rRNA isimportantinribosomal functionProtected byP-tRNA三CINear EF-GbindingCTSubunitassociationCentral domainCTERMsuppressionC三m3'majordomain530loopatAsiteNeededforP-tRNAbindingTERMsuppressionNeededforP-tRNAbinding1400atAsite公Colicin E3 cleavage1492atAsiteNomethylationinksgAComplementarytomRNAAccuracyC3'minordomain5'domainC具二IDevirtualtexf.www.ergito.comFigure6.45Some sitesin16SrRNAareprotectedfrom chemical probeswhen50S subunits join30S subunitsorwhen aminoacyl-tRNAbindstotheAsite.Othersarethe sites ofmutationsthat affectproteinsynthesis.TERMsuppressionsitesmayaffecttermination atsomeor severaltermination codons.Thelargecolored blocksindicate the four domains of therRNA
(二)真核生物tRNA和 rRNA的加工1.tRNA的加工真核tRNA的基因与原核不同：真核的前体分子tRNA数目多、单顺反子，成簇排列、有基因间隔区，例如爪蟾tRNA基因数目>200拷贝，酵母约有400个tRNA基因，是一种重复序列真核的前体分子RNA中含有内含子其加工过程要剪接内含子，都要加CCA
（二）真核生物 tRNA 和 rRNA的加工 1. tRNA的加工 真核tRNA的基因与原核不同： 真核的前体分子 tRNA数目多、单顺反子、 成簇排列、有基因间隔区，例如 爪蟾 tRNA基因 数目＞200拷贝，酵母约有400个tRNA基因，是一 种重复序列； 真核的前体分子 tRNA中含有内含子。 其加工过程要剪接内含子，都要加CCA
Figure22.23 The intron in yeast tRNAPhe base pairswith the anticodon to change the structure oftheanticodon arm.Pairing between an excluded base inthe stem and the intron loop inthe precursormayberequiredforsplicing.Mature tRNAPrecursortRNACGCGGAGAAUAUGcmGcmAyAymcACGU拉AUYAUGCGAAAUGG.CGAnticodonUAIntronpairs withUAGanticodon loopC-AAAIntron
Figure 22.24 Splicing of yeast tRNA in vitro can befollowed by assaying the RNA precursor and productsbygelelectrophoresis.酵母tRNA前体的体Incubatewithsplicingextract外剪接invitro十F在未处理前电泳只出现一条带Gelelectrophoresis加入核酸内切酶Precursor出现两条带tRNAIntron
酵母tRNA前体的体 外剪接 在未处理前电泳 只出现一条带 加入核酸内切酶 出现两条带
Figure 22.26 SplicingoftRNArequiresseparate nuclease andOH3'Pligaseactivities.Theexon-intron boundariesare cleaved by thenuclease to generate 2NucleaseFolding十3-cyclicphosphateand?25-OHtermini.ThecyclicOHphosphateisopenedtoLOgenerate 3-OH and2-OH53phosphategroups.The2-3'P5-OHisphosphorylatedAfterreleasingtheintronthetRNAhalfmoleculestRNA半分子fold intoatRNA-likestructurethatnowhasa3-OH,5-Pbreak.Thisissealed bya ligase.PhosphodiesteraseopensphosphateringKinasephosphorylates5-OHterminusStRNAtRNAchainBaseBasechainOOOHOBaseBaseCH2CH,OHO-P-OOHOHOtRNA111520042,3cyclicphosphate3-OH,2-phosphatechain
tRNA半分子 tRNA半分子