细胞分化的分子机制

一、绪论细胞分化的分子机制细胞分化

多细胞生物个体生长发育过程中，细胞在结构、形态、生理功能及生化特征等方面逐步产生稳定的差异，形成不同的细胞类型，形成不同的组织器官和系统。细胞类型分化基因表达调控细胞类型形成组织器官形成个体发育细胞分化：渐进过程；贯穿生命过程细胞决定：细胞在发育的某个时刻被定向被定向的细胞最终发育成为成熟细胞细胞表型全能性细胞（totipotentcell)多潜能细胞（pluripotentcell)分化的细胞（differentiatedcell)能够产生全部细胞表型；具有分化形成全部细胞类型的能力发育潜能一定的局限性;具有分化形成大多数细胞类型的能力发育命运在一定程度上被限定；基因表达受到一定程度的限定由多潜能细胞分裂分化发育成的特殊细胞表型；有丝分裂频率明显降低甚至停止分裂；一般仅5%～10%的基因表达细胞分化过程全能性细胞多潜能细胞分化细胞基因选择性表达的结果随着个体发育的进行，细胞核指导发育的潜能被限定，甚至丧失指导全部发育的能力豹蛙囊胚期细胞核→激活的去核卵60%正常发育成囊胚其中80～85%形成正常蝌蚪豹蛙原肠胚早期内胚层细胞核→激活的去核卵50%的胚胎能正常发育成正常蝌蚪豹蛙神经胚内胚层细胞核→激活的去核卵10%以下的胚胎能正常发育

全能性及多能性的维持由外部信号和内部决定因素控制外部信号内部决定因素各种细胞因子各种转录调节因子细胞分化是基因差异表达的结果细胞内环境影响差异表达

卵质不均匀分布细胞外环境的影响

胚胎细胞处于不同区域，接受不同位置信息；邻近细胞相互关系；信号转导细胞诱导细胞诱导细胞生长因子邻近细胞表面分子细胞不对称分裂不对称分裂受体及信号转导分子基因差异表达是最重要的调控机制差异表达基因

时间特异性：只在发育的某个特定时期表达空间特异性：组织、细胞特异性基因表达在时间和空间上的特异性时间特异性空间特异性在发育的某个或某些时期表达检测方法：转录组测序；消减杂交；RT-PCR；real-timePCR基因表达的组织特异性、细胞特异性检测方法：原位杂交胚胎诱导胚胎发育过程中一部分细胞影响相邻细胞向一定方向分化不对称分裂mRNA不对称分配视胞诱导外胚层形成晶体晶体诱导外胚层形成角膜细胞环境影响转录抑制剂（放线菌素D）处理受精卵，胚胎发育仍能进行至囊胚期蛋白质翻译抑制剂处理受精卵（嘌呤霉素），受精卵停止发育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、基因的差异表达机制细胞分化的分子机制差异基因表达的调控机制差异基因转录RNA选择性加工选择性翻译差别蛋白加工真核生物基因表达受转录调节因子的组合调控真核生物基因的调控区启动子+DNA调控序列Figure7-41.Thegenecontrolregionofatypicaleucaryoticgene.ThepromoteristheDNAsequencewherethegeneraltranscriptionfactorsandthepolymeraseassemble(seeFigure6-16).Theregulatorysequencesserveasbindingsitesforgeneregulatoryproteins,whosepresenceontheDNAaffectstherateoftranscriptioninitiation.Thesesequencescanbelocatedadjacenttothepromoter,farupstreamofit,orevenwithinintronsordownstreamofthegene.DNAloopingisthoughttoallowgeneregulatoryproteinsboundatanyofthesepositionstointeractwiththeproteinsthatassembleatthepromoter.WhereasthegeneraltranscriptionfactorsthatassembleatthepromoteraresimilarforallpolymeraseIItranscribedgenes,thegeneregulatoryproteinsandthelocationsoftheirbindingsitesrelativetothepromoteraredifferentforeachgene.©2002byBruceAlberts,AlexanderJohnson,JulianLewis,MartinRaff,KeithRoberts,andPeterWalter.基因组中有成千种不同转录调节因子

