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1、Bacterial GeneticsXiao-Kui GUO PhDBacterial GenomicsMicrobial Genomics68%Deinococcus radioduransMicrobial Genome FeaturesG+C content29%Borrelia burgdorferiGenome organizationsingle circular chromosomelarge linear chromosome plus21 extrachromosomal elementscircular chromosome plus one or moreextrachr

2、omosomal elementstwo circular chromosomesPLASMIDS PLASMIDS Plasmids are extrachromosomal genetic elements capable of Plasmids are extrachromosomal genetic elements capable of autonomous replication.An autonomous replication.An episomeepisome is a plasmid that can integrate into the bacterial is a pl

3、asmid that can integrate into the bacterial chromosomechromosomeClassification of PlasmidsClassification of Plasmids Transfer properties Transfer properties Conjugative plasmidsConjugative plasmids Nonconjugative plasmidsNonconjugative plasmids Phenotypic effects Phenotypic effects Fertility plasmid

4、(F factor)Fertility plasmid(F factor)Bacteriocinogenic plasmidsBacteriocinogenic plasmids.Resistance plasmids 7 factors)Resistance plasmids 7 factors).Insertion sequences(IS)-Insertion sequences are transposable genetic Insertion sequences are transposable genetic elements that carry no known genes

5、except those that are required for transposition.elements that carry no known genes except those that are required for transposition.a.a.NomenclatureNomenclature-Insertion sequences are given the designation IS followed by a number.-Insertion sequences are given the designation IS followed by a numb

6、er.e.g.IS1 e.g.IS1 b.b.StructureStructure Insertion sequences are small stretches of DNA that have at their ends repeated sequences,Insertion sequences are small stretches of DNA that have at their ends repeated sequences,which are involved in transposition.In between the terminal repeated sequences

7、 there are genes involved in which are involved in transposition.In between the terminal repeated sequences there are genes involved in transposition and sequences that can control the expression of the genes but no other nonessential genes transposition and sequences that can control the expression

8、 of the genes but no other nonessential genes are present.are present.c.c.ImportanceImportance i)Mutation-The introduction of an insertion sequence into a bacterial gene will result in the inactivation of i)Mutation-The introduction of an insertion sequence into a bacterial gene will result in the i

9、nactivation of the gene.the gene.ii)Plasmid insertion into chromosomes-The sites at which plasmids insert into the bacterial chromosome ii)Plasmid insertion into chromosomes-The sites at which plasmids insert into the bacterial chromosome are at or near insertion sequence in the chromosome.are at or

10、 near insertion sequence in the chromosome.iii)Phase Variation-The flagellar antigens are one of the main antigens to which the immune response is iii)Phase Variation-The flagellar antigens are one of the main antigens to which the immune response is directed in our attempt to fight off a bacterial

11、infection.In Salmonella there are two genes which code for two directed in our attempt to fight off a bacterial infection.In Salmonella there are two genes which code for two antigenically different flagellar antigens.The expression of these genes is regulated by an insertion antigenically different

12、 flagellar antigens.The expression of these genes is regulated by an insertion sequences.In one orientation one of the genes is active while in the other orientation the other flagellar sequences.In one orientation one of the genes is active while in the other orientation the other flagellar gene is

13、 active.Thus,Salmonella can change their flagella in response to the immune systems attack.gene is active.Thus,Salmonella can change their flagella in response to the immune systems attack.Phase variation is not unique to Salmonella flagellar antigens.It is also seen with other bacterial surface Pha

14、se variation is not unique to Salmonella flagellar antigens.It is also seen with other bacterial surface antigens.Also the mechanism of phase variation may differ in different species of bacteria(e.g.Neisseria;antigens.Also the mechanism of phase variation may differ in different species of bacteria

15、(e.g.Neisseria;transformation).transformation).Transposons(Tn)-Transposons are transposable genetic elements Transposons are transposable genetic elements that carry one or more other genes in addition to those which are essential for that carry one or more other genes in addition to those which are

16、 essential for transposition.transposition.NomenclatureNomenclature-Transposons are given the designation Tn followed by a -Transposons are given the designation Tn followed by a number.number.StructureStructure-The structure of a transposon is similar to that of an insertion -The structure of a tra

17、nsposon is similar to that of an insertion sequence.The extra genes are located between the terminal repeated sequence.The extra genes are located between the terminal repeated sequences.In some instances(composite transposons)the terminal repeated sequences.In some instances(composite transposons)t

18、he terminal repeated sequences are actually insertion sequences.sequences are actually insertion sequences.ImportanceImportance-Many antibiotic resistance genes are located on transposons.-Many antibiotic resistance genes are located on transposons.Since transposons can jump from one DNA molecule to

