細(xì)胞分化的分子機(jī)制

一、緒論細(xì)胞分化的分子機(jī)制細(xì)胞分化

多細(xì)胞生物個(gè)體生長發(fā)育過程中，細(xì)胞在結(jié)構(gòu)、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(xì)胞類型，形成不同的組織器官和系統(tǒng)。細(xì)胞類型分化基因表達(dá)調(diào)控細(xì)胞類型形成組織器官形成個(gè)體發(fā)育細(xì)胞分化：漸進(jìn)過程；貫穿生命過程細(xì)胞決定：細(xì)胞在發(fā)育的某個(gè)時(shí)刻被定向被定向的細(xì)胞最終發(fā)育成為成熟細(xì)胞細(xì)胞表型全能性細(xì)胞（totipotentcell)多潛能細(xì)胞（pluripotentcell)分化的細(xì)胞（differentiatedcell)能夠產(chǎn)生全部細(xì)胞表型；具有分化形成全部細(xì)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(xì)胞類型的能力發(fā)育命運(yùn)在一定程度上被限定；基因表達(dá)受到一定程度的限定由多潛能細(xì)胞分裂分化發(fā)育成的特殊細(xì)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(dá)細(xì)胞分化過程全能性細(xì)胞多潛能細(xì)胞分化細(xì)胞基因選擇性表達(dá)的結(jié)果隨著個(gè)體發(fā)育的進(jìn)行，細(xì)胞核指導(dǎo)發(fā)育的潛能被限定，甚至喪失指導(dǎo)全部發(fā)育的能力豹蛙囊胚期細(xì)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(nèi)胚層細(xì)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(nèi)胚層細(xì)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(nèi)部決定因素控制外部信號內(nèi)部決定因素各種細(xì)胞因子各種轉(zhuǎn)錄調(diào)節(jié)因子細(xì)胞分化是基因差異表達(dá)的結(jié)果細(xì)胞內(nèi)環(huán)境影響差異表達(dá)

卵質(zhì)不均勻分布細(xì)胞外環(huán)境的影響

胚胎細(xì)胞處于不同區(qū)域，接受不同位置信息；鄰近細(xì)胞相互關(guān)系；信號轉(zhuǎn)導(dǎo)細(xì)胞誘導(dǎo)細(xì)胞誘導(dǎo)細(xì)胞生長因子鄰近細(xì)胞表面分子細(xì)胞不對稱分裂不對稱分裂受體及信號轉(zhuǎn)導(dǎo)分子基因差異表達(dá)是最重要的調(diào)控機(jī)制差異表達(dá)基因

