細胞分化的分子機制

一、緒論細胞分化的分子機制細胞分化

多細胞生物個體生長發(fā)育過程中，細胞在結構、形態(tài)、生理功能及生化特征等方面逐步產(chǎn)生穩(wěn)定的差異，形成不同的細胞類型，形成不同的組織器官和系統(tǒng)。細胞類型分化基因表達調控細胞類型形成組織器官形成個體發(fā)育細胞分化：漸進過程；貫穿生命過程細胞決定：細胞在發(fā)育的某個時刻被定向被定向的細胞最終發(fā)育成為成熟細胞細胞表型全能性細胞（totipotentcell)多潛能細胞（pluripotentcell)分化的細胞（differentiatedcell)能夠產(chǎn)生全部細胞表型；具有分化形成全部細胞類型的能力發(fā)育潛能一定的局限性;具有分化形成大多數(shù)細胞類型的能力發(fā)育命運在一定程度上被限定；基因表達受到一定程度的限定由多潛能細胞分裂分化發(fā)育成的特殊細胞表型；有絲分裂頻率明顯降低甚至停止分裂；一般僅5%～10%的基因表達細胞分化過程全能性細胞多潛能細胞分化細胞基因選擇性表達的結果隨著個體發(fā)育的進行，細胞核指導發(fā)育的潛能被限定，甚至喪失指導全部發(fā)育的能力豹蛙囊胚期細胞核→激活的去核卵60%正常發(fā)育成囊胚其中80～85%形成正常蝌蚪豹蛙原腸胚早期內胚層細胞核→激活的去核卵50%的胚胎能正常發(fā)育成正常蝌蚪豹蛙神經(jīng)胚內胚層細胞核→激活的去核卵10%以下的胚胎能正常發(fā)育

全能性及多能性的維持由外部信號和內部決定因素控制外部信號內部決定因素各種細胞因子各種轉錄調節(jié)因子細胞分化是基因差異表達的結果細胞內環(huán)境影響差異表達

卵質不均勻分布細胞外環(huán)境的影響

胚胎細胞處于不同區(qū)域，接受不同位置信息；鄰近細胞相互關系；信號轉導細胞誘導細胞誘導細胞生長因子鄰近細胞表面分子細胞不對稱分裂不對稱分裂受體及信號轉導分子基因差異表達是最重要的調控機制差異表達基因

時間特異性：只在發(fā)育的某個特定時期表達空間特異性：組織、細胞特異性基因表達在時間和空間上的特異性時間特異性空間特異性在發(fā)育的某個或某些時期表達檢測方法：轉錄組測序；消減雜交；RT-PCR；real-timePCR基因表達的組織特異性、細胞特異性檢測方法：原位雜交胚胎誘導胚胎發(fā)育過程中一部分細胞影響相鄰細胞向一定方向分化不對稱分裂mRNA不對稱分配視胞誘導外胚層形成晶體晶體誘導外胚層形成角膜細胞環(huán)境影響轉錄抑制劑（放線菌素D）處理受精卵，胚胎發(fā)育仍能進行至囊胚期蛋白質翻譯抑制劑處理受精卵（嘌呤霉素），受精卵停止發(fā)育Figure21-18.AsymmetricdivisionssegregatingPgranulesintothefoundercelloftheC.elegansgermline.Themicrographsintheupperrowshowthepatternofcelldivisions,withcellnucleistainedbluewithaDNA-specificfluorescentdye;belowarethesamecellsstainedwithanantibodyagainstPgranules.Thesesmallgranules(0.5–1μmindiameter)aredistributedrandomlythroughoutthecytoplasmintheunfertilizedegg(notshown).Afterfertilization,ateachcelldivisionuptothe16-cellstage,boththeyandtheintracellularmachinerythatlocalizesthemasymmetricallyaresegregatedintoasingledaughtercell.(CourtesyofSusanStrome.)theParproteinsservetobringasetofribonucleoproteinparticlescalledPgranulestotheposteriorpole,sothattheposteriordaughtercellinheritsPgranulesandtheanteriordaughtercelldoesnot.atthe16-cellstage,thereisjustonecellthatcontainsthePgranules.Thisonecellgivesrisetothegermline.二、基因的差異表達機制細胞分化的分子機制差異基因表達的調控機制差異基因轉錄RNA選擇性加工選擇性翻譯差別蛋白加工真核生物基因表達受轉錄調節(jié)因子的組合調控真核生物基因的調控區(qū)啟動子+DNA調控序列Figure7-41.Thegenecontrolregionofatypicaleucaryoticgene.ThepromoteristheDNAsequencewherethegeneraltranscriptionfactorsandthepolymeraseassemble(seeFigure6-16).Theregulatorysequencesserveasbindingsitesforgeneregulatoryproteins,whosepresenceontheDNAaffectstherateoftranscriptioninitiation.Thesesequencescanbelocatedadjacenttothepromoter,farupstreamofit,orevenwithinintronsordownstreamofthegene.DNAloopingisthoughttoallowgeneregulatoryproteinsboundatanyofthesepositionstointeractwiththeproteinsthatassembleatthepromoter.WhereasthegeneraltranscriptionfactorsthatassembleatthepromoteraresimilarforallpolymeraseIItranscribedgenes,thegeneregulatoryproteinsandthelocationsoftheirbindingsitesrelativetothepromoteraredifferentforeachgene.?2002byBruceAlberts,AlexanderJohnson,JulianLewis,MartinRaff,KeithRoberts,andPeterWalter.基因組中有成千種不同轉錄調節(jié)因子

