SomepredictedintermediatesoftheSzostakmodelsuchasDSBsanddHJshavebeen - PDF document

SomepredictedintermediatesoftheSzostakmodel,suchasDSBsanddHJs,havebeenobservedinthebuddingSaccharomycescerevisiaeandDSBshavealsobeendetectedintheÞssionyeastSchizosaccharomycespombeandinferredinmicemice.However,recentworkindiffer-entmodelorganismshasledtotheconclusionthattheSzostakmodelisnotageneralmodelformeioticrecombi-nationbutapplies,withrevision,onlytoCOformation.COsandNCOsarisethroughdifferentbranchesoftherecombinationpathwayMutationspreventingDSBformationorprocessingeliminatebothCOsandNCOs,indicatingthatearlystepsinCOandNCOformationproceedbythesamepathway.IntheSzostakandpreviousmodels,thebranchingoftherecombinationpathwaytoproduceCOsorNCOswashypothesizedtooccurataverylatestage.Inthesemodels,NCOsandCOsbotharisefromthesameHJ-containingintermediate,dependingontheorientationofHJresol-ution(Figure1a).Animportantconclusionofrecentstu-diesisthatthisideaisincorrectandthat,instead,thepathwaybranchessoonafterDSBformationandproceedsthroughdifferentDNAintermediatestoproduceeitherCOsorNCOsNCOs.ThesenewconclusionsarebasedonthestudyofmutationsaffectingstepsinrecombinationoccurringafterDSBformationandprocessingandonthestudyofphysicalrecombinationintermediatesandpro-OnepredictionoftheSzostakmodelisthatCOsandNCOsshouldoccuratthesametime.Usingphysicalassaysinbuddingyeast,astudyofectopicrecombinationrecombinationshowedthatthisisnottrueandthatNCOsformbeforeCOs.Similarly,repairDNAsynthesisassociatedwithNCOsinbuddingyeastoccursearlierthandoesthatassociatedwithCOsCOs.DSBsareprecursorstobothCOsandNCOs,buttheotherobservedintermediatesoftheSzostakmodel(D-loopsanddHJs)appeartolieonlyontheCOpathway.Inbuddingyeast,thetimingofdHJappearanceanddis-appearancecomparedwiththetimingofCOandNCOformationsuggeststhatdHJsareprecursorsonlyofCOsCOs.MutantswithreducedSEIordHJformationhavereducedCObutnotNCOfrequencyfrequency.InbothÞssionÞssionandbuddingbuddingyeasts,mutationscausingdefects Figure1.SzostakmodelofCOandNCOformationandthesynthesis-dependentstrand-annealing(SDSA)modelofNCOformation.IntheSzostakmodel,aDSBoccursononehomologouschromosome(blue)andthetwoduplexendsofthisDSBareprocessedtoformsingle-strand3overhangs(i).OneendoftheDSBinvadesanotherhomologouschromosome(red)andsetsupaSEIordisplacement-loop(D-loop)structure(ii).DNAsynthesisextendstheD-loopandenablesannealingofthesecondsideoftheDSB.FurtherDNAsynthesisfillsintheremaininggapsandadoubleHJstructureisformed(iii).Cleavageandre-annealing,or‘resolution’,ofeitherpairoflike-orientedstrandsineachHJ(‘1’or‘2’),occursatrandom.Inthefigure,theleftwardHJhasbeenresolvedinorientation‘1’;resolutionoftherightwardHJinorientation‘1’thenproducesaNCO(iv)andinorientation‘2’aCO(v).Notethat,intheSzostakmodel,newlyreplicatedDNA(brokenlines)occursonbothhomologouscterminatedattheDSBsiteoneach(iii).IntheSDSAmodel,aDSBisformedandprocessedasintheSzostakmodel(i).OneendoftheDSBtransientlyinvadesanotherhomologouschromosomeandisextendedbyDNAsynthesis(ii).ThisendthenpullsoutandthenewlysynthesizedextensionannealswiththeotherendoftheDSB(iii).FurtherrepairDNAsynthesisoccursandaNCOisformed(iv).NoHJresolutionhasoccurred.Notethat,intheSDSAmodel,newlyreplicatedDNA(brokenlines)occursonlyononehomologouschromosomeandspanstheDSBsite(iii).Inbothmodels,geneconversionscanoccurbymismatchcorrectioninanyregionofheteroduplexDNA,shownasduplexescontainingoneblueandoneredstrand. TRENDSinCellBiologyVol.xxxNo.x