時(shí)間特異性：只在發(fā)育的某個(gè)特定時(shí)期表達(dá)空間特異性：組織、細(xì)胞特異性基因表達(dá)在時(shí)間和空間上的特異性時(shí)間特異性空間特異性在發(fā)育的某個(gè)或某些時(shí)期表達(dá)檢測方法：轉(zhuǎn)錄組測序；消減雜交；RT-PCR；real-timePCR基因表達(dá)的組織特異性、細(xì)胞特異性檢測方法：原位雜交胚胎誘導(dǎo)胚胎發(fā)育過程中一部分細(xì)胞影響相鄰細(xì)胞向一定方向分化不對稱分裂mRNA不對稱分配視胞誘導(dǎo)外胚層形成晶體晶體誘導(dǎo)外胚層形成角膜細(xì)胞環(huán)境影響轉(zhuǎn)錄抑制劑（放線菌素D）處理受精卵，胚胎發(fā)育仍能進(jìn)行至囊胚期蛋白質(zhì)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(dá)機(jī)制細(xì)胞分化的分子機(jī)制差異基因表達(dá)的調(diào)控機(jī)制差異基因轉(zhuǎn)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(dá)受轉(zhuǎn)錄調(diào)節(jié)因子的組合調(diào)控真核生物基因的調(diào)控區(qū)啟動子+DNA調(diào)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(zhuǎn)錄調(diào)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(dá)調(diào)控的保守機(jī)制蛋白-核酸相互作用真核生物基因表達(dá)需要基礎(chǔ)轉(zhuǎn)錄因子通常搭建轉(zhuǎn)錄起始復(fù)合體時(shí)是低效的，需要轉(zhuǎn)錄激活中介體遠(yuǎn)距離互作轉(zhuǎn)錄調(diào)節(jié)因子與中介體相互作用轉(zhuǎn)錄調(diào)節(jié)因子與基礎(chǔ)轉(zhuǎn)錄因子相互作用Thegeneregulatoryproteinsallowtheindividualgenesofanorganismtobeturnedonoroffspecifically.Differentselectionsofgeneregulatoryproteinsarepresentindifferentcelltypesandtherebydirectthepatternsofgeneexpressionthatgiveeachcelltypeitsuniquecharacteristics.Eachgeneinaeucaryoticcellisregulateddifferentlyfromnearlyeveryothergene.轉(zhuǎn)錄調(diào)節(jié)因子介導(dǎo)基因轉(zhuǎn)錄調(diào)節(jié)不同細(xì)胞類型，有不同的轉(zhuǎn)錄調(diào)節(jié)蛋白組，因而特定基因表達(dá)譜不同基因調(diào)控序列不同順式元件啟動子UPE增強(qiáng)子沉默子TATA盒和Inr主要決定轉(zhuǎn)錄方向，起始位點(diǎn)，只能引起相當(dāng)?shù)退降霓D(zhuǎn)錄；UPE元件影響轉(zhuǎn)錄起始的頻率，但不具有組織特異性調(diào)控的性能增強(qiáng)子(enhancer)具有正調(diào)控功能的順式元件，主要見于真核生物增強(qiáng)子可以位于啟動子上游、內(nèi)含子中、下游Inhighereucaryotesitisnotunusualtofindtheregulatorysequencesofagenedottedoverdistancesasgreatas50,000nucleotidepairs.Figure9.26.Activatorsofeukaryotictranscriptioninitiation.Theblueactivatorisattachedtoaregulatorymoduleupstreamofagene,andinfluencestranscriptioninitiationonlyatthatsinglegene.Thegreenactivatorisattachedtoasitewithinanenhancerandisinfluencingtranscriptionofallthreegenes.?2002GarlandScience時(shí)間特異增強(qiáng)子人γ-珠蛋白基因在胚胎期表達(dá)，如果將β-珠蛋白基因的時(shí)間特異性增強(qiáng)子移到γ-珠蛋白基因附近，該基因可在成體中表達(dá)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(qiáng)子人β-珠蛋白基因的時(shí)間特異性增強(qiáng)子位于AATAAAA下游600～900bp增強(qiáng)子能使基因轉(zhuǎn)錄頻率增加10～200倍組織特異性增強(qiáng)子主增強(qiáng)子β-珠蛋白基因的第三個(gè)內(nèi)含子內(nèi)有組織特異性增強(qiáng)子使β-珠蛋白基因只能在紅細(xì)胞中轉(zhuǎn)錄所有位于人11號染色體上的β-珠蛋白基因家族的轉(zhuǎn)錄都受主增強(qiáng)子控制，缺失主增強(qiáng)子基因家族所有成員都沉默區(qū)域染色質(zhì)結(jié)構(gòu)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(yīng)不是單個(gè)激活因子效應(yīng)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(yīng)基因轉(zhuǎn)錄活性在不同細(xì)胞中可由不同增強(qiáng)子控制果蠅卵黃基因有兩個(gè)增強(qiáng)子，一個(gè)負(fù)責(zé)卵黃基因在卵巢中表達(dá)，另一個(gè)負(fù)責(zé)該基因在脂肪體中表達(dá)silencer