-varyfromonegenecontrolregiontothenextusuallypresentinverysmallamountsinacell,oftenlessthan0.01%ofthetotalprotein.mostofthemrecognizetheirspecificDNAsequencesusingoneoftheDNA-bindingmotifs,althoughsomedonotrecognizeDNAdirectlybutinsteadassembleonotherDNA-boundproteins.Forexample,oftheroughly30,000humangenes,anestimated5–10%encodegeneregulatoryproteins.NameDNASequenceRecognized*Bacterialacrepressor5′AATTGTGAGCGGATAACAATT3′TTAACACTCGCCTATTGTTAACAPTGTGAGTTAGCTCACT

ACACTCAATCGAGTGA

lambdarepressorTATCACCGCCAGAGGTA

ATAGTGGCGGTCTCCAT

YeastGal4CGGAGGACTGTCCTCCG

GCCTCCTGACAGGAGGC

Matα2CATGTAATT

GTACATTAA

Gcn4ATGACTCAT

TACTGAGTADrosophila

KruppelAACGGGTTAA

TTGCCCAATT

BicoidGGGATTAGA

CCCTAATCTMammals

Sp1GGGCGG

CCCGCCOct-1PoudomainATGCAAAT

TACGTTTA

GATA-1TGATAG

ACTATC

MyoDCAAATGGTTTAC

p53GGGCAAGTCT

CCCGTTCAGA基因表達調控的保守機制蛋白-核酸相互作用真核生物基因表達需要基礎轉錄因子通常搭建轉錄起始復合體時是低效的，需要轉錄激活中介體遠距離互作轉錄調節(jié)因子與中介體相互作用轉錄調節(jié)因子與基礎轉錄因子相互作用Thegeneregulatoryproteinsallowtheindividualgenesofanorganismtobeturnedonoroffspecifically.Differentselectionsofgeneregulatoryproteinsarepresentindifferentcelltypesandtherebydirectthepatternsofgeneexpressionthatgiveeachcelltypeitsuniquecharacteristics.Eachgeneinaeucaryoticcellisregulateddifferentlyfromnearlyeveryothergene.轉錄調節(jié)因子介導基因轉錄調節(jié)不同細胞類型，有不同的轉錄調節(jié)蛋白組，因而特定基因表達譜不同基因調控序列不同順式元件啟動子UPE增強子沉默子TATA盒和Inr主要決定轉錄方向，起始位點，只能引起相當?shù)退降霓D錄；UPE元件影響轉錄起始的頻率，但不具有組織特異性調控的性能增強子(enhancer)具有正調控功能的順式元件，主要見于真核生物增強子可以位于啟動子上游、內含子中、下游Inhighereucaryotesitisnotunusualtofindtheregulatorysequencesofagenedottedoverdistancesasgreatas50,000nucleotidepairs.Figure9.26.Activatorsofeukaryotictranscriptioninitiation.Theblueactivatorisattachedtoaregulatorymoduleupstreamofagene,andinfluencestranscriptioninitiationonlyatthatsinglegene.Thegreenactivatorisattachedtoasitewithinanenhancerandisinfluencingtranscriptionofallthreegenes.?2002GarlandScience時間特異增強子人γ-珠蛋白基因在胚胎期表達，如果將β-珠蛋白基因的時間特異性增強子移到γ-珠蛋白基因附近，該基因可在成體中表達兩棲類動物胚胎MBT基因在胚囊中期被激活閱讀框上游500bp有增強子人β-珠蛋白基因的時間特異性增強子位于AATAAAA下游600～900bp增強子能使基因轉錄頻率增加10～200倍組織特異性增強子主增強子β-珠蛋白基因的第三個內含子內有組織特異性增強子使β-珠蛋白基因只能在紅細胞中轉錄所有位于人11號染色體上的β-珠蛋白基因家族的轉錄都受主增強子控制，缺失主增強子基因家族所有成員都沉默區(qū)域染色質結構打開？