-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基因表达调控的保守机制蛋白-核酸相互作用真核生物基因表达需要基础转录因子通常搭建转录起始复合体时是低效的，需要转录激活中介体远距离互作转录调节因子与中介体相互作用转录调节因子与基础转录因子相互作用Thegeneregulatoryproteinsallowtheindividualgenesofanorganismtobeturnedonoroffspecifically.Differentselectionsofgeneregulatoryproteinsarepresentindifferentcelltypesandtherebydirectthepatternsofgeneexpressionthatgiveeachcelltypeitsuniquecharacteristics.Eachgeneinaeucaryoticcellisregulateddifferentlyfromnearlyeveryothergene.转录调节因子介导基因转录调节不同细胞类型，有不同的转录调节蛋白组，因而特定基因表达谱不同基因调控序列不同顺式元件启动子UPE增强子沉默子TATA盒和Inr主要决定转录方向，起始位点，只能引起相当低水平的转录；UPE元件影响转录起始的频率，但不具有组织特异性调控的性能增强子(enhancer)具有正调控功能的顺式元件，主要见于真核生物增强子可以位于启动子上游、内含子中、下游Inhighereucaryotesitisnotunusualtofindtheregulatorysequencesofagenedottedoverdistancesasgreatas50,000nucleotidepairs.Figure9.26.Activatorsofeukaryotictranscriptioninitiation.Theblueactivatorisattachedtoaregulatorymoduleupstreamofagene,andinfluencestranscriptioninitiationonlyatthatsinglegene.Thegreenactivatorisattachedtoasitewithinanenhancerandisinfluencingtranscriptionofallthreegenes.©2002GarlandScience时间特异增强子人γ-珠蛋白基因在胚胎期表达，如果将β-珠蛋白基因的时间特异性增强子移到γ-珠蛋白基因附近，该基因可在成体中表达两栖类动物胚胎MBT基因在胚囊中期被激活阅读框上游500bp有增强子人β-珠蛋白基因的时间特异性增强子位于AATAAAA下游600～900bp增强子能使基因转录频率增加10～200倍组织特异性增强子主增强子β-珠蛋白基因的第三个内含子内有组织特异性增强子使β-珠蛋白基因只能在红细胞中转录所有位于人11号染色体上的β-珠蛋白基因家族的转录都受主增强子控制，缺失主增强子基因家族所有成员都沉默区域染色质结构打开？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协同作用的效应不是单个激活因子效应之和而是其乘积不同的激活因子也具有这样的协同效应基因转录活性在不同细胞中可由不同增强子控制果蝇卵黄基因有两个增强子，一个负责卵黄基因在卵巢中表达，另一个负责该基因在脂肪体中表达silencer

Aregulatorysequencethatreducestherateoftranscriptionofageneorgeneslocatedsomedistanceawayineitherdirection.

restrictingthetranscriptionofaparticulargenetoaparticulargroupofcellsregulatingthetimingofthegene'sexpression真核生物以正调控为主沉默子：基因的负调控元件，可介导基因表达的组织特异性和时序性neuralrestrictivesilencerelement(NRSE)