Aregulatorysequencethatreducestherateoftranscriptionofageneorgeneslocatedsomedistanceawayineitherdirection.

restrictingthetranscriptionofaparticulargenetoaparticulargroupofcellsregulatingthetimingofthegene'sexpression真核生物以正調(diào)控為主沉默子：基因的負(fù)調(diào)控元件，可介導(dǎo)基因表達(dá)的組織特異性和時(shí)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(dá)基因中；限制靶基因在除神經(jīng)元以外其它組織中的表達(dá)；轉(zhuǎn)錄調(diào)節(jié)因子NRSF（識別NRSE)在除神經(jīng)元以外其它組織中表達(dá)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(zhuǎn)錄A：消化道內(nèi)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(gè)轉(zhuǎn)錄調(diào)節(jié)因子，既可以是轉(zhuǎn)錄激活因子，也可以是阻遏蛋白真核生物基因表達(dá)調(diào)控的特點(diǎn)是組合調(diào)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(diào)控序列不保守Theprotein-codingsequencesareunmistakablysimilar,butthecorrespondingregulatoryDNAsequencesappearverydifferent.

eve基因調(diào)控區(qū)一組調(diào)節(jié)模塊(regulatorymodules),每個(gè)模塊含multipleregulatorysequences，決定一條eve基因表達(dá)的條帶

每個(gè)模塊，決定一條eve基因表達(dá)的條帶阻遏蛋白:GiantandKrüppel激活因子:Bicoid、HunchbackEve基因表達(dá)調(diào)控，提供了真核基因表達(dá)組合調(diào)控的例子，七種調(diào)控蛋白組合用于激活eve表達(dá)，每種組合決定一條帶，在帶與帶之間的位置，轉(zhuǎn)錄調(diào)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(xì)胞分化涉及轉(zhuǎn)錄因子的次序表達(dá)中胚層祖細(xì)胞成肌細(xì)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(guān)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(xì)胞分化產(chǎn)生腹側(cè)子宮前體細(xì)胞Lin-2-胚胎Z1.Ppp和Z4Aaa細(xì)胞分化產(chǎn)生子宮頸細(xì)胞果蠅具有能分化上皮細(xì)胞或神經(jīng)母細(xì)胞潛能的細(xì)胞正常情況：1/4分化為神經(jīng)母細(xì)胞；3/4分化為上皮組織的前體Notch轉(zhuǎn)錄缺乏的胚胎中：全部細(xì)胞分化為神經(jīng)母細(xì)胞，胚胎非正常發(fā)育，至死亡肌母細(xì)胞決定基因Mydo1基因僅在肌細(xì)胞中表達(dá)肌細(xì)胞分化的開關(guān)基因具有肌細(xì)胞分化表型特異性調(diào)控基因功能細(xì)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(dá)模式轉(zhuǎn)錄調(diào)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(zhuǎn)錄因子次序表達(dá)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(xì)胞中存在的調(diào)節(jié)蛋白，MyoD,Myf5,myogenin,Mrf4識別許多muscle-specificgenes的調(diào)控序列在離體培養(yǎng)的雞胚表皮成纖維細(xì)胞中誘導(dǎo)MyoD表達(dá)，成纖維細(xì)胞轉(zhuǎn)變成肌細(xì)胞。MyoD,Myf5,myogenin,Mrf4中任何一個(gè)在成纖維細(xì)胞中表達(dá)都能誘導(dǎo)肌細(xì)胞分化正反饋回路推測：成纖維細(xì)胞中可能已經(jīng)積累一些調(diào)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(diào)節(jié)蛋白組合決定肌肉分化Ey基因編碼的調(diào)節(jié)蛋白引發(fā)果蠅眼形成果蠅的眼由成千的細(xì)胞組成Ey基因控制許多基因，包括一些調(diào)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(gè)調(diào)節(jié)蛋白的作用可以啟動轉(zhuǎn)錄調(diào)節(jié)蛋白基因表達(dá)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(dá)協(xié)調(diào)調(diào)節(jié)一種類型細(xì)胞中表達(dá)多種細(xì)胞類型特異基因激素誘導(dǎo)幾種基因表達(dá)（如類固醇受體激活的基因）饑餓、劇烈運(yùn)動→糖皮質(zhì)素激素→肝細(xì)胞中氨基酸→葡萄糖糖皮質(zhì)素激素受體（轉(zhuǎn)錄調(diào)節(jié)因子）多個(gè)基因表達(dá)

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(xì)胞對類固醇、甲狀腺素，維生素D、維甲酸的響應(yīng)不僅由信號決定，也與細(xì)胞類型有關(guān)；雖然許多細(xì)胞具有相同的細(xì)胞內(nèi)受體，但由于組合調(diào)控機(jī)制的緣故，類固醇、甲狀腺素，維生素D、維甲酸等受體調(diào)控的基因可不同細(xì)胞內(nèi)受體只有在與其它轉(zhuǎn)錄調(diào)節(jié)因子形成完整組合時(shí)才能激活靶基因的表達(dá)，而調(diào)節(jié)蛋白組合中的許多成員具有細(xì)胞類型特異性Figure21-4.HowregulatoryDNAdefinesthesuccessionofgeneexpressionpatternsindevelopment.ThegenomesoforganismsAandBcodeforthesamesetofproteinsbuthavedifferentregulatoryDNA.Thetwocellsinthecartoonstartinthesamestate,expressingthesameproteinsatstage1,butsteptoquitedifferentstatesatstage2becauseoftheirdifferentarrangementsofregulatorymodules.regulatoryDNAcandefinethesequentialprogramofdevelopment:

therulesforsteppingfromonestatetothenext,asthecellsproliferateandreadtheirpositionsintheembryo,switchingonnewsetsofgenesaccordingtotheactivitiesoftheproteinsthattheycurrentlycontain組合調(diào)控在發(fā)育中的意義轉(zhuǎn)錄調(diào)節(jié)因子的組合產(chǎn)生細(xì)胞類型在發(fā)育過程中，細(xì)胞感知位置信號產(chǎn)生不同的轉(zhuǎn)錄因子組合在發(fā)育過程中，細(xì)胞可以累積一系列的調(diào)節(jié)蛋白，它們開始并不改變基因的表達(dá)，直到所需的調(diào)節(jié)因子組合中最后的成員加入，調(diào)節(jié)信息完全，導(dǎo)致基因表達(dá)大的改變。小結(jié)轉(zhuǎn)錄調(diào)節(jié)因子組合調(diào)控真核生物基因表達(dá)基因表達(dá)的時(shí)序性和組織特異性常由增強(qiáng)子和沉默子決定不同細(xì)胞類型具有不同的轉(zhuǎn)錄調(diào)節(jié)因子組在個(gè)體發(fā)育過程中細(xì)胞命運(yùn)決定基因常為自激活的轉(zhuǎn)錄調(diào)節(jié)因子在系統(tǒng)進(jìn)化過程中，順式元件是不保守的選擇性基因轉(zhuǎn)錄與染色質(zhì)變化組蛋白乙?；哂屑せ罨蜣D(zhuǎn)錄的作用核小體是真核生物染色質(zhì)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(jié)構(gòu)eachofthecorehistoneshasalong

N-terminalaminoacid“tail”,

whichextendsoutfromtheDNA-histonecore.Thesehistonetailsaresubjectto

severaldifferenttypesofcovalentmodifications,whichcontrolmanyaspectsofchromatinstructure.

組蛋白N端“尾”可被多種共價(jià)修飾，與染色質(zhì)結(jié)構(gòu)的控制有關(guān)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.組蛋白乙酰化：減少組蛋白與DNA之間的親和力影響30nm結(jié)構(gòu)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