Figure7-59.Modelforthecontrolofthehumanβ-globingene.Thediagramshowssomeofthegeneregulatoryproteinsthoughttocontrolexpressionofthegeneduringredbloodcelldevelopment.Someofthegeneregulatoryproteinsshown,suchasCP1,arefoundinmanytypesofcells,whileothers,suchasGATA-1,arepresentinonlyafewtypesofcellsincludingredbloodcellsandthereforearethoughttocontributetothecell-typespecificityofβ-globingeneexpression.Asindicatedbythedouble-headedarrows,severalofthebindingsitesforGATA-1overlapthoseofothergeneregulatoryproteins;itisthoughtthatoccupancyofthesesitesbyGATA-1excludesbindingofotherproteins.OnceboundtoDNA,thegeneregulatoryproteinsrecruitchromatinremodelingcomplexes,histonemodifyingenzymes,thegeneraltranscriptionfactorsandRNApolymerasetothepromoter.(AdaptedfromB.Emerson,inGeneExpression:GeneralandCell-TypeSpecific[M.Karin,ed.],pp.116161.Boston:Birkhauser,1993.)thejointeffectisgenerallynotmerelythesumoftheenhancementscausedbyeachfactoralone,buttheproduct.Figure7-47.Transcriptionalsynergy.Inthisexperiment,therateoftranscriptionproducedbythreeexperimentallyconstructedregulatoryregionsiscomparedinaeucaryoticcell.Transcriptionalsynergy,thegreaterthanadditiveeffectoftheactivators,isobservedwhenseveralmoleculesofgeneactivatorproteinareboundupstreamofthepromoter.Synergyisalsotypicallyobservedbetweendifferentgeneactivatorproteinsfromthesameorganismandevenbetweenactivatorproteinsfromwidelydifferenteucaryoticspecieswhentheyareexperimentallyintroducedintothesamecell.Thislastobservationreflectsthehighdegreeofconservationofthetranscriptionmachinery協(xié)同作用的效應不是單個激活因子效應之和而是其乘積不同的激活因子也具有這樣的協(xié)同效應基因轉錄活性在不同細胞中可由不同增強子控制果蠅卵黃基因有兩個增強子，一個負責卵黃基因在卵巢中表達，另一個負責該基因在脂肪體中表達silencer

Aregulatorysequencethatreducestherateoftranscriptionofageneorgeneslocatedsomedistanceawayineitherdirection.

restrictingthetranscriptionofaparticulargenetoaparticulargroupofcellsregulatingthetimingofthegene'sexpression真核生物以正調控為主沉默子：基因的負調控元件，可介導基因表達的組織特異性和時序性neuralrestrictivesilencerelement(NRSE)

foundinseveralmousegeneswhoseexpressionislimitedtothenervoussystem；

preventsthepromoter'sactivationinanytissueexceptneurons;NRSF(azincfingertranscriptionfactor)bindstotheNRSE，appearstobeexpressedineverycellthatisnotamatureneuron存在于幾種小鼠神經(jīng)系統(tǒng)特異性表達基因中；限制靶基因在除神經(jīng)元以外其它組織中的表達；轉錄調節(jié)因子NRSF（識別NRSE)在除神經(jīng)元以外其它組織中表達Figure5.16.

