Gene-X-ppt(Gene10-基因十)--Chapter27

Chapter27PhageStrategies27.1Introduction•bacteriophage(orphage)–Abacterialvirus.•lyticinfection–Infectionofabacteriumbyaphagethatendsinthedestructionofthebacteriumwithreleaseofprogenyphage.•lysis–Thedeathofbacteriaattheendofaphageinfectivecyclewhentheyburstopentoreleasetheprogenyofaninfectingphage(becausephageenzymesdisruptthebacterium’scytoplasmicmembraneorcellwall).27.1IntroductionFIGURE01:Aphagemayfollowthelyticorlysogenicpathway27.1Introduction•virulentphage–Abacteriophagethatcanonlyfollowthelyticcycle.•prophage–Aphagegenomecovalentlyintegratedasalinearpartofthebacterialchromosome.•lysogeny–Theabilityofaphagetosurviveinabacteriumasastableprophagecomponentofthebacterialgenome.27.1Introduction•temperatephage–Abacteriophagethatcanfollowthelyticorlysogenicpathway.•integration–InsertionofaviraloranotherDNAsequenceintoahostgenomeasaregioncovalentlylinkedoneithersidetothehostsequences.•excision–ReleaseofphagefromthehostchromosomeasanautonomousDNAmolecule.27.1Introduction•inductionofphage–Aphage’sentryintothelytic(infective)cycleasaresultofdestructionofthelysogenicrepressor,whichleadstoexcisionoffreephageDNAfromthebacterialchromosome.•plasmid–Circular,extrachromosomalDNA.Itisautonomousandcanreplicateitself.•episome–AplasmidabletointegrateintobacterialDNA.27.2LyticDevelopmentIsDividedintoTwoPeriods•Aphageinfectivecycleisdividedintotheearlyperiod(beforereplication)andthelateperiod(aftertheonsetofreplication).•Aphageinfectiongeneratesapoolofprogenyphagegenomesthatreplicateandrecombine.FIGURE02:Phagesreproduceinlyticdevelopment27.3LyticDevelopmentIsControlledbyaCascade•cascade–Asequenceofevents,eachofwhichisstimulatedbythepreviousone.–Transcriptionalregulationisdividedintostages,andateachstageoneofthegenesthatisexpressedencodesaregulatorneededtoexpressthegenesofthenextstage.FIGURE03:Lyticdevelopmentisaregulatorycascade27.3LyticDevelopmentIsControlledbyaCascade•Theearly(orimmediateearly)genestranscribedbyhostRNApolymerasefollowinginfectioninclude,orcomprise,regulatorsrequiredforexpressionofthemiddle(ordelayedearly)setofphagegenes.•Themiddlegroupofgenesincludesregulatorstotranscribethelategenes.•Thisresultsintheorderedexpressionofgroupsofgenesduringphageinfection.27.4TwoTypesofRegulatoryEventsControltheLyticCascade•Regulatorproteinsusedinphagecascadesmaysponsorinitiationatnew(phage)promotersorcausethehostpolymerasetoreadthroughtranscriptionterminators.FIGURE06:RNApolymerasecontrolspromoterrecognition.27.5ThePhageT7andT4GenomesShowFunctionalClustering•Genesconcernedwithrelatedfunctionsareoftenclustered.FIGURE08:T4genesshowfunctionalclustering27.5ThePhageT7andT4GenomesShowFunctionalClustering•PhagesT7andT4areexamplesofregulatorycascadesinwhichphageinfectionisdividedintothreeperiods.FIGURE09:T4genesfallintotwogeneralgroups27.6LambdaImmediateEarlyandDelayedEarlyGenesAreNeededforBothLysogenyandtheLyticCycle•Lambdahastwoimmediateearlygenes,Nandcro,whicharetranscribedbyhostRNApolymerase.•TheNgeneisrequiredtoexpressthedelayedearlygenes.•Threeofthedelayedearlygenesareregulators.27.6LambdaImmediateEarlyandDelayedEarlyGenesAreNeededforBothLysogenyandtheLyticCycle•LysogenyrequiresthedelayedearlygenescII-cIII.•ThelyticcyclerequirestheimmediateearlygenecroandthedelayedearlygeneQ.FIGURE10:Lambdahastwolifestyles27.7TheLyticCycleDependsonAntiterminationbypN•pNisanantiterminationfactorthatallowsRNApolymerasetocontinuetranscriptionpasttheendsofthetwoimmediateearlygenes.•pQistheproductofadelayedearlygeneandisanantiterminatorthatallowsRNApolymerasetotranscribethelategenes.FIGURE12:Similarcontrolsapplytoleftandrighttranscription27.7TheLyticCycleDependsonAntiterminationbypN•LambdaDNAcircularizesafterinfection;asaresult,thelategenesformasingletranscriptionunit.FIGURE13:Lambdahasthreestagesofdevelopment27.8LysogenyIsMaintainedbytheLambdaRepressorProtein•Thelambdarepressor,encodedbythecIgene,isrequiredtomaintainlysogeny.•ThelambdarepressoractsattheOLandORoperatorstoblocktranscriptionoftheimmediateearlygenes.•Theimmediateearlygenestriggeraregulatorycascade;asaresult,theirrepressionpreventsthelyticcyclefromproceeding.FIGURE15:Repressormaintainslysogeny27.9TheLambdaRepressorandItsOperatorsDefinetheImmunityRegion•immunity–Inphages,theabilityofaprophagetopreventanotherphageofthesametypefrominfectingacell.•virulentmutations–Phagemutantsthatareunabletoestablishlysogeny.27.9TheLambdaRepressorandItsOperatorsDefinetheImmunityRegion•Severallambdoidphageshavedifferentimmunityregions.•Alysogenicphageconfersimmunitytofurtherinfectionbyanyotherphagewiththesameimmunityregion.FIGURE16:RNApolymeraseinitiatesatPlandPrbutnotatPrmduringthelyticcycle.27.10TheDNA-BindingFormoftheLambdaRepressorIsaDimer•Arepressormonomerhastwodistinctdomains.•TheN-terminaldomaincontainstheDNA-bindingsite.•TheC-terminaldomaindimerizes.•BindingtotheoperatorrequiresthedimericformsothattwoDNA-bindingdomainscancontacttheoperatorsimultaneously.27.10TheDNA-BindingFormoftheLambdaRepressorIsaDimer•Cleavageoftherepressorbetweenthetwodomainsreducestheaffinityfortheoperatorandinducesalyticcycle