19、 another,these Since transposons can jump from one DNA molecule to another,these antibiotic resistance transposons are a major factor in the development of antibiotic resistance transposons are a major factor in the development of plasmids which can confer multiple drug resistance on a bacterium pla

20、smids which can confer multiple drug resistance on a bacterium harboring such a plasmid.These multiple drug resistance plasmids have harboring such a plasmid.These multiple drug resistance plasmids have become a major medical problem because the indiscriminate use of become a major medical problem b

21、ecause the indiscriminate use of antibiotics have provided a selective advantage for bacteria harboring these antibiotics have provided a selective advantage for bacteria harboring these plasmidsplasmids.MechanismofbacterialvariationGene mutationGene transfer and recombination TransformationTransfor

22、mationConjugationConjugationTransductionTransductionLysogenicconversionLysogenicconversionProtoplastfusionProtoplastfusionTypes of mutationBase substitutionFrame shefitInsertion sequencesWhat can cause mutation?Chemicals:nitrous acid;alkylating agents5-bromouracilbenzpyreneRadiation:X-rays and Ultra

23、violet lightVirusesBacterial mutationMutation rateMutation and selectivityBackward mutationTransformationTransformation is gene transfer resulting from the uptake by a recipient cell of naked DNA from a donor cell.Certain bacteria(e.g.Bacillus,Haemophilus,Neisseria,Pneumococcus)can take up DNA from

24、the environment and the DNA that is taken up can be incorporated into the recipients chromosome.Conjugation Transfer of DNA from a donor to a recipient by direct physical contact between the cells.In bacteria there are two mating types a donor(male)and a recipient(female)and the direction of transfe

25、r of genetic material is one way;DNA is transferred from a donor to a recipient.Physiological States of F FactorAutonomous(F+)Characteristics of FCharacteristics of F+x F x F-crosses crossesF F-becomes F becomes F+while F while F+remains F remains F+Low transfer of donor Low transfer of donor chromo

26、somal geneschromosomal genesF+Physiological States of F FactorIntegrated(Hfr)Characteristics of Characteristics of Hfr x FHfr x F-crosses crossesF F-rarely becomes rarely becomes Hfr while Hfr Hfr while Hfr remains Hfrremains HfrHigh transfer of High transfer of certain donor certain donor chromosom

27、al chromosomal genesgenesF+HfrPhysiological States of F FactorAutonomous with Autonomous with donor genes(Fdonor genes(F)Characteristics of FCharacteristics of F x Fx F-crosses crossesF F-becomes F becomes F while Fwhile F remains remains F F High transfer of High transfer of donor genes on donor ge

28、nes on F F and low and low transfer of other transfer of other donor donor chromosomal chromosomal genesgenesHfrFMechanism of F+x F-CrossesDNA transferOrigin of Origin of transfertransferRolling circle Rolling circle replicationreplication Pair formation Conjugation bridgeF+F-F+F-F+F+F+F+Mechanism o

29、f Hfr x F-CrossesDNA transferDNA transfer Origin of transferOrigin of transfer Rolling circle Rolling circle replicationreplicationHomologous Homologous recombinationrecombination Pair formation Conjugation bridgeHfrF-HfrF-HfrF-HfrF-Mechanism of F x F-CrossesDNA transferDNA transfer Origin of Origin

30、 of transfertransfer Rolling circle Rolling circle replicationreplication Pair formation Conjugation bridgeFFFFFF-FF-RPlasmidTransduction:Transductionisdefinedasthetransferofgeneticinformationbetweencellsthroughthemediationofavirus(phage)particle.Ittherefore does not require cell to cellcontactandis

31、DNaseresistant.Generalized Transduction Generalized transduction is transduction in which potentially any bacterial gene from the donor can be transferred to the recipient.The mechanism of generalizedtransductionGeneralized transduction1.It is relatively easy.It is relatively easy.2.It is rather eff

32、icient(10It is rather efficient(10-3-3 per recipient with per recipient with P22HT,10P22HT,10-6-6 with P22 or P1),using the with P22 or P1),using the correct phage.correct phage.3.It moves only a small part of the It moves only a small part of the chromosome which allows you to change chromosome whi

33、ch allows you to change part of a strains genotype without affecting part of a strains genotype without affecting the rest of the chromosome.the rest of the chromosome.4.The high frequency of transfer and the The high frequency of transfer and the small region transferred allows fine-small region transferred allows fine-structure mappingstructure mappingSpecialized transductionSpecialized transduc