Theimportanceofsilencersinliver-specificgenetranscription.(A)Intheearlydigestivetubeendoderm,mostofthetranscriptionfactorsarenotboundtotheirsitesontheenhancerforserumalbumin.(B)Asendodermdevelopmentproceeds,thesitesontheenhancerbecomeoccupiedbyfiveproteinswhosepresenceisessentialforactivatingthegene,andoneprotein,boundtothesilencer(siteeY),thatcaninhibittranscription.(C)Astheliverforms,theinhibitoryproteinisnolongerfoundontheenhancer,andtheserumalbumingeneistranscribed.Interestingly,thischangemaytakeplaceshortlyaftertheassociationofthepre-liverendodermalregionwithheart-formingtissue.Atthistime,thechromatininthisregionclumpstogethertoformanucleoproteinactivationcomplexthatspans180basepairsofDNAandactivatesthealbuminpromoter.(AfterGualdietal.1996.)Theimportanceofsilencersinliver-specificgenetranscription小鼠血清白蛋白基因在肝臟專一性轉錄A：消化道內皮層B：肝原基;C：小鼠肝臟Figure7-50.EucaryoticgeneregulatoryproteinsoftenassembleintocomplexesonDNA.Sevengeneregulatoryproteinsareshownin(A).ThenatureandfunctionofthecomplextheyformdependsonthespecificDNAsequencethatseedstheirassembly.In(B),someassembledcomplexesactivategenetranscription,whileanotherrepressestranscription.Notethattheredproteinissharedbybothactivatingandrepressingcomplexes對不同的基因而言，某一個轉錄調節(jié)因子，既可以是轉錄激活因子，也可以是阻遏蛋白真核生物基因表達調控的特點是組合調控Manysetsofgeneregulatoryproteinscanbeboundsimultaneouslyandinfluencethepromoterofagene.ThepromoterintegratesthetranscriptionalcuesprovidedbyalloftheboundproteinsFigure7-57.Integrationatapromoter.Multiplesetsofgeneregulatoryproteinscanworktogethertoinfluencetranscriptioninitiationatapromoter,astheydointheevestripe2moduleillustratedpreviouslyinFigure7-55.Itisnotyetunderstoodindetailhowtheintegrationofmultipleinputsisachieved,butitislikelythatthefinaltranscriptionalactivityofthegeneresultsfromacompetitionbetweenactivatorsandrepressorsthatactbythemechanismssummarizedinFigures7-43,7-44,7-45,7-46,and7-49.基因編碼序列保守；DNA調控序列不保守Theprotein-codingsequencesareunmistakablysimilar,butthecorrespondingregulatoryDNAsequencesappearverydifferent.

eve基因調控區(qū)一組調節(jié)模塊(regulatorymodules),每個模塊含multipleregulatorysequences，決定一條eve基因表達的條帶