TICB-453;NoofPages8 Pleasecitethisarticleinpressas:Cromie,G.A.andSmith,G.R.,Branchingout:meioticrecombinationanditsregulation,TrendsCellBiol.(2007),doi:10.1016/j.tcb.2007.07.007www.sciencedirect.com inHJresolutionreduceonlyCOfrequency,arguingthatCOsbutnotNCOsarisefromHJresolution.MutationsspeciÞcallyreducingCObutnotNCOformationhavealsobeenidentiÞedinmicemiceandthefruitßyßy.ItthereforeappearsthattheÔdecisionÕtoformCOsorNCOsisearly,afterDSBformationbutbeforeHJresolutionorevenSEIformation.Todate,DNAintermediatesspeciÞctotheNCOpathwayhavenotbeenIthasbeensuggestedthatNCOsoccurbysynthesis-dependentstrand-annealing(SDSA),whichisamechanisminvolvingstrandinvasion,DNAsynthesisandstrandÔpulloutÕthat,cruc

ially,doesnotinvolveHJs(Figure1b).Arecentstudyinbuddingyeastyeastusedthymidine-analogincorporationtodetectDNAsynthesisoccurringduringmeioticrecombination.Thisstudydeter-minedthepatternofDNAsynthesisataDSBhotspotusingDNAcombing,inwhichindividualDNAmoleculesareextendedonaglasscoverslip.ThecombedDNAwasprobedforthymidine-analogincorporationandposition-deÞninglociandthenexaminedbymicroscopy.Usingthistechnique,itwasobservedthatthepatternofDNAsyn-thesisassociatedwithCOsisthatexpectedfromtheSzostakmodel,whereasthepatternofDNAsynthesisassociatedwithNCOsisthatexpectedfromSDSAFigure1).Therefore,itappearsthattheSDSAmodelisvalidforNCOrecombinationandtheSzostakmodel,modiÞedtoassumestronglybiasedHJresolutiontoCOs,isessentiallyvalidforCOrecombination.Thereismorethanonepathwaytocrossing-overAswellasCOsandNCOsarisingbyseparatepathways,itisalsobecomingclearthatthereareatleasttwodifferentpathwaystoformingCOs,atleastinbuddingyeast.OnepathwaydependsontheMsh4ÐMsh5complexandissub-jecttoaformofspatialCOregulationcalledinterference(discussedlater),inwhichoneCOpreventsadditionalCOsfromoccurringnearby.TheotherpathwaydependsontheMus81ÐEme1complexandisnotsubjecttointerference.Bothpathways,aswellasthepathwaytoformingNCOs,areinitiatedbyDSBs(Figure2).Comparisonofexper-imentalresultsfromdifferentmodelorganismshasbeenparticularlyimportantindecipheringthisstory.InÞssionyeast,meioticHJresolutionappearstobecarriedoutsolelybytheMus81ÐEme1endonucleasecom-com-.Mus81andEme1(calledMms4inbuddingyeast)arewidelyconservedandinvitroofcomplexesfromÞssionyeast,buddingyeastandhumansindicatesthattheyallhaveHJresolutionactivityactivity19].However,miceareviableandfertilefertileandtheeffectofamutationinbuddingyeastisonlyamildreductioninCOfrequencyfrequency.TheexplanationforthesephenotypicdifferencesisthatÞssionyeastandsomeothereukaryotes,includingfruitfruit,lackanalternativeCOpathwaythatappearstobeactiveinmammalsandbuddingyeast.ThispathwaydependsonagroupofproteinscalledÔZMMproteinsÕ(seelater