,isnot.ThisdifferenceisduetoachangeinasinglenucleotidepairthatpreventsIS50Lfromspecifyingtheactivetransposase. Sagiv精英名校考 WhenabacterialcellisinfectedwithnonlyticbacteriophagethatcarriesTn5onitschromosome,thefrequencyofTn5transpositionisdramaticallyreducediftheinfectedcellalreadycarriesacopyofThisreductionimpliesthattheresidenttransposoninhibitsthetranspositionof ingtransposon,possiblybysynthesizingarepressor. Sagiv精英名校考 TheIS50RelementofSagiv精英名校考 One,transposase,catalyzestransposition,whereastheother,atruncatedtransposase(截短的转座酶createdbytranslationfromastartcodonwithinthetransposase,preventsBecausetheshorterproteinisthemoreabundant,Tn5transpositiontendstobe SaglioQ75674 InfectionofE.colicellswithbacteriophagecarryingTn5.CellsthatalreadypossessacopyofTnrepressGeneticbasisofTn5regulation.OneoftheproteinsproducedbyIS50Risatransposasethatcatalyzestransposi-tion,buttheotherisarepressorthatinhibitstransposition.Theeffectoftherepressorusuallyprevails(优先的Sag精英名校考研 /Tn3TheelementsinthisgroupoftransposonsarelargerthanISelementsandusuallycontaingenesthatarenotnecessaryfortransposition—afeaturethatisalsocharacteristicofcompositetransposons(与复合转座子的相同点).However,unlikethecompositetransposons,theelementsdonothaveISelementsateachoftheirInstead,theTn3elementshavesimpleinvertedrepeats3040bplongattheirTheTn3elementsalsoproducetargetsiteduplicationwhentheyinsertintoDNA(与复合转座子的相同点). Sag精英名校考研 /GeneticorganizationofTherearethreegenes,TrpA,TrpB,andbla,respectively,atransposase,aresovase(解离酶)/repressor,andanenzymecalledbetalactamase(内酰胺酶).Thebetalactamaseconfers totheantibioticampicillin,andtheothertwoproteinsplayimportantrolesin Sag精英名校考研 /First,themediatesthefusionoftwocircularmolecules,onecarryingTn3andtheothernotTheresultingstructureiscalledacointegrate(共合体).Duringthisprocess,transposonisreplicated,andonecopyisinsertedateachjunctioninthecointegrate;thetwoTn3elementsinthecointegrateareorientedinthesame Thetranspositionof

occursintwo Sag精英名校考研 /Inthesecondstageoftransposition,thetnpR-encodedresolvasemediatesasite-specificbinationeventbetweenthetwoTn3ThiseventoccursatasequenceinTn3calledres,theresolutionsite,generatingtwomolecules,eachwithacopyofthetransposon.Tn3elementsthereforetransposebyareplicativemechanismthatinvolvestheformationofthetransitionalcointegratestructure.Theyareclassifiedasreplicative Sag精英名校考研 / ThetnpRgeneproductofTn3hasyetanotherfunction—torepresssynthesisofboththetransposaseandresolvaseproteins.RepressionoccursbecausetheressiteislocatedbetweenthetnpAandtnpRgenes.Bybindingtothissite,thetnpRproteininterfereswiththetranscriptionofbothgenes,leavingtheirproductsinchronicshortsupply.Asaresult,theTn Sagiv精英名校考 TheMedicalSagiv精英名校考 Manybacterialtransposonscarrygenesfor Consequently,itisarelativelysimplematterforthesegenestomovefromoneDNAmoleculetoanother—forinstance,fromachromosometoThisgeneticfluxhasaprofoundmedicalsignificancebecausemanyoftheDNAmoleculesthatacquire genescanbepassedintoother Sag精英名校考研 / toaparticularantibioticcanthereforebespreadhorizontallybetweenindividualsaswellasverticallyfromonegenerationtothenextinabacterialEventually,allornearlyallthebacterial eresistant,andtheantibioticisnolongerusefulincombatingtheorganism. Sag精英名校考研 /Thisprocesshasoccurredinseveralspeciesgenictohumans,includingstrainsofStaphylococcus(葡萄球菌),Enterococcus(肠球菌),Neisseria( Shigella(志贺氏菌andSalmonella(沙门氏菌Todaymanybacteriainfectionscausingdiseasessuchasdysentery(痢疾),tuberculosis(结核),and )aredifficulttotreat.Insomecases,thepathogenhastoseveraldifferentantibiotics,makingitparticularly Sag精英名校考研 /Thespreadofmultiple inpopulationshasbeenacceleratedbytheevolutionofconjugativeRplasmidsthatcarrytheTheseplasmidshavetwo transferfactor(抗性转移因子)orRTF,containsthegenesneededforconjugativetransferbetweencells.R-determinant(抗性决定),containsthegenesfor Oftenthese genesarecarriedbyatransposon,orasetoftransposons,thathasbeeninsertedintotheplasmid. al canbetransferredrapidlybetweencellsinabacterialConjugativeplasmidscan,infact,bepassedfromonespeciestoanother,evenbetweenquitedissimilarcelltypes—forexample,betweenacoccusandaThus,oncemultiplehasevolvedininpartofthemicrobialkingdom,itcanspreadtootherpartswith

Evolutionofplasmidscarryingfor Sagiv

Key/Insertionsequences(ISelements)arecut-and-pastertransposonsthatresideinbacterialchromosomeandplasmid./CompositetransposonsconsistsoftwoISelementsflankingaregionthatcontainsoneormoregenesforantibiotic.Tn3isareplicativetransposonthattransposesbytemporarilyfusingDNAmoleculesintoacointergrate;whenthecointegrateisresolved,eachoftheconstituentDNAmoleculesemergeswithacopyofTn3.Bacterialtransposonsaredemarcatedbyinvertedterminalrepeats;whentheyinsertintoaDNAmolecule,theycreateaduplicationattheinsertionsite.Conjugativeplasmidscanmovetransposonsthatcontaingenesforantibiotc fromonebacterialcellto Sag精英名校考研 /Section3.Cut-and-TransposonsinTransposableelementswerediscoveredbyyzinggeneticinstabilitiesinmaize; yseshavealsorevealedtransposableelementsinDrosophilia.Therearemanydifferenttypesoftransposonsineukaryotes.Theseelementsvaryinsize,structureandbehavior.Someareabundantinthegenome,othersrare. Sag精英名校考研 /AcandDsElementsinTheAcandDselementsinmaizewerediscoveredthroughthepioneeringworkofMcClintockin1948.SheshowedthattheactivitiesoftheseelementsareresponsibleforthestriandspottingofmaizeManyyearslater,theelementswereisolatedand

theirmolecularstructurewas Sag精英名校考研 /McClintockdiscoveredtheACandDselementsstudyingchromosomeSheusedgeneticmarkersthatcontrolledthecolorofmaizekernelstodetectthebreakageevents.WhenaparticularmarkerwaslostMcClintockinferred(推断thatthechromosomesegmentonwhichitwaslocatedhadalsobeenlost,anindicationthatabreakageeventhadoccurred.Theloseofamarkerwasdetectedbyachangeinthecolororaleurone(糊粉theoutermostlayerofthetriploidendosperm(胚乳)ofmaizekernels. Sag精英名校考研 /Inonesetofexperiments,thegeneticmarkerthatMcClintockfollowedwasanalleleoftheClocusontheshortarmofchromosome9.Becausethisallele,CI,isadominantinhibitorofaleuronecoloration,anykernelpossessingifisMcClintockfertilizedCCears(雌穗)withpollenfromCICItassels(雄穗),producingkernelsinwhichtheendospermwasCICC.Thetriploidendospermreceivestwoallelesfromfemaleparentandonefrommaleparent.Althoughmostofthesekernelswerecolorlessasexpected,someshowedpatchesofbrownish-purple(褐紫色)pigment. SagivMcClintockguessedthatinsuchmosaics,theinhibitoryCIallelehadbeenlostsometime

Xduringendosperm(胚乳

Maizekernel(topdevelopment,leadingtoashowinglossoftheCIallelecloneoftissuethatwasabletomakepigment.Thegenotypeinsuchaclonewouldbe–CC.

theinhibitionofinthealeurone.Thepigmentedpatchesare–CC,whereasthecolorlesspatchesareCICC. Sag精英名校考研 /ChromosomebreakagecausedbythetransposableelementDSinMaize.ThealleleContheshortarmofchromosome9producesnormalpigmentationinthealeurone;thealleleCIinhibitsthispigmentation. Sag精英名校考研 /Abreakatthesitelabeledbythearrowdetachesasegmentofthechromosomefromitscentromere,creatinganacentric(端着丝粒的)fragment.Suchafragmenttendstobelostduringcelldivision;thus,allthedescendantsofthiscellwilllackpartofthepaternallyderivedchromosome.BecausethelostfragmentcarriestheCIallele,noneofthecellsinthiscloneisinhibitedfromformingIfanyofthemproducesapartofthealeurone,apatchofcolorwillappear,creatingamosaickernel. Sag精英名校考研 /McClintockfoundthatthebreakageresponsibleforthesemosaickernelsoccurredataparticularsiteonchromosome9.ShenamedthefactorthatproducedthesebreaksDSforDissociation(分离).However,byitself,thisfactorwasunabletoinducechromosomebreakage.Onfact,Dshadtobestimulatedbyanotherfactor,calledAc,forActivator.TheAcfactorwaspresentinsomemaizestocksbutabsentinothers.Whendifferentstockswerecrossed,AccouldbecombinedwithDstocreatetheconditionthatledtochromosome Sag精英名校考研 /ManydifferentgeneticinstabilitiespresentmaizeBecausebreakageatthesesitesdependedonactivationbyAc,andtheDsfactorwerealsoinvolved,DscouldexistatmanydifferentsitesinthegenomeandthatitcouldmovefromonesitetoTheAcandDsbelongtoafamilyoftransposons.TheseelementsarestructurallyrelatedtoeachotherandcaninsertatmanydifferentsitesontheMultiplecopiesofAcandDselementsareoftenpresentinthemaizegenome. Sag精英名校考研 /BothAcandDscanWhenoneoftheseelementsinsertsinornearagene,thegene’sfunctionisaltered—sometimescomple yabolished.Thus,AcandDscaninducemutationsbyinsertingintogenes.Toemphasizethiseffectongeneexpression,theAcandDstransposonswerecalledcontrollingelements(控制元件). r7ph35aoo/familyoftransposableelementsinEachAcelementisalsoflankedbydirectrepeats8bplong.Becausethedirectrepeatsarecreatedatthetimetheelementisinsertedintothechro-mosometheyaretargetsiteduplications,notintegral(主要)partsofthe Sag精英名校考研 /UnlikeAc,Dselementsareheterogeneous(Ds元件具结构多样性TheyallpossessthesameinvertedterminalrepeatsasAcelements,demonstratingthatbelongtothesametransposonfamily,buttheirinternalsequencevary.SomeDselementsappeartohavebeenderivedfromAcelementsbylossofinternalsequences.Thedeletionintheseelementsmayhavebeencausedby pleteDNAsynthesisduringreplicationortransposition. Sag精英名校考研 /OtherDselementscontainnon-AcDNAbetweentheirinvertedterminalrepeats.TheseunusualmembersoftheAs/Dcfamilyarecalledaberrant(异常的)Dselements.AthirdclassofDselementsischaracterizedbyapeculiarpiggybackingarrangement(打包排列);oneDselementisinsertedintoanotherbutinaninvertedTheseso-calleddoubleDselementsareapparentlyresponsibleforthechromosomebreakagethatMcClintockobservedinherexperiments. Sagiv精英名校考 TheactivitiesoftheAc/Dselements—excisionandtransposition,andalltheirgeneticcorrelates,includingmutationandchromosomebreakage—arecausedbyatransposaseencodedbytheAcelements.TheActransposaseapparentlyinteractswithsequencesatorneartheendsofAcandDselements,catalyzingtheirmovement.Deletionormutationsinthegenethatencodesthetransposaseabolishthiscatalytic Sag精英名校考研 /Dselements,whichhavesuchlesions,cannotactivatethemselves.However,theycanbeactivatedifatransposase-producingAcelementispresentsomewhereinthegenome.Thetransposasemadebythiselementcandiffusethroughthenucleus,bindtoaDselement,andactivateit.TheActransposaseis,therefore,atrans-actingprotein. TransposesrelatedtotheAc/Dselementshavebeenfoundinotherspecies,includinganimals.Forexamplehobo(流浪者in很严 Sagiv精英名校考 PElementsandHybridSagiv精英名校考 SomeofthemostextensiveresearchdiscoveredcrossesbetweencertainstrainsofDrosophilaproducehybridswithanassortmentofaberranttraitsincludingfrequentmutation,chromosomebreakage,andsterility(不育).Thetermhybriddysgenesis,derivedfromGreekrootsmeaningadeteriorationinquality”(品质下降),wasusedtodenotethissyndromeof Sag精英名校考研 /Drosophilastrainscouldbeclassifiedintotwomaintypesbasedonwhetherornottheyproducedysgenichybridsintest-crosses.ThetwotypesofstrainsaredenotedMandP.OnlycrossesbetweenMandPstrainsproducedysgenichybrids,andtheydosoonlyifthemaleinthecrossisfromthePstrain.CrossesbetweentwodifferentPstrains,orbetweentwodifferentMstrains,producehybridsthatarenormal. Sag精英名校考研 /ThesefindingsuggestedthatthechromosomesofstrainscarrygeneticfactorsthatareactivatedwhentheyentereggsmadebyMfemales,andthatonceactivated,thosefactorsinducemutationsandchromosomebreakage.Inspiredbythiswork,Engelsbegantostudymutationsinducedindysgenichybrids.In1979Engelsfoundaparticularmutationthatrevertedtowild-typeatahighrate.Thisinstability,whichisreminiscentofbehaviorofISinducedmutationinE.coli,stronglysuggestedthatatransposableelementwasinvolved. Tey47133698/nesis-inducedmutationsinthewhitegeneallowedthetransposonhypothesistobetested.In1980,SimmonsandLimrespectivelysentthenewlydiscoveredwhitemutationstoPaulBingham,ageneticistinNorthCarolina.BinghamandGeraldRubin,ageneticistinMaryland,hadjustfinishedisolatingDNAfromthewhitegene.UsingwhitegeneDNAasaprobe,BinghamandRubinwereabletocompareittothewild-typewhiteDNA.Ineachmutation,theyfoundthatasmallelementhadbeeninsertedintothecodingregionofthewhitegene. Sag精英名校考研 /AdditionalexperimentsdemonstratedthattheseelementsarepresentinmultiplecopiesandatdifferentlocationsinthegenomesofPstrains;however,theyarecomple yabsentfromthegenomesofMstrains.GeneticiststhereforebegancallingthesePstrain-specifictransposonsPelements. ysishasshownthatPelementsvaryinsize.Thelargestelementsare2907bplong,includingterminalinvertedrepeatsof31bp.ThesecompletePelementscarryagenethatencodedatransposase. Sag精英名校考研 /WhentransposasebindsneartheendsofacompletePelement,itcanmovethatelementtoanewlocationinthegenome. pletePelementlacktheabilitytoproducethetransposasebecausesomeoftheirinternalsequencesaredetected;however,theydopossesstheterminalandsubterminal(靠近末端的)sequencesthatbindtheConsequently,theseelementscanbemobilizedifatransposase-producingcompleteelementispresentsomewhereinthegenome. Sag精英名校考研 /StructureofPelementsinDrosophilashowingorientationsandlengthsofDNAsequences. Sag精英名校考研 /SurveysofnaturalpopulationsofconductedbyDominiqueAnxolabéhèreinFrancehavedemonstratedthatthereisconsiderablevariationinthenumberofPelementsinthechromosomes.Someflieshaveasmanyas50,whereasothershaveonlyafew.Perhapsthemostsurprisingdiscoveryisthatfliesderivedfromstrainscapturedbefore1950havenoPelementsatall(1950年以前的果蝇没有P元件).Kidwellhassuggestedthatthese“empty”strainsrepresenttheprimitiveconditionandthatPelementshaveinvadednaturalpopulationsofDrosophiladuringrecenttimes. Sag精英名校考研 /Curiously,theclosestrelativeofD.melanogasterhavep the“empty”condition,butother,moredistantlyrelatedspecieshaveacquiredPItisnotpossibletosayhowthesespeciesacquiredtheirPelements,butonepossibilityisthattheelementswerecarriedintothegenomeby thatnaturallyinfectDrosophila.Suchaprocesswouldbe ogous(类似于)tothetransductionofEcolicellsbyabacteriophagethatcarriedanISelement.PopulationofDrosophilathatpossessPelementshaveevolvedmechanismstoregulatetheir Sag精英名校考研 /Insomestrains,thisregulationdependsoncytotype(细胞型),acellularconditionthatistransmittedmaternallythoughtheeggcytoplasm.ThePcytotyperepressesPelementmovement,andtheMcytotypepermitsit.PcytotypeischaracteristicofstrainsthathavePelementsontheirchromosomes,whereasMcytotypeischaracteristicofstrainsthatlackPWhenPelementsarecombinedwiththeMcytotypebymakingappropriatecrosses,theyareinducedtotranspose. 精lthghtheregulatoryroleofcytotypeh/asbeenstudiedbynumerousresearchers,itsmolecularnatureisstillunknown.ThematernaltransmissionofcytotypecanbeseenintheoffspringofreciprocalcrossesbetweenPandMcytotypestrain.Therearetwo①PcytotypefemaleXMcytotype②PcytotypemaleXMcytotypeTheoffspringfrombothcrossesinheritPelementsontheirchromosomeandare,infact,genotypicallyidentical;however,onlythosefromthesecondcrossallowPmovement. Sag精英名校考研 /Pelement-mediatedhybriddysgenesisinCytotypeisinheritedmaternally.ThePcytotyperepressedPelementsmovement,whereastheMcytotypeDysgenesisoccursonlyinMcyrotypehybridsofCross2. Sag精英名校考研 /ThisdifferencebetweenthereciprocalcrossesindicatesthattheconditionthatpermitsorrepressesPmovementistransmittedonlybythefemaleparent,presumableintheeggcytoplasm.Incross1,thefemalestransmitthePcytotypetotheirprogeny,whichthenrepressesPmovement,whereasinCross2,thefemalestransmittheMcytotype,whichallowsPmovement.GiventhedamagethatcanbecausedbyextensivePelementmovement,itmayseemsurprisingthatPmaleXMfemalecrossesproduceanyviableprogenyatall. Sag精英名校考研 /ManyresearchershaveshownthattheseprogenyarehealthbecausePelementsmoveonlyinthegermline.Inthesomatictissues,wherePelementmovementwouldcauseseriousproblems,thereisalittle,ifany,transpositionbecausethePtransposaseisnotsynthesizedthere.ThemetabolicblockoccursatthelevelofRNAsplicing;oneoftheintronsremainsinthetranscriptofthetransposasegene,creatingastopcodonthatprematurelyterminatestranslation.Thus,insteadofmakingthetransposase,somaticcellssythesizeashorterproteinthatdoesnothavethetransposase.scatalyticfunction. Sag精英名校考研 /GeneticistsroutinelyusehybriddysgenesisobtainPinsertionmutationsintheDysgenesishybridsproducedbycrossingPmalewithMfemalesarematedtorecovermutationsthathaveoccurredintheirgermline.Thesesmutationsaredetectedbyappropriatetechniques,suchasMuller’sCIBcrossingHybriddysgenesishasanadvantageovertraditionalmethodsofinducingmutationsbecauseagenethathasbeenmutatedbytheinsertionofatransposableelementis“tagged”withaknownDNAsequence. Sag精英名校考研 /Thetransposetagmaysubsequentlybeisolatethegenefromalarge,heterogeneousmixtureofDNA.Mutagenesisbytransposontagging(通过转座子标记发生的突变)isthereforea