每個模塊，決定一條eve基因表達的條帶阻遏蛋白:GiantandKrüppel激活因子:Bicoid、HunchbackEve基因表達調控，提供了真核基因表達組合調控的例子，七種調控蛋白組合用于激活eve表達，每種組合決定一條帶，在帶與帶之間的位置，轉錄調節(jié)因子的組合使Eve基因沉默。Figure7-56.Distributionofthegeneregulatoryproteinsresponsibleforensuringthateveisexpressedinstripe2.ThedistributionsoftheseproteinswerevisualizedbystainingadevelopingDrosophilaembryowithantibodiesdirectedagainsteachofthefourproteins(seeFigures7-52and7-53).Theexpressionofeveinstripe2occursonlyatthepositionwherethetwoactivators(BicoidandHunchback)arepresentandthetworepressors(GiantandKrüppel)areabsent.InflyembryosthatlackKrüppel,forexample,stripe2expandsposteriorly.Likewise,stripe2expandsposteriorlyiftheDNA-bindingsitesforKrüppelinthestripe2module(seeFigure7-55)areinactivatedbymutationandthisregulatoryregionisreintroducedintothegenome.Theevegeneitselfencodesageneregulatoryprotein,which,afteritspatternofexpressionissetupinsevenstripes,regulatestheexpressionofotherDrosophilagenes.Asdevelopmentproceeds,theembryoisthussubdividedintofinerandfinerregionsthateventuallygiverisetothedifferentbodypartsoftheadultfly,asdiscussedinChapter21.ThisexamplefromDrosophilaembryosisunusualinthatthenucleiareexposeddirectlytopositionalcuesintheformofconcentrationsofgeneregulatoryproteins.Inembryosofmostotherorganisms,individualnucleiareinseparatecells,andextracellularpositionalinformationmusteitherpassacrosstheplasmamembraneor,moreusually,generatesignalsinthecytosolinordertoinfluencethegenome.細胞分化涉及轉錄因子的次序表達中胚層祖細胞成肌細胞多核肌管肌纖維externalsignaldeterminationdifferentiationmaturationmyoDMyf-5MRF-4myogeninGrowthregulatormuslespecificgenesFigure21-7.ThestandardtestforcelldeterminationFigure21-8.Prospectivethightissuegraftedintothetipofachickwingbudformstoes.(AfterJ.W.Saundersetal.,Dev.Biol.1:281–301,1959.)“開關基因”（switchgene)如線蟲的lin-12；果蠅的notch；脊椎動物的myod1決定兩種分化方向Lin-2+胚胎Z1.Ppp和Z4Aaa細胞分化產(chǎn)生腹側子宮前體細胞Lin-2-胚胎Z1.Ppp和Z4Aaa細胞分化產(chǎn)生子宮頸細胞果蠅具有能分化上皮細胞或神經(jīng)母細胞潛能的細胞正常情況：1/4分化為神經(jīng)母細胞；3/4分化為上皮組織的前體Notch轉錄缺乏的胚胎中：全部細胞分化為神經(jīng)母細胞，胚胎非正常發(fā)育，至死亡肌母細胞決定基因Mydo1基因僅在肌細胞中表達肌細胞分化的開關基因具有肌細胞分化表型特異性調控基因功能細胞記憶裝置

Positivefeedbackloops—簡單的、普遍的策略Figure7-68.Schematicdiagramshowinghowapositivefeedbackloopcancreatecellmemory.

ProteinAisageneregulatoryproteinthatactivatesitsowntranscription.Allofthedescendantsoftheoriginalcellwilltherefore“remember”thattheprogenitorcellhadexperiencedatransientsignalthatinitiatedtheproductionoftheprotein.

建立和遺傳基因表達模式轉錄調節(jié)蛋白，激活自身編碼基因InArabidopsis,LEAFY(LFY),APETALA1(AP1),andCAULIFLOWER(CAL)are

floralmeristemidentitygenes

Floralmeristemidentitygenesinitiateacascadeofgeneexpressionthatturnson

region-specifying

(cadastral)

genes；SUPERMAN(SUP)isanexampleofacadastralgeneinArabidopsisthatplaysaroleinspecifyingboundariesfororganidentitygeneexpression轉錄因子次序表達Threeclasses(A,B,andC)oforganidentitygenesarenecessarytospecifythefourwhorlsoffloralorgans(CoenandMeyerowitz1991).TheyincludeAP2,AGAMOUS(AG),AP3,andPISTILLATA(PI)inArabidopsis.Wild-typeandmutantphenotypesoftheArabidopsis

classA(ap2),classB(ap3,pi)classC(ag)TheABCgenescodefortranscriptionfactorsthatinitiateacascadeofeventsleadingtotheactualproductionoffloralpartsLeavesrepresenta“groundstate”inwhichnoneofthesehomeoticselectorgenesareexpressed,whiletheothertypesoforganresultfromexpressingthegenesindifferentcombinations.肌原性蛋白(myogenicprotein)為僅在肌細胞中存在的調節(jié)蛋白，MyoD,Myf5,myogenin,Mrf4識別許多muscle-specificgenes的調控序列在離體培養(yǎng)的雞胚表皮成纖維細胞中誘導MyoD表達，成纖維細胞轉變成肌細胞。MyoD,Myf5,myogenin,Mrf4中任何一個在成纖維細胞中表達都能誘導肌細胞分化正反饋回路推測：成纖維細胞中可能已經(jīng)積累一些調節(jié)蛋白，它們可以與肌原性蛋白協(xié)作，開啟muscle-specificgenes。特定的調節(jié)蛋白組合決定肌肉分化Ey基因編碼的調節(jié)蛋白引發(fā)果蠅眼形成果蠅的眼由成千的細胞組成Ey基因控制許多基因，包括一些調節(jié)蛋白的編碼基因，有些基因反過來激活Ey形成正反饋，可確保持續(xù)合成Ey蛋白，這種方式下，一個調節(jié)蛋白的作用可以啟動轉錄調節(jié)蛋白基因表達的cascadeTheEyproteinisknowntobinddirectlytonumeroustargetgenesforeyedevelopment,includingthoseencodinglenscrystallins(seeFigure7-119),rhodopsins,andotherphotoreceptorproteins.(AdaptedfromT.Czernyetal.,Mol.Cell3:297–307,1999.)Figure7-75.GeneregulatoryproteinsthatspecifyeyedevelopmentinDrosophila.