foundinseveralmousegeneswhoseexpressionislimitedtothenervoussystem；

preventsthepromoter'sactivationinanytissueexceptneurons;NRSF(azincfingertranscriptionfactor)bindstotheNRSE，appearstobeexpressedineverycellthatisnotamatureneuron存在于几种小鼠神经系统特异性表达基因中；限制靶基因在除神经元以外其它组织中的表达；转录调节因子NRSF（识别NRSE)在除神经元以外其它组织中表达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脏专一性转录A：消化道内皮层B：肝原基;C：小鼠肝脏Figure7-50.EucaryoticgeneregulatoryproteinsoftenassembleintocomplexesonDNA.Sevengeneregulatoryproteinsareshownin(A).ThenatureandfunctionofthecomplextheyformdependsonthespecificDNAsequencethatseedstheirassembly.In(B),someassembledcomplexesactivategenetranscription,whileanotherrepressestranscription.Notethattheredproteinissharedbybothactivatingandrepressingcomplexes对不同的基因而言，某一个转录调节因子，既可以是转录激活因子，也可以是阻遏蛋白真核生物基因表达调控的特点是组合调控Manysetsofgeneregulatoryproteinscanbeboundsimultaneouslyandinfluencethepromoterofagene.ThepromoterintegratesthetranscriptionalcuesprovidedbyalloftheboundproteinsFigure7-57.Integrationatapromoter.Multiplesetsofgeneregulatoryproteinscanworktogethertoinfluencetranscriptioninitiationatapromoter,astheydointheevestripe2moduleillustratedpreviouslyinFigure7-55.Itisnotyetunderstoodindetailhowtheintegrationofmultipleinputsisachieved,butitislikelythatthefinaltranscriptionalactivityofthegeneresultsfromacompetitionbetweenactivatorsandrepressorsthatactbythemechanismssummarizedinFigures7-43,7-44,7-45,7-46,and7-49.基因编码序列保守；DNA调控序列不保守Theprotein-codingsequencesareunmistakablysimilar,butthecorrespondingregulatoryDNAsequencesappearverydifferent.

eve基因调控区一组调节模块(regulatorymodules),每个模块含multipleregulatorysequences，决定一条eve基因表达的条带

每个模块，决定一条eve基因表达的条带阻遏蛋白:GiantandKrüppel激活因子:Bicoid、HunchbackEve基因表达调控，提供了真核基因表达组合调控的例子，七种调控蛋白组合用于激活eve表达，每种组合决定一条带，在带与带之间的位置，转录调节因子的组合使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.细胞分化涉及转录因子的次序表达中胚层祖细胞成肌细胞多核肌管肌纤维externalsignaldeterminationdifferentiationmaturationmyoDMyf-5MRF-4myogeninGrowthregulatormuslespecificgenesFigure21-7.ThestandardtestforcelldeterminationFigure21-8.Prospectivethightissuegraftedintothetipofachickwingbudformstoes.(AfterJ.W.Saundersetal.,Dev.Biol.1:281–301,1959.)“开关基因”（switchgene)如线虫的lin-12；果蝇的notch；脊椎动物的myod1决定两种分化方向Lin-2+胚胎Z1.Ppp和Z4Aaa细胞分化产生腹侧子宫前体细胞Lin-2-胚胎Z1.Ppp和Z4Aaa细胞分化产生子宫颈细胞果蝇具有能分化上皮细胞或神经母细胞潜能的细胞正常情况：1/4分化为神经母细胞；3/4分化为上皮组织的前体Notch转录缺乏的胚胎中：全部细胞分化为神经母细胞，胚胎非正常发育，至死亡肌母细胞决定基因Mydo1基因仅在肌细胞中表达肌细胞分化的开关基因具有肌细胞分化表型特异性调控基因功能细胞记忆装置

Positivefeedbackloops—简单的、普遍的策略Figure7-68.Schematicdiagramshowinghowapositivefeedbackloopcancreatecellmemory.

ProteinAisageneregulatoryproteinthatactivatesitsowntranscription.Allofthedescendantsoftheoriginalcellwilltherefore“remember”thattheprogenitorcellhadexperiencedatransientsignalthatinitiatedtheproductionoftheprotein.

建立和遗传基因表达模式转录调节蛋白，激活自身编码基因InArabidopsis,LEAFY(LFY),APETALA1(AP1),andCAULIFLOWER(CAL)are

floralmeristemidentitygenes

Floralmeristemidentitygenesinitiateacascadeofgeneexpressionthatturnson

region-specifying

(cadastral)

