生物化学英文版课件：Chapter41 Regrogrammed genetic decoding and translational pause

Chapter41Regrogrammedgeneticdecoding&translationalpauseOutlineRegrogrammedgeneticdecodingTranslationalframeshiftingRead-throughJump-translation&translationalintronTheIncorporationofSecandPylTranslationalpauseAtalowconcentrationofRF2,Frameshiftingoccursandfull-lengthRF2istranslated.AtahighconcentrationofRF2,Frameshiftingdoesn’toccurandshortenedRF2istranslated.+1FrameshiftingofRF2inE.coliRead-throughInMo-MuLVRNA,gagandpolareinthesamereadingframebutseparatedbyaterminationcodon.TranslationofthepolycistronisaccomplishedbysuppressionbyaGln-tRNA.Similarly,theRNAoftobaccomosaicviruscontainsasuppressiblestopcodon.ThereadthroughproductcontainstheRNApolymerasedomain.Sequencecontextdetermineswhethersuppressionoccursatagivenstopcodon.Acomplexsignalpreventsnormalstopcodonsfrombeingsuppressed.Bothupstreamanddownstreamofthesuppressiblestopcodonarerequiredforsuppression.

RegionsdownstreamseemmoreconservedthanthoseupstreamTerminationsuppressionhasthesameeffectasframeshifting:theproductionoftwoproteins,onelongerthantheother.Theefficiencyofterminationsuppressiondeterminesthestoichiometricratioofthelongertotheshorterproduct

Jumptranslation&translationalintronBacteriophageT4gene60(18ksubunitofDNATopoisomerase).BycomparisonbetweencDNAandAAsequences,50ntsofthecodingregionofthemRNAareskippedasthemRNA"slips"betweentwoidenticalGly

codons(GGA)50ntsapart,withcloseto100%efficiencyofbypass.AGene60-lacZfusionisskippedwithalmost100%efficiency.Thisdependsonthenascentpeptide,andaUAGstopcodoncontainedinastablestem-loopjustpastthe5'GGAcodon.Ifyoudestabilizetheloop,nobypassofinterveningcodons.Ifyouputincompensatorynucleotidestocreateanewloop,bypassagain.PuttingGGAinto50ntsdropsefficiency.Increasing50ntdistancedropsefficiency.ChangeGGAstoGCAs,stillworksalthoughalittlelessbypass.Aminoacidscodedbyupstreamnucleotides17-42arecrucial,since1baseinsertionordeletiontoknockoutofframepreventsbypass,butinvisiblecodonswappinghasnoeffect.E.coli

trpR(trprepressorprotein,controls4E.coli

operons).bypass55ntJumptranslationofBacteriophageT4gene60TheIncorporationofSecandPylSelenocysteinehasthesamestructureascysteineexceptitcontainsseleniuminsteadofsulfur.Itisformedbymodifyingserineafterithasbeenattachedtoselenocysteine

tRNA.Pylisalysinederivativewithanextraaromaticring.Pylisfullysynthesizedandonlythenattachedtopyl

tRNA.BothSecandPylareencodedbystopcodons(UGAandUAG,respectively).BothhavetheirowntRNAsthatcontainanticodonsthatreadthesestopcodons.BothSecandPylalsohavespecificaminoacyl

tRNA

synthetasestochargethetRNAwiththeaminoacids.Moststopcodonsinorganismsthatuseselenocysteineandpyrrolysinedoindeedindicatestop.However,occasionalstopcodonsarerecognizedasencodingSecorPyl.Codoncontextcanchangecodonmeaning.Severalenzymes(forexample,glutathioneperoxidase,T4-5'deiodinaseandformate

dehydrogenase)containtheunusualaminoacidselenocysteine

TheyuseUGAsinthespecialcontextasthecodonforSec.Intheabsenceofselenium,proteinsynthesisfromthesemRNAsterminatesprematurely.ForselenocysteinethisdependsonarecognitionsequencejustdownstreamofthespecialUGAcodon.Thisformsastem–loopthatbindstheSelBprotein.TheSelBproteinalsobindschargedselenocysteine

tRNAandbringsittotheribosomewhenneeded.TheIncorporationofSecGeneticencodingofselenocysteineModelofredefinitionofUGAasSecasunderstoodforE.coli

formate

dehydrogenasePyrrolysineisrarerstill.IthasbeenfoundincertainArchaeaandBacteriabutwasfirstdiscoveredinspeciesofmethanogenic

Archaea,organismsthatgeneratemethane.Incertainmethanogenstheenzymemethylaminemethyltransferasec