toy(twinofeyeless)andey(eyeless)encodesimilargeneregulatoryproteins,ToyandEy,eitherofwhich,whenectopicallyexpressed,cantriggereyedevelopment.Innormaleyedevelopment,expressionofeyrequiresthetoygene.OnceitstranscriptionisactivatedbyToy,Eyactivatestranscriptionofso(sineoculis)andeya(eyesabsent)whichacttogethertoexpressthedac(dachshund)gene.Asindicatedbythegreenarrows,someofthegeneregulatoryproteinsformpositivefeedbackloopswhichreinforcetheinitialcommitmenttoeyedevelopment.TheEyproteinisknowntobinddirectlytonumeroustargetgenesforeyedevelopment,includingthoseencodinglenscrystallins(seeFigure7-119),rhodopsins,andotherphotoreceptorproteins.(AdaptedfromT.Czernyetal.,Mol.Cell3:297–307,1999.)Theconversionofonecelltype(fibroblast)toanother(skeletalmuscle)byasinglegeneregulatoryproteinreemphasizesoneofthemostimportantprinciples:dramaticdifferencesbetweencelltypes—insize,shape,chemistry,andfunction—canbeproducedbydifferencesingeneexpression發(fā)育過程中經(jīng)常出現(xiàn)基因表達協(xié)調調節(jié)一種類型細胞中表達多種細胞類型特異基因激素誘導幾種基因表達（如類固醇受體激活的基因）饑餓、劇烈運動→糖皮質素激素→肝細胞中氨基酸→葡萄糖糖皮質素激素受體（轉錄調節(jié)因子）多個基因表達

