Gene-X-ppt(Gene10-基因十)--Chapter13

Chapter13BacterialReplicationIsConnectedtotheCellCycle13.1IntroductionFIGURE01:Replicationinitiatesatthebacterialoriginwhenacellpassesacriticalthresholdofsize13.2ReplicationIsConnectedtotheCellCycle•ThedoublingtimeofE.colicanvaryovera10range,dependingongrowthconditions.•Itrequires40minutestoreplicatethebacterialchromosome(atnormaltemperature).•Completionofareplicationcycletriggersabacterialdivision20minuteslater.13.2ReplicationIsConnectedtotheCellCycle•Ifthedoublingtimeis~60minutes,areplicationcycleisinitiatedbeforethedivisionresultingfromthepreviousreplicationcycle.•Fastratesofgrowththereforeproducemultiforkedchromosomes.FIGURE02:Thefixedintervalof60minutesbetweeninitiationofreplicationandcelldivisionproducesmultiforkedchromosomesinrapidlygrowingcells13.3TheSeptumDividesaBacteriumintoProgenyThatEachContainaChromosome•anucleatecell–Bacteriathatlackanucleoid,butareofsimilarshapetowild-typebacteria.•Bacterialchromosomesarespecificallyarrangedandpositionedinsidecells.•poles–Theendsofabacterialcell,usuallycontainingspecializedstructureforpositioningstructures.•Septumformationisinitiatedmid-cell,50%ofthedistancefromtheseptumtoeachendofthebacterium.13.3TheSeptumDividesaBacteriumintoProgenyThatEachContainaChromosomeFIGURE03:AttachmentofbacterialDNAtothemembranecouldprovideamechanismforsegregation13.3TheSeptumDividesaBacteriumintoProgenyThatEachContainaChromosome•Theseptumconsistsofthesamepeptidoglycansthatcomprisethebacterialenvelope.•TherodshapeofE.coliisdependentonMreB,PBP2,andRodA.•FtsZisnecessarytorecruittheenzymesneededtoformtheseptum.13.4MutationsinDivisionorSegregationAffectCellShape•ftsmutantsformlongfilamentsbecausetheseptumthatdividesthedaughterbacteriafailstoform.•Minicellsforminmutantsthatproducetoomanysepta.–TheyaresmallandlackDNA.•Anucleatecellsofnormalsizearegeneratedbypartitionmutants,inwhichtheduplicatechromosomesfailtoseparate.13.5FtsZIsNecessaryforSeptumFormation•TheproductofftsZisrequiredforseptumformationatpreexistingsites.•FtsZisaGTPasethatformsaring(theZ-ringorseptalring)ontheinsideofthebacterialenvelope.–Itisconnectedtoothercytoskeletalcomponents.13.6minandnoc/slmGenesRegulatetheLocationoftheSeptum•ThelocationoftheseptumiscontrolledbyminC,-D,and-Eandbynoc/slmA.•ThenumberandlocationofseptaisdeterminedbytheratioofMinE/MinC,D.•DynamicmovementoftheMinproteinsinthecellsetsupapatterninwhichinhibitionofZ-ringassemblyishighestatthepolesandlowestatmid-cell.•SlmA/Nocproteinspreventseptationfromoccurringinthespaceoccupiedbythebacterialchromosome.13.6minandnoc/slmGenesRegulatetheLocationoftheSeptumFIGURE08:MinC/DisadivisioninhibitorwhoseactionisconfinedtothepolarsitesbyMinE13.7ChromosomalSegregationMayRequireSite-SpecificRecombination•TheXersite-specificrecombinationsystemactsonatargetsequencenearthechromosometerminus.–Itrecreatesmonomersifageneralizedrecombinationeventhasconvertedthebacterialchromosometoadimer.FIGURE09:Intermolecularrecombinationmergesmonomersintodimers,andintramolecularrecombinationreleasesindividualunitsfromoligomers13.7ChromosomalSegregationMayRequireSite-SpecificRecombinationFIGURE10:Acircularchromosomereplicatestoproducetwomonomericdaughtersthatsegregatetodaughtercells13.7ChromosomalSegregationMayRequireSite-SpecificRecombinationFIGURE11:Arecombinationeventcreatestwolinkedchromosomes13.8PartitionInvolvesSeparationoftheChromosomes•Repliconoriginsareattachedtotheinnerbacterialmembrane.•Chromosomesmakeabruptmovementsfromthemid-centertotheonequarterandthreequarterpositions.FIGURE12:TheDNAofasingleparentalnucleoidbecomesdecondensedduringreplication13.9Single-CopyPlasmidsHaveaPartitioningSystem•copynumber–Thenumberofcopiesofaplasmidthatismaintainedinabacterium(relativetothenumberofcopiesoftheoriginofthebacterialchromosome).•Single-copyplasmidsexistatoneplasmidcopyperbacterialchromosomeorigin.•Multicopyplasmidsexistat1plasmidcopyperbacterialchromosomeorigin.13.9Single-CopyPlasmidsHaveaPartitioningSystem•Partitionsystemsensurethatduplicatedplasmidsaresegregatedtodifferentdaughtercellsproducedbyadivision.FIGURE14:ThepartitionofplasmidR1involvespolymerizationoftheParMATPasebetweenplasmids13.9Single-CopyPlasmidsHaveaPartitioningSystem•addictionsystem–Asurvivalmechanismusedbyplasmids.–Themechanismkillsthebacteriumuponlossoftheplasmid.FIGURE15:Plasmidsmayensurethatbacteriacannotlivewithoutthembysynthesizingalong-livedkillerandashort-livedantidote13.10PlasmidIncompatibilityIsDeterminedbytheReplicon•Plasmidsinasinglecompatibilitygrouphaveoriginsthatareregulatedbyacommoncontrolsystem.FIGURE16:Twoplasmidsareincompatible(theybelongtothesamecompatibilitygroup)iftheiroriginscannotbedistinguishedatthestageofinitiation13.11TheColE1CompatibilitySystemIsControlledbyanRNARegulator•ReplicationofColE1requirestranscriptiontopassthroughtheorigin,wherethetranscriptiscleavedbyRNAaseHtogenerateaprimerend.FIGURE17:ReplicationofColE1DNAisinitiatedbycleavingtheprimerRNAtogeneratea3’–OHend13.11TheColE1CompatibilitySystemIsControlledbyanRNARegulator•TheregulatorRNAIisashortantisenseRNAthatpairswiththetranscriptand