Gene Transfer

There are three fundamentally distinct mechanisms (1) Transformation (2) Conjugation (3)Transduction (1) Transformation (2) Conjugation (3)Transduction

Note: Conjugation is most readily demonstrated in Gram- negative bacteria but does occur in some Gram-positive genera such as Streptomyces and Streptococcus Conjugation is most readily demonstrated in Gram- negative bacteria but does occur in some Gram-positive genera such as Streptomyces and Streptococcus

Transformation the uptake ofDNAby a bacterial Cell Avery MacLeod and McCarty demonstrated that the ‘transforming principle’ was DNA Transformation has been important in genetic analysis of some species and more recently (and to a much greater extent) because of its key role in gene cloning the uptake ofDNAby a bacterial Cell Avery MacLeod and McCarty demonstrated that the ‘transforming principle’ was DNA Transformation has been important in genetic analysis of some species and more recently (and to a much greater extent) because of its key role in gene cloning

Note: Competence generally occurs at a specific stage of growth, most commonly in late log phase, just as the cells are entering stationary phaseThis may be a response to cell density rather than (or as well as) growth phase. For example, in Bacillus subtilis, some of the genes involved in the development of competence are also involved in the early stages of sporulation Competence generally occurs at a specific stage of growth, most commonly in late log phase, just as the cells are entering stationary phaseThis may be a response to cell density rather than (or as well as) growth phase. For example, in Bacillus subtilis, some of the genes involved in the development of competence are also involved in the early stages of sporulation

Following the development of competence, double-stranded DNA fragments bind to receptors on the cell surface, but only one of the strands enters the cell For example:  Neisseria meningitides Haemophilus influenzae Following the development of competence, double-stranded DNA fragments bind to receptors on the cell surface, but only one of the strands enters the cell For example:  Neisseria meningitides Haemophilus influenzae

Note: B. subtilis and Str. pneumoniae can take up virtually any linear DNA molecule If the cell is to become transformed in a stable manner, the new DNA has to be replicated andinherited Natural transformation is of limited usefulness for artificial genetic modification of bacteria B. subtilis and Str. pneumoniae can take up virtually any linear DNA molecule If the cell is to become transformed in a stable manner, the new DNA has to be replicated andinherited Natural transformation is of limited usefulness for artificial genetic modification of bacteria

Conjugation Conjugation is the direct transmission of DNA from one bacterial cell to another. In most cases, this involves the transfer of plasmid DNA, although with some organisms chromosomal transfer can also occur Conjugation is the direct transmission of DNA from one bacterial cell to another. In most cases, this involves the transfer of plasmid DNA, although with some organisms chromosomal transfer can also occur

Conjugation Conjugation is the direct transmission of DNA from one bacterial cell to another. In most cases, this involves the transfer of plasmid DNA, although with some organisms chromosomal transfer can also occurSeveral genera of Gram-positive bacteria possess reasonably well-characterized conjugation systemsthese include Streptomyces specieEnterococcus faecaliss lactic streptococci The most obvious significance of conjugation is that it enables the transmission of plasmids from one strain to another ، Conjugation is the direct transmission of DNA from one bacterial cell to another. In most cases, this involves the transfer of plasmid DNA, although with some organisms chromosomal transfer can also occurSeveral genera of Gram-positive bacteria possess reasonably well-characterized conjugation systemsthese include Streptomyces specieEnterococcus faecaliss lactic streptococci The most obvious significance of conjugation is that it enables the transmission of plasmids from one strain to another ،

Mechanism of conjugation Formation of mating pairs: In the vast majority of cases, the occurrence of conjugation is dependent on the presence, in the donor strain, of a plasmid that carries the genes required for promoting DNA transfer E. coli and other Gram-negative bacteria RP4 Formation of mating pairs: In the vast majority of cases, the occurrence of conjugation is dependent on the presence, in the donor