Unformatted text preview: Bacterial Genetics
XiaoKui GUO PhD Bacterial Genomics Microbial Genomics Microbial Genome Features
29% Borrelia burgdorferi G+C content 68% Deinococcus radiodurans single circular chromosome two circular chromosomes circular chromosome plus one or more extrachromosomal elements Genome organization large linear chromosome plus 21 extrachromosomal elements 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 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 transfer Gene recombination
Transformation Conjugation Transduction Lysogenic conversion Protoplast fusion and Types of mutation Base substitution Frame shefit Insertion sequences What can cause mutation? Chemicals: nitrous acid; alkylating agents 5bromouracil benzpyrene Radiation: Xrays 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
F+ Physiological States of F Integrated (Hfr) Factor Characteristics of Hfr x F crosses F rarely becomes Hfr while Hfr remains Hfr High transfer of certain donor chromosomal genes F+ Hfr 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 Hfr F' Mechanism of F+ x F Crosses Pair formation Conjugation bridge Origin of transfer Rolling circle replication DNA transfer F+ F- F+ F- F+ F+ F+ F+ Mechanism of Hfr x F Crosses Pair formation Conjugation bridge DNA transfer Hfr F- Hfr F- Homologous Origin of transfer Rolling circle replication
Hfr FHfr F- recombination Mechanism of F' x F Crosses Pair formation Conjugation bridge DNA transfer F' F- F' F- Origin of transfer Rolling circle replication
F' F' F' F' R Plasmid Transduction: Transduction is defined as the transfer of genetic information between cells through the mediation of a virus (phage) particle. It therefore does not require cell to cell contact and is DNase resistant. Generalized Transduction Generalized transduction is transduction in which potentially any bacterial gene from the donor can be transferred to the recipient. The mechanism of generalized transduction Generalized transduction
1. It is relatively easy. 2. It is rather efficient (103 per recipient with 3. 4. P22HT, 106 with P22 or P1), using the correct phage. It moves only a small part of the chromosome which allows you to change part of a strain's genotype without affecting the rest of the chromosome. The high frequency of transfer and the small region transferred allows finestructure mapping Specialized transduction Specialized transduction is transduction in which only certain donor genes can be transferred to the recipient. Different phages may transfer different genes but an individual phage can only transfer certain genes Specialized transduction is mediated by lysogenic or temperate phage and the genes that get transferred will depend on where the prophage has inserted in the chromosome. The mechanism of specialized transduction Specialized transduction 1. Very efficient transfer of a small regioncan be 2. 3. 4.
useful for finestructure mapping Excellent source of DNA for the chromosomal region carried by the phage, since every phage carries the same DNA. Can often be used to select for deletions of some of the chromosomal genes carried on the phage. Merodiploids generated using specialized phage can be quite useful in complementation analyses. Lysogenic conversion The prophage DNA as a gene recombined with chromosome of host cell. Protoplast Fusion Fusion of two protoplasts treated with lysozyme and penicillin. Application of Bacterial Variation Use in medical clinic: Diagnosis,
Treatment, Prophylaxis. Use in Genetic Engineering ...
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