酶Figure15-12.Somesignalingmoleculesthatbindtonuclearreceptors.Notethatallofthemaresmallandhydrophobic.Theactive,hydroxylatedformofvitaminD3isshown.Estradiolandtestosteronearesteroidsexhormones.?2002byBruceAlberts,AlexanderJohnson,JulianLewis,MartinRaff,KeithRoberts,andPeterWalter.核受體的信號類固醇激素：甾醇、類固醇性激素、維生素D、蛻皮素；前體為膽固醇Figure12.5.Geneactivationbyasteroidhormone.Estradiolisoneoftheestrogensteroidhormones.Afterenteringthecell,estradiolattachestoitsreceptorproteinandthecomplexentersthenucleuswhereitbindstothe15-bpestrogenresponseelement(abbreviation:N,anynucleotide),whichislocatedupstreamofthosegenesactivatedbyestradiolandotherestrogens.Othersteroidhormonereceptorsrecognizeotherresponseelements.Forexample,glucocorticoidhormonestargetthesequence5′-AGAACANNNTGTTCT-3′.Notethatthissequence,andthatoftheestrogenresponseelement,isaninvertedpalindrome.TheresponseelementforvitaminD3,whichisasteroidderivativethatactivatestranscriptionviaanuclearreceptor(seethetext),hasthesequence5′-AGGTCANNNAGGTCA-3′,whichisadirectrepeatratherthananinvertedpalindrome.?2002GarlandScienceOnceinsidethecell,eachhormonebindstoaspecificsteroidreceptorprotein,whichisusuallylocatedinthecytoplasmAfterbinding,theactivatedreceptormigratesintothenucleus,whereitattachestoaresponseelementupstreamofatargetgene.雌二醇雌二醇受體Figure15-14.Responsesinducedbytheactivationofanuclearhormonereceptor.(A)Earlyprimaryresponseand(B)delayedsecondaryresponse.Thefigureshowstheresponsestoasteroidhormone,butthesameprinciplesapplyforallligandsthatactivatethisfamilyofreceptorproteins.Someoftheprimary-responseproteinsturnonsecondary-responsegenes,whereasothersturnofftheprimary-responsegenes.Theactualnumberofprimary-andsecondary-responsegenesisgreaterthanshown.Asexpected,drugsthatinhibitproteinsynthesissuppressthetranscriptionofsecondary-responsegenesbutnotprimary-responsegenes,allowingthesetwoclassesofgenetranscriptionresponsestobereadilydistinguished.?2002byBruceAlberts,AlexanderJohnson,JulianLewis,MartinRaff,KeithRoberts,andPeterWalter.Responsesinducedbytheactivationofanuclearhormonereceptorearlyprimaryresponsedelayedsecondaryresponse.Figure9.13.Thesteroidreceptorzincfinger.TheRgroupsoftheaminoacidsinvolvedintheinteractionswiththezincatomsareshownassolidgreenlines.‘N'and‘C'indicatetheN-andC-terminiofthemotif,respectively.ReprintedfromDNA-ProteinInteractionsbyAndrewTravers,publishedbyChapman&Hall,1993.ReprintedwithkindpermissionofA.Travers.?2002GarlandScienceThetypicalhormoneresponseelementisa15bpsequencecomprisinga6bpinvertedpalindromeseparatedbya3bpspacertowhichthesteroidreceptorbindsviaaspecialversionofthezincfinger.Responseelementsforeachreceptorarelocatedupstreamof50–100genesand,oncebound,thereceptoractsasatranscriptionactivator.Asteroidhormonecanthereforeinducealarge-scalechangeinthebiochemicalpropertiesofthecell.細胞對類固醇、甲狀腺素，維生素D、維甲酸的響應不僅由信號決定，也與細胞類型有關；雖然許多細胞具有相同的細胞內受體，但由于組合調控機制的緣故，類固醇、甲狀腺素，維生素D、維甲酸等受體調控的基因可不同細胞內受體只有在與其它轉錄調節(jié)因子形成完整組合時才能激活靶基因的表達，而調節(jié)蛋白組合中的許多成員具有細胞類型特異性Figure21-4.HowregulatoryDNAdefinesthesuccessionofgeneexpressionpatternsindevelopment.ThegenomesoforganismsAandBcodeforthesamesetofproteinsbuthavedifferentregulatoryDNA.Thetwocellsinthecartoonstartinthesamestate,expressingthesameproteinsatstage1,butsteptoquitedifferentstatesatstage2becauseoftheirdifferentarrangementsofregulatorymodules.regulatoryDNAcandefinethesequentialprogramofdevelopment:

therulesforsteppingfromonestatetothenext,asthecellsproliferateandreadtheirpositionsintheembryo,switchingonnewsetsofgenesaccordingtotheactivitiesoftheproteinsthattheycurrentlycontain組合調控在發(fā)育中的意義轉錄調節(jié)因子的組合產(chǎn)生細胞類型在發(fā)育過程中，細胞感知位置信號產(chǎn)生不同的轉錄因子組合在發(fā)育過程中，細胞可以累積一系列的調節(jié)蛋白，它們開始并不改變基因的表達，直到所需的調節(jié)因子組合中最后的成員加入，調節(jié)信息完全，導致基因表達大的改變。小結轉錄調節(jié)因子組合調控真核生物基因表達基因表達的時序性和組織特異性常由增強子和沉默子決定不同細胞類型具有不同的轉錄調節(jié)因子組在個體發(fā)育過程中細胞命運決定基因常為自激活的轉錄調節(jié)因子在系統(tǒng)進化過程中，順式元件是不保守的選擇性基因轉錄與染色質變化組蛋白乙酰化具有激活基因轉錄的作用核小體是真核生物染色質的基本單位Figure4-27.Theassemblyofahistoneoctamer.ThehistoneH3–H4dimerandtheH2A-H2Bdimerareformedfromthehandshakeinteraction.AnH3-H4tetramerformsthescaffoldoftheoctamerontowhichtwoH2A-H2Bdimersareadded,tocompletetheassembly.ThehistonesarecoloredasinFigure4-26.NotethatalleightN-terminaltailsofthehistonesprotrudefromthedisc-shapedcorestructure.Inthex-raycrystal(Figure4-25),mostofthehistonetailswereunstructured(andthereforenotvisibleinthestructure),suggestingthattheirconformationsarehighlyflexible.(AdaptedfromfiguresbyJ.Waterborg.)?2002byBruceAlberts,AlexanderJohnson,JulianLewis,MartinRaff,KeithRoberts,andPeterWalter.H2A-H2B二聚體→H3H4四聚體→八聚體；N端在外，無結構eachofthecorehistoneshasalong