genes；SUPERMAN(SUP)isanexampleofacadastralgeneinArabidopsisthatplaysaroleinspecifyingboundariesfororganidentitygeneexpression转录因子次序表达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)为仅在肌细胞中存在的调节蛋白，MyoD,Myf5,myogenin,Mrf4识别许多muscle-specificgenes的调控序列在离体培养的鸡胚表皮成纤维细胞中诱导MyoD表达，成纤维细胞转变成肌细胞。MyoD,Myf5,myogenin,Mrf4中任何一个在成纤维细胞中表达都能诱导肌细胞分化正反馈回路推测：成纤维细胞中可能已经积累一些调节蛋白，它们可以与肌原性蛋白协作，开启muscle-specificgenes。特定的调节蛋白组合决定肌肉分化Ey基因编码的调节蛋白引发果蝇眼形成果蝇的眼由成千的细胞组成Ey基因控制许多基因，包括一些调节蛋白的编码基因，有些基因反过来激活Ey形成正反馈，可确保持续合成Ey蛋白，这种方式下，一个调节蛋白的作用可以启动转录调节蛋白基因表达的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发育过程中经常出现基因表达协调调节一种类型细胞中表达多种细胞类型特异基因激素诱导几种基因表达（如类固醇受体激活的基因）饥饿、剧烈运动→糖皮质素激素→肝细胞中氨基酸→葡萄糖糖皮质素激素受体（转录调节因子）多个基因表达

酶Figure15-12.Somesignalingmoleculesthatbindtonuclearreceptors.Notethatallofthemaresmallandhydrophobic.Theactive,hydroxylatedformofvitaminD3isshown.Estradiolandtestosteronearesteroidsexhormones.©2002byBruceAlberts,AlexanderJohnson,JulianLewis,MartinRaff,KeithRoberts,andPeterWalter.核受体的信号类固醇激素：甾醇、类固醇性激素、维生素D、蜕皮素；前体为胆固醇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.雌二醇雌二醇受体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.细胞对类固醇、甲状腺素，维生素D、维甲酸的响应不仅由信号决定，也与细胞类型有关；虽然许多细胞具有相同的细胞内受体，但由于组合调控机制的缘故，类固醇、甲状腺素，维生素D、维甲酸等受体调控的基因可不同细胞内受体只有在与其它转录调节因子形成完整组合时才能激活靶基因的表达，而调节蛋白组合中的许多成员具有细胞类型特异性Figure21-4.HowregulatoryDNAdefinesthesuccessionofgeneexpressionpatternsindevelopment.ThegenomesoforganismsAandBcodeforthesamesetofproteinsbuthavedifferentregulatoryDNA.Thetwocellsinthecartoonstartinthesamestate,expressingthesameproteinsatstage1,butsteptoquitedifferentstatesatstage2becauseoftheirdifferentarrangementsofregulatorymodules.regulatoryDNAcandefinethesequentialprogramofdevelopment:

therulesforsteppingfromonestatetothenext,asthecellsproliferateandreadtheirpositionsintheembryo,switchingonnewsetsofgenesaccordingtotheactivitiesoftheproteinsthattheycurrentlycontain组合调控在发育中的意义转录调节因子的组合产生细胞类型在发育过程中，细胞感知位置信号产生不同的转录因子组合在发育过程中，细胞可以累积一系列的调节蛋白，它们开始并不改变基因的表达，直到所需的调节因子组合中最后的成员加入，调节信息完全，导致基因表达大的改变。小结转录调节因子组合调控真核生物基因表达基因表达的时序性和组织特异性常由增强子和沉默子决定不同细胞类型具有不同的转录调节因子组在个体发育过程中细胞命运决定基因常为自激活的转录调节因子在系统进化过程中，顺式元件是不保守的选择性基因转录与染色质变化组蛋白乙酰化具有激活基因转录的作用核小体是真核生物染色质的基本单位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二聚体→H3H4四聚体→八聚体；N端在外，无结构eachofthecorehistoneshasalong

N-terminalaminoacid“tail”,

whichextendsoutfromtheDNA-histonecore.Thesehistonetailsaresubjectto

severaldifferenttypesofcovalentmodifications,whichcontrolmanyaspectsofchromatinstructure.

组蛋白N端“尾”可被多种共价修饰，与染色质结构的控制有关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.组蛋白乙酰化：减少组蛋白与DNA之间的亲和力影响30nm结构的稳定性differenttypesofcelldisplaydifferentpatternsofhistoneacetylation,Indications:

histoneacetylationplaysaprominentroleinregulatinggenomeexpression.启动子DNA序列甲基化修饰使基因沉默Ineukaryotes,cytosinebasesinchromosomalDNAmoleculesaresometimeschangedto5-methyl