细菌的遗传和变异_3

Bacterial Genetics,Xiao-Kui GUO PhD,Bacterial Genomics,Microbial Genomics,68% Deinococcus radiodurans,Microbial Genome Features,G+C content,29% Borrelia burgdorferi,Genome organization,single circular chromosome,large linear chromosome plus 21 extrachromosomal elements,circular chromosome plus one or more extrachromosomal elements,two circular chromosomes,PLASMIDS Plasmids are extrachromosomal genetic elements capable of autonomous replication. An episome is a plasmid that can integrate into the bacterial chromosome,Classification of Plasmids Transfer properties Conjugative plasmids Nonconjugative plasmids Phenotypic effects Fertility plasmid (F factor) Bacteriocinogenic plasmids. Resistance plasmids 7 factors) .,Insertion sequences (IS)- Insertion sequences are transposable genetic elements that carry no known genes except those that are required for transposition.,a. Nomenclature - Insertion sequences are given the designation IS followed by a number. e.g. IS1 b. Structure 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 there are genes involved in transposition and sequences that can control the expression of the genes but no other nonessential genes are present. c. Importance i) Mutation - The introduction of an insertion sequence into a bacterial gene will result in the inactivation of the gene. ii) Plasmid insertion into chromosomes - The sites at which plasmids insert into the bacterial chromosome are at or near insertion sequence in the chromosome. 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 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 sequences. In one orientation one of the genes is active while in the other orientation the other flagellar 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 antigens. Also the mechanism of phase variation may differ in different species of bacteria (e.g. Neisseria; transformation).,Transposons (Tn) - Transposons are transposable genetic elements that carry one or more other genes in addition to those which are essential for transposition.,Nomenclature - Transposons are given the designation Tn followed by a number. Structure - The structure of a transposon is similar to that of an insertion sequence. The extra genes are located between the terminal repeated sequences. In some instances (composite transposons) the terminal repeated sequences are actually insertion sequences. Importance - Many antibiotic resistance genes are located on transposons. Since transposons can jump from one DNA molecule to another, these antibiotic resistance transposons are a major factor in the development of plasmids which can confer multiple drug resistance on a bacterium harboring such a plasmid. These multiple drug resistance plasmids have become a major medical problem because the indiscriminate use of antibiotics have provided a selective advantage for bacteria harboring these plasmids.,Mechanism of bacterial variation,Gene mutation Gene transfer and recombination Transformation Conjugation Transduction Lysogenic conversion Protoplast fusion,Types of mutation,Base substitution Frame shefit Insertion sequences,What can cause mutation?,Chemicals: nitrous acid; alkylating agents 5-bromouracil benzpyrene Radiation: X-rays and Ultraviolet light Viruses,Bacterial mutation,Mutation rate Mutation and selectivity Backward mutation,Transformation,Transformation 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 the environment and the DNA that is taken up can be incorporated into the recipient's 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 transfer of genetic material is one way; DNA is transferred from a donor to a recipient.,Physiological States of F Factor,Autonomous (F+) Characteristics of F+ x F- crosses F- becomes F+ while F+ remains F+ Low transfer of donor chromosomal genes,Physiological States of F Factor,Integrated (Hfr) Characteristics of Hfr x F- crosses F- rarely becomes Hfr while Hfr remains Hfr High transfer of certain donor chromosomal genes,Physiological States of F Factor,Autonomous with donor genes (F) Characteristics of F x F- crosses F- becomes F while F remains F High transfer of donor genes on F and low transfer of other donor chromosomal genes,Mec