)thatincludeMsh4andMsh5,sothat,inbuddingyeast,mutationsaffectingboththeMus81andMsh4ÐMsh5pathwayshaveamoreseverereductioninCOfre-quencythandotheindividualmutationsmutations.Msh4andMsh5arehomologuesofthebacterialMutSmismatchrepairproteinbut,insteadofactinginmismatchrepair,theyformaslidingclampthatrecognizesHJsspeciÞcallyspeciÞcally.Msh4andMsh5areapparentlyabsentfromÞssionyeastandfruitßies.ItappearsthattheMsh4ÐMsh5,butnottheMus81ÐEme1,pathwayissubjecttoCOinterference.Thisaffectsthepatternofinterferenceseenineachorganism,dependingonwhethertheorganismhasbothpathwaysoronlyone(Figure2).InthewormCaenorhabditiselegansonlytheMsh4ÐMsh5pathwayappearstobeactiveandCOsaresubjecttoexceptionallystronginterferenceinterference(seelater).Bycontrast,inÞssionyeast,whereonlytheMu81ÐEme1pathwayispresent,thereisnono.Inbuddingyeast,theMus81ÐEme1-de-pendentCOslackinterference,whereasthosedependentonMsh4ÐMsh5aresubjecttointerferenceinterference.PlantsandmammalsappeartohavetheproteinsrequiredforbothCOpathwaysand,consistentwiththis,appeartohavebothinterference-sensitiveand-insensitiveclassesofCOsCOs.Inorganismsinwhichbothpathwaysarepre-sent,theirrelativeimportancecanvary.Mutationsaffect-ingeitherpathwayinbuddingyeasthavesimilar,moderatedefectsinrecombinationrecombinationbutmus81/micearefertilefertile,whereasmiceareare.SingleanddoubleHollidayjunctionsWhatistherelationshipbetweenthetwoCOpathwaysandtheSzostakpathwayofDNAintermediates?BothCOpathwaysareinitiatedbyDSBsandtwo-dimensionalgelelectrophoreticstudiesinbuddingyeast,inwhichbothCOpathwaysoperate,identiÞedtheSEIanddHJintermedi-atespredictedbytheSzostakmodelmodel.BuddingyeastdHJshavealsobeenseenbyelectronmicroscopy Figure2.ThreedifferentpathwaysproduceCOs(withorwithoutinterference)andNCOs.AllthreepathwaysareinitiatedbyDSBs.NCOsappeartobeproducedbytheSDSAmechanismbuttheproteinsspecificallyinvolvedinthispathwayareunknown.WorkfrombuddingyeastindicatesthatCOswithinterferenceareproducedbytheSzostakmechanism,progressingthroughdHJsandspecificallyusingtheZMMproteins,includingMsh4–Msh5.Workfromfissionyeastandbuddingyeas

tindicatesthatCOswithoutinterferenceareproducedbyamechanismthatmightinvolvesHJsandspecificallyrequirestheMus81–Eme1proteins.ThismechanismmightbeessentiallythesameastheSzostakmodel,modifiedtoproducesHJs. TRENDSinCellBiologyVol.xxxNo.x3 TICB-453;NoofPages8 Pleasecitethisarticleinpressas:Cromie,G.A.andSmith,G.R.,Branchingout:meioticrecombinationanditsregulation,TrendsCellBiol.(2007),doi:10.1016/j.tcb.2007.07.007www.sciencedirect.com Bycontrast,electronmicroscopicandtwo-dimensionalgelelectrophoreticstudiesinÞssionyeast,inwhichonlytheMus81pathwayisactive,observedmostlysingleHJs(sHJs),recombinationintermediatesnotpredictedbytheSzostakmodel,andonlyaminorityofdHJsdHJs.ThesmallnumberofÞssionyeastdHJsmightrepresentcloselyspacedsHJsbecauseCOinterferenceisnotactiveinthatorganism.Interestingly,asizableminorityofsHJsisalsoseenbyelectronmicroscopyinbuddingyeast.Itisthereforepossiblethat,althoughthetwoCOpathwaysarebothinitiatedbyDSBs,theymightproceedthroughdiffer-entDNAintermediates:sHJsfortheMus81pathwayanddHJsfortheMsh4ÐMsh5pathway(Figure2).AsHJmightarisebycuttingoftheD-loop(Figure1aii)before,ratherthanafter,captureofthesecondendoftheDSB.Similarproposals,thatis,thatMus81ÐEme1actsonstructuresdifferenttothedHJsoftheMsh4ÐMsh5pathway,speciÞ-callyunligatedorpartialHJs,havebeenadvancedbasedontheinvitroactivitiesoftheMus81ÐEme1complexcomplex.RegulationofCOnumberandpositionCOcontrol,thatis,theregulationofCOpositionandnumber,isspeciÞedatseveraldifferentlevels.Mostfun-damentally,theDSBsthatinitiaterecombinationaredis-tributednon-randomly.Inmanyorganisms,thereisevidencethatDSBsareconcentratedatparticularsitescalledhotspotshotspots.Inrecentyears,severalstudieshaveexaminedthegenome-widedistributionofmeioticDSBsinbuddingyeastusingSpo11-basedchromatinimmunopre-cipitation(ChIP)andmicroarrayanalysis,atechniquetermedChIPÐchip.Inthesestudiesmicroarrayswithalargenumberofoligonucleotideprobes,givinggenome-widecoverage,areusedtodeterminewhichlociareenrichedspeciÞcallyduringmeiosisbyChIPofSpo11.Spo11isthetopoisomerase-likeactive-siteproteinthatgeneratesmeioticDSBsand,hence,theChIP-enrichedlociareinferredtobethesitesatwhichDSBshaveoccurred.Suchgenome-widestudiesinbuddingyeastindicatethatDSBhotspotstendtooccurinpromoterregionsandaresomewhatsuppressedincentromericandtelomerictelomeric.ThisconÞrmspreviousconclusionsdrawnfromSouthernblotanalysisofselectedregionsofthebuddingyeastgenomegenome.Bycontrast,SouthernblotandChIPanalysesoftheÞssionyeastgenomeindicatethatDSBsitesareseparatedmorewidelywidely.COsandNCOsbotharisefromDSBsandthechoiceofwhichDSBsgenerateCOsprovidesanadditionallevelofCOcontrol.Inmost,butnotall,organisms,meioticCOsaredistributedhighlynon-randomly.ThetotalnumberofCOsislowbutevensmallchromosomesreceiveatleastone.Two,possiblylinked,effectscontributetothisdistri-bution.First,anobligateCOperchromosomeoccursand,second,agradientofsuppressionaroundeachCOdecreasestheprobabilityofadditionalCOsbutnotDSBsorNCOs.ThesuppressiveeffectistermedCOinterferenceFigure3Forsomeyears,itseemedlikelythatCOinterferencewasmediatedbythesynaptonemalcomplex(SC).Theonlyorganismsknowntolackinterference,thefungiS.pombeAspergillusnidulans,alsolackSCs,andabuddingyeastmutationaffectingZip1,anSCcomponent,abolishesboththeSCandinterferenceinterference.TheSCisaprotein-aceousstructurethat,inmostorganisms,linksthecoresofpairedhomologouschromosomesduringmeiosisII.Itconsistsoftwolateralelements,onefromeachhomologouschromosome,heldtogetherbyacentralelement(Figure3Eachlateralelementisaproteinaceousstructureformingasinglecoreoraxisencompassingbothsisterchromatidsofonehomologouschromosome.BeforeformationofthematureSC,lateralelementsarecalledaxialelements. Figure3.COinterferenceandthesynaptonemalcomplex.COinterference.Recombinationoccursbetweenhomologouschromosomes,eachconsistingoftwosisterchromatids,asshown.Azoneofinterference(blackelongatedtriangles)iscenteredatpositionswhereDSBsareprocessedtogiveCOs.Inthiszone,furtherCOsaresuppressedtovaryingdegrees,withDSBsinsteadbeingprocessedtogivemoreNCOs.Theinterferenceeffectisstrongestatthesiteofth

eCOanddeclineswithdistance.Structureofmeioticchromosomesheldtogetherbythesynaptonemalcomplex.Eachchromosome,consistingoftwosisterchromatids,formsasingleaxis.TheaxialelementsareheldtogetherbyacentralelementinthematureSC,formingatripartiteproteinaceousstructure,whichjoinsthetwohomologouschromosomestogether.Inthisstructure,thedescendentsoftheaxialelementsarecalledlateralelements.LoopsofDNAareshownemergingfromtheselateralelementsfortwoofthefourchromatids(DNAloopsintheothertwoarenotshown). TRENDSinCellBiologyVol.xxxNo.x TICB-453;NoofPages8 Pleasecitethisarticleinpressas:Cromie,G.A.andSmith,G.R.,Branchingout:meioticrecombinationanditsregulation,TrendsCellBiol.(2007),doi:10.1016/j.tcb.2007.07.007www.sciencedirect.com RecentresearchindicatesthatthepatternofinterferenceisactuallysetupbeforetheSCisformed.TheZip2andZip3proteins,possiblycomponentsofaubiquitin-conjugatingcomplex[48],aremembersofthegroupofÔZMMÕproteinsthatarerequiredspeciÞcallytoproduceCOsthataresubjecttointerference[10].Msh4ÐMsh5isamemberofthisgroupandcolocalizeswithZip2ÐZip3[49].BuddingyeastZip2ÐZip3fociappeartomarkthesiteswhereinterference-sen-sitiveCOswilloccur.ThesefocihavearelativefrequencyanddisplayadegreeofinterferencematchingthoseoftheCOs,evenwhenthenumberofCOsisincreasedordecreasedbyappropriatemutations[50Ð52].MouseMsh4ÐMsh5focialsoshowinterference[53].BecauseZip2ÐZip3fociarealsothesitesatwhichSCformationisnucleated[54,55]thisindicatesthatatleastthisaspectofinterferencepre-cedesformationoftheSC.Instead,itappearsthattheSCisinitiatedatthesitesofCOssubjecttointerference.IfnotthroughtheSC,howisinterferencemediated?RecentevidencefromthewormC.eleganssuggeststhatthephysicalstructureofthechromosomalaxiscanpropa-gateinterference.InC.elegans,onlytheMsh4ÐMsh5pathwayisactiveandanextremeformofinterferenceensuresthatessentiallyeverychromosomehasoneandonlyoneCOduringmeiosismeiosis.However,whentwooreventhreechromosomesarefusedend-to-end,theresult-ingfusions,ratherthanexperiencingtwoorthreemeioticCOs,respectively,stillusuallyexperienceoneandonlyoneone(Figure4).ItappearsthatC.elegansCOcontrolactsonchromosomesasaunit,independentoflength,toensurethateachreceivesonlyoneCO.Continuityofchromosomeaxesisessentialtothemechanismofinterference.Whenfusedchromo-somesundergorecombinationwithtwounfusedchromo-somes,sothatoneofthetwoÔhomologuesÕisÔbrokenÕandthereforedoesnothaveacontinuousaxis,thenumberofCOsincreases(Figure4).Inaddition,amutationlimitingthelevelofthemeiosis-speciÞcchromosome-axiscom-ponentHim3increasesthefrequencyofchromosomepairsthatexperiencedoubleCOsCOs.Acurrentmodelsuggeststhatstressforcesintheaxesofmeioticchromo-somespromotemeioticCOs,which,inturn,relieveaxialstresslocally,suppressingfurtherCOs,thatis,generatinggenerating.However,howthiswouldaccountforotheraspectsofinterference,suchasthatofZip2ÐZip3foci,isunclear.Recently,anentirelynewlevelofCOregulationinbuddingyeasthasbeenreported:aphenomenontermedÔcrossoverhomeostasisÕ.ThisworkwascarriedoutusinghypomorphicallelesofSpo11,whichreducetheDSBfre-quencytovaryingdegrees.AstheDSBfrequencyisreduced,thefrequencyofCOsdoesnotfallintandem;rather,COnumbersaremaintainedandNCOsarereducedpreferen-tiallyinstead[58].ThissupportsmodelsinwhichaÞxednumberofCOsarederivedfromtheÔpoolÕofallDSBs,withtheremainderoftheDSBsproducingNCOs[57,58]WhetherCOhomeostasisexistsinorganismsotherthanbuddingyeasthasnotyetbeendetermined.Severaldifferentnon-randomdistributionscanbeobservedinmeiosis.TheseincludethepositionsofDSBs,whichsubsetofDSBsformCOs,thenumberandlocationsofCOs,andthepositioningofproteincomplexes,suchasZip2ÐZip3.Unifyingmodelsthatexplainmultiplenon-randomdistributions,seenacrossarangeofmodelorgan-isms,intermsofoneorasmallnumberofconserved,underlyingprocessesareattractiveattractive.However,itisalsopossiblethatmorethanonefundamentalmechanismisinvolvedand,hence,theremightbemorethanonetypeofinterferenceactingduringmeiosis.Fundamental Figure4.IntactchromosomeaxesarerequiredfortransmissionofCOinterferenceinthewormC.elegans.EachpairofhomologouschromosomesusuallyundergoesoneandonlyoneCOpermeiosi

s.Whentwo,oreventhree,chromosomesarefused,theresultinglargechromosomesstillusuallyundergooneandonlyonecrossoverperpairduringmeiosis.Thisindicatesthat,forthepurposesofinterference,intactchromosomesaretreatedasasingleunit,irrespectiveofsize.Bycontrast,ifafusionoftwoorthreechromosomesundergoesrecombinationwithtwounfusedhomologouschromosomes,morethanoneCOpermeiosisoccurs.Thisisequivalenttorecombinationbetweentwofusedchromosomesbutwhereonehasanaxialdiscontinuity.Therefore,anintactchromosomeaxisisnecessaryfortransmissionof TRENDSinCellBiologyVol.xxxNo.x5 TICB-453;NoofPages8 Pleasecitethisarticleinpressas:Cromie,G.A.andSmith,G.R.,Branchingout:meioticrecombinationanditsregulation,TrendsCellBiol.(2007),doi:10.1016/j.tcb.2007.07.007www.sciencedirect.com mechanisticdifferencesmightalsoexistamongspecies.Forexample,inmice,proteincomplexesinvolvedmostlyinNCOormostlyinCOformationbothshowinterference,ofdifferentstrengthsstrengths.Inaddition,theproteincomplexesassociatedwiththesitesofCOsinmiceshowinterferenceintheabsenceofintactaxialelements,aswellasintheabsenceofintactSCSC.Choiceofpartner-DNAmoleculeforcrossing-overAnimportantfunctionofmeioticrecombinationisthegenerationofCOsbetweenhomologouschromosomes.How-ever,whenaDSBoccurs,itdoesnothavetoberepairedagainstthehomologouschromosome:recombinationwiththesisterchromatidcanalsooccur(Figure5).Inaddition,ifhomologyexistselsewhereinthegenome,ectopicrecombi-nationwithanon-alleliclocuscantakeplace.Whatdeter-minesthechoiceofpartnerinmeioticrecombination?PartnerchoiceinmeioticrecombinationcanbestudiedatDSBhotspotsusinghomologouschromosomeswithdifferentpatternsofrestrictionsitesatahotspotlocus.Thisenablesintersisterrecombinationintermediatestobedistinguishedfrominterhomologuerecombinationinter-mediates.SuchphysicalstudiesofDNAintermediatesatabuddingyeastmeioticrecombinationhotspotindicatedthatinterhomologuerecombinationisgreatlyfavoredoverintersisterrecombination,incontrasttothegeneralbiasforintersistereventsseenduringmitosismitosis.Becausethismakessenseintermsofpromotingproductiveinterhomo-logueevents,itwaspresumedtobeauniversalfeatureofmeiosis.However,recentresearchinÞssionyeasthasdisprovedtheuniversalityofthisconclusion:intersisterrecombinationisfavoredoverinterhomologuerecombina-tionatameioticrecombinationhotspothotspot.Mechanistically,thedifferenceinpartnerchoicebetweenbuddingandÞssionyeastappearstobeexplainedbythepresenceofabarriertointersisterrecombinationthatispresentintheformerbutnotinthelatter(Figure5ThisisseenmostclearlywhencomparingthebehaviorofmutationsaffectingDmc1,ameiosis-speciÞcstrand-exchangeprotein.Inmutantsofbothyeasts,inter-homologuerecombinationisreduced.However,inÞssionyeast,essentiallyeveryDSBisstillrepaired,presumablyagainstasisterchromatidchromatid,whereas,inbuddingyeast,DSBsremainunrepaired,indicatingtheexistenceofabarriertointersisterrepairrepair.LikeÞssionyeast,plantsappeartolacktheintersisterrecombinationbarrierseeninbuddingyeastyeast.ThenatureofthebuddingyeastbarrierisunclearbutinvolvestheMek1proteinkinaseandtheHop1andRed1axial-elementcomponents.Mutationofremovesthebarriertointersisterrecombina-tioninabuddingyeastyeast.Inadditiontotheintersisterrecombinationbarrierinbuddingyeast,interhomologuerecombinationmightbeupregulatedspeciÞcallybytheHed1proteininthatorgan-ism.ThisproteinpromotestheuseofDmc1bysuppressingrecombinationcarriedoutsolelybytheconstitutivelyexpressedRad51strand-exchangeproteinprotein.Hed1hasnotyetbeencharacterizedinothermodelorganisms.Asmentionedearlier,meioticrecombinationcanoccurbetweenhomologousDNAsequencespresentatdifferentplacesonthegenome(ectopicrecombination),aswellasbetweenthesamelocusonsisterchromatidsorhomolo-gouschromosomes(allelicrecombination).Generally,ecto-piceventsoccurlessfrequentlythanallelicevents,although,inbuddingyeast,someectopicrecombinationratesapproachallelicfrequenciesfrequencies.Ingeneral,buddingyeastappearstohavemoreformsofcrossovercontrol,suchasinterferenceandthebarriertointersisterrecombination,thandoesÞssionyeast.WhyaretheseformsofregulationnecessarywhenmeiosisinÞssio

nyeastoccurssuccessfullywithoutthem?COcontrolappearstoexisttolimitthetotalamountofrecombination,implyingthatCOscanbedeleteriousifoccurringtoofrequently.Fissionyeast,withanunusuallysmallnumberofrelativelylargechromosomes,needsfewertotalCOs,intheabsenceofCOcontrol,toensureatleastoneCOperchromosomepermeiosis.Thisnumberispresumablylowenoughtoavoiddeleteriouseffectsand,hence,theneces-sityforCOcontrol.However,theseissuesarefarfromsettledandremaintopicsofdebate.ConcludingremarksRecentresearchhasgreatlyexpandedourunderstandingofhowmeioticCOsform,howtheirdistributionandnum-berareregulatedandhowCOpartnerchoiceismade.NovelpathwaysofCOandNCOformationhavebeenidentiÞed,alongwithnewformsofCOregulation.Inparticular,thesingleSzostakmodelforCOandNCOformationhasbeenreplacedbyatleastthreebranchesleadingfromaninitiatingDSBtoproduceNCOsorinter-feringornon-interferingCOs.Thefullerpicturenowemer-ginghighlightsthemechanisticdifferencesbetweenmeioticrecombinationindifferentorganismsandindicatesthattheideaofasinglepathwayofmeioticrecombinationisobsolete.Futurestudieswillconcentrateonexploringthenewpathwaysthatrecentresearchhasrevealedand Figure5.Partnerchoiceduringmeioticrecombination.RecombinationinitiatedbyaDSBononechromatidcanoccurwiththehomologouslocusontheother(sister)chromatidofthesamechromosomeorwiththehomologouslocioneitherchromatidoftheotherhomologouschromosome.Inbuddingyeast,astrongbiastointerhomologuerecombinationisseeninmeiosismeiosisbut,bycontrast,infissionyeast,abiastointersisterrecombinationisseenseen.Thebiastointerhomologueeventsinbuddingyeastseemstobetheresultofabarriertointersisterrecombinationthatisabsentfromfissionyeast. TRENDSinCellBiologyVol.xxxNo.x TICB-453;NoofPages8 Pleasecitethisarticleinpressas:Cromie,G.A.andSmith,G.R.,Branchingout:meioticrecombinationanditsregulation,TrendsCellBiol.(2007),doi:10.1016/j.tcb.2007.07.007www.sciencedirect.com understandingthecontrastingmeioticrecombinationmechanismsandtheircontrolindifferentorganisms.WethankSueAmundsenandLutherDavisforcriticalreadingofthemanuscript.OurresearchissupportedbygrantsR01GM031693andR01GM032194fromtheNationalInstitutesofHealth.1Szostak,J.W.etal.(1983)Thedouble-strand-breakrepairmodelfor33,25Ð352Sun,H.etal.(1989)Double-strandbreaksataninitiationsiteformeioticgeneconversion.338,87Ð903Schwacha,A.andKleckner,N.(1995)IdentiÞcationofdoubleHollidayjunctionsasintermediatesinmeioticrecombination.83,783Ð7914Cervantes,M.D.etal.(2000)MeioticDNAbreaksassociatedwithrecombinationinS.pombeMol.Cell5,883Ð8885Qin,J.etal.(2004)MousestrainswithanactiveH2-EameioticrecombinationhotspotexhibitincreasedlevelsofH2-Ea-speciÞcDNAbreaksintesticulargermcells.Mol.Cell.Biol.24,1655Ð16666Mahadevaiah,S.K.etal.(2001)RecombinationalDNAdouble-strandbreaksinmiceprecedesynapsis.Nat.Genet.27,271Ð2767Allers,T.andLichten,M.(2001)Differentialtimingandcontrolofnoncrossoverandcrossoverrecombinationduringmeiosis.8Guillon,H.etal.(2005)CrossoverandnoncrossoverpathwaysinmouseMol.Cell20,563Ð5739Terasawa,M.etal.(2007)Meioticrecombination-relatedDNAsynthesisanditsimplicationsforcross-overandnon-cross-overrecombinantformation.Proc.Natl.Acad.Sci.U.S.A.10Borner,G.V.etal.(2004)Crossover/noncrossoverdifferentiation,synaptonemalcomplexformation,andregulatorysurveillanceattheleptotene/zygotenetransitionofmeiosis.117,29Ð4511Osman,F.etal.(2003)GeneratingcrossoversbyresolutionofnickedHollidayjunctions:aroleforMus81-Eme1inmeiosis.Mol.Cell12Smith,G.R.etal.(2003)FissionyeastMus81ÐEme1Hollidayjunctionresolvaseisrequiredformeioticcrossingoverbutnotforgene165,2289Ð229313Bhagat,R.etal.(2004)Studiesoncrossover-speciÞcmutantsandthedistributionofcrossingoverinDrosophilaCytogenet.Genome107,160Ð17114Boddy,M.N.etal.(2001)Mus81ÐEme1areessentialcomponentsofaHollidayjunctionresolvase.107,537Ð54815Cromie,G.A.etal.(2006)SingleHollidayjunctionsareintermediatesofmeioticrecombination.127,1167Ð117816Chen,X.B.etal.(2001)HumanMus81-associatedendonucleasecleavesHollidayjunctionsinvitroMol.Cell8,1117Ð112717Gaillard,P.H.etal.(2003)TheendogenousMus81ÐEme1complexresolvesHollidayjunctionsbyanickandco

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