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1、12POST-TRANSCRIPTIONALGENECONTROLANDNUCLEARTRANSPORTPortionofa“lampbrushchromosome”fromanoocyteofthenewtNophthalmusviridescens;hnRNPproteinassociatedwithnascentRNAtranscriptsfluorescesredafterstainingwithamonoclonalantibody.[CourtesyofM.RothandJ.Gall.]nthepreviouschapter,we
2、sawthatmostgenesareregu-12.1ProcessingofEukaryoticlatedatthefirststepingeneexpression,namely,theini-Itiationoftranscription.However,oncetranscriptionhasPre-mRNAbeeninitiated,synthesisoftheencodedRNArequiresthatInthissection,wetakeacloserlookathoweukaryoticcellsRNApolymerase
3、transcribetheentiregeneandnottermi-converttheinitialprimarytranscriptsynthesizedbyRNAnateprematurely.Moreover,theinitialprimarytranscriptspolymeraseIIintoafunctionalmRNA.Threemajoreventsproducedfromeukaryoticgenesmustundergovariouspro-occurduringtheprocess:5capping,3cleav
4、age/polyadenyl-cessingreactionstoyieldthecorrespondingfunctionalation,andRNAsplicing(Figure12-2).ProcessingoccursinRNAs.Onceformedinthenucleus,mature,functionalthenucleusasthenascentmRNAprecursorisbeingtran-RNAsaretransportedtothecytoplasmascomponentsofri-scribed,andthefunc
5、tionalmRNAproducedistransportedbonucleoproteins.BothprocessingofRNAsandtheirexporttothecytoplasmbymechanismsdiscussedlater.fromthenucleusofferopportunitiesforfurtherregulatinggeneexpressionatstagessubsequenttotranscriptioninitia-tion.Additionalcontrolofgeneexpressioncanoccu
6、rintheThe5CapIsAddedtoNascentRNAsShortlycytoplasm.Inthecaseofprotein-codinggenes,forinstance,AfterInitiationbyRNAPolymeraseIItheamountofproteinproduceddependsonthestabilityofthecorrespondingmRNAsinthecytoplasmandtherateofAfternascentRNAmoleculesproducedbyRNApolymerasetheir
7、translation.Inaddition,thecellularlocationsofsomeIIreachalengthof25–30nucleotides,7-methylguanosinemRNAsareregulated,sothatnewlysynthesizedproteinisandtheothercomponentsofthe5capfoundoneukaryoticconcentratedwhereitisneeded.Werefertoallthemechanismsthatregulategeneexpres-OU
8、TLINEsionfollowingtranscriptionaspost-transcriptionalgenecon-trol(Figure12-1).Alth