N-terminalaminoacid“tail”,

whichextendsoutfromtheDNA-histonecore.Thesehistonetailsaresubjectto

severaldifferenttypesofcovalentmodifications,whichcontrolmanyaspectsofchromatinstructure.

組蛋白N端“尾”可被多種共價修飾，與染色質結構的控制有關Figure4-35.Covalentmodificationofcorehistonetails.(A)Knownmodificationsofthefourhistonecoreproteinsareindicated:Me=methylgroup,Ac=acetylgroup,P=phosphate,u=ubiquitin.Notethatsomepositions(e.g.,lysine9ofH3)canbemodifiedinmorethanoneway.Mostofthesemodificationsaddarelativelysmallmoleculeontothehistonetails;theexceptionisubiquitin,a76aminoacidproteinalsousedinothercellularprocesses(seeFigure6-87).Thefunctionofubiquitininchromatinisnotwellunderstood:histoneH2Bcanbemodifiedbyasingleubiquitinmolecule;H2Acanbemodifiedbytheadditionofseveralubiquitins.(B)Ahistonecodehypothesis.Histonetailscanbemarkedbydifferentcombinationsofmodifications.Accordingtothishypothesis,eachmarkingconveysaspecificmeaningtothestretchofchromatinonwhichitoccurs.Onlyafewofthemeaningsofthemodificationsareknown.InChapter7,wediscussthewayadoubly-acetylatedH4tailis“read”byaproteinrequiredforgeneexpression.Inanotherwell-studiedcase,anH3tailmethylatedatlysine9isrecognizedbyasetofproteinsthatcreateanespeciallycompactformofchromatin,whichsilencesgeneexpression.Theacetylationoflysine14ofhistoneH3andlysines8and16ofhistoneH4—usuallyassociatedwithgeneexpression—isperformedbythetypeAhistoneacetylases(HATs)inthenucleus.Incontrast,theacetylationoflysines5and12ofhistoneH4andalysineofhistoneH3takesplaceinthecytosol,afterthehistoneshavebeensynthesizedbutbeforetheyhavebeenincorporatedintonucleosomes;thesemodificationsarecatalyzedbytypeBHATs.ThesemodifiedhistonesaredepositedontoDNAafterDNAreplication(seeFigure5-41),andtheiracetylgroupsaretakenoffshortlyafterwardsbyhistonedeacetylases(HDACs).Thus,theacetylationatthesepositionssignalsnewlyreplicatedchromatin.Modificationofaparticularpositioninahistonetailcantakeondifferentmeaningsdependingonotherfeaturesofthelocalchromatinstructure.Forexample,thephosphorylationofposition10ofhistoneH3isassociatednotonlywiththecondensationofchromosomesthattakesplaceinmitosisandmeiosisbutalsowiththeexpressionofcertaingenes.Somehistonetailmodificationsareinterdependent.ForexamplemethylationofH3position9blocksthephosphorylationofH3position10,andviceversa.?2002byBruceAlberts,AlexanderJohnson,JulianLewis,MartinRaff,KeithRoberts,andPeterWalter.removesthepositivechargefromthelysine,makingitmoredifficultforhistonestoneutralizethechargesonDNAaschromatiniscompacted.組蛋白乙?；簻p少組蛋白與DNA之間的親和力影響30nm結構的穩(wěn)定性differenttypesofcelldisplaydifferentpatternsofhistoneacetylation,Indications:

histoneacetylationplaysaprominentroleinregulatinggenomeexpression.啟動子DNA序列甲基化修飾使基因沉默Ineukaryotes,cytosinebasesinchromosomalDNAmoleculesaresometimeschangedto5-methyl