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Unformatted text preview: B ac te rial Ge ne tic s
XiaoKui GUO PhD Bacterial Genomics
Bacterial Genomics Microbial Genomics
Microbial Genomics Microbial Genome Features
burgdorferi G+C content 68%
radiodurans single circular chromosome
plus one or more
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
Conjugative Nonconjugative plasmids
Nonconjugative Phenotypic effects Fertility plasmid (F factor)
Fertility Bacteriocinogenic plasmids.
Bacteriocinogenic Resistance plasmids 7 factors) .
Resistance 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.
ii) Mutation - The introduction of an insertion sequence into a bacterial gene will result in the inactivation of
iii) Plasmid insertion into chromosomes - The sites at which plasmids insert into the bacterial chromosome
are at or near insertion sequence in the chromosome.
iiii) 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
plasmids Mechanism of bacterial variation
Mechanism of bacterial variation
• Gene mutation
Lysogenic conversion Protoplast fusion and Type s o f mutatio n
• Base substitution
Insertion sequences What can cause mutation?
What can cause mutation?
• Chemicals: nitrous acid; alkylating agents
• Radiation: Xrays and Ultraviolet light
• Viruses B ac te rial mutatio n
• 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 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
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 Physiological Factor • Integrated (Hfr) – 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
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
Mechanism of F
• Pair formation
bridge • DNA transfer
– Origin of transfer
– Rolling circle replication F+ F- F+ F- F+ F+ F+ F+ Mechanism of Hfr x F Crosses
Mechanism of Hfr x F
• Pair formation
bridge • DNA transfer Hfr F- Hfr F- – Origin of transfer
– Rolling circle replication • Homologous recombination Hfr F- Hfr F- Mechanism of F’ x F Crosses
Mechanism of F’ x F
• Pair formation
• DNA transfer F’ F- F’ F- F’ F’ F’ F’ – Origin of transfer
– Rolling circle replication R Plasmid
P lasmid Transduction: 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 • Generalized transduction is transduction in which potentially any bacterial gene from the donor can be transferred to the recipient. The mechanism of generalized
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 The mechanism of specialized transduction Specialized transduction Specialized transduction 1. Very efficient transfer of a small regioncan be 2.
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 Lysogenic
• The prophage DNA as a gene recombined with chromosome of host cell. Protoplast Fusion P rotoplast
• Fusion of two protoplasts treated with
lysozyme and penicillin. Application of Bacterial Variation
Application of Bacterial Variation
• Use in medical clinic: Diagnosis,
• Use in Genetic Engineering
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This note was uploaded on 12/27/2011 for the course STEP 1 taught by Professor Dr.aslam during the Fall '11 term at Montgomery College.
- Fall '11