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 are extrachromosomal genetic elements capable of autonomous replication. An episome is a plasmid that can integrate into the bacterial chromosomeClassification of Plasmids Transfer properties Conjugative plasmids Nonconjugative plasmids Phen
3、otypic 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 ar
4、e 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 th
5、at 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
6、 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
7、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
8、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 ge
9、netic 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 locat
10、ed 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 antibio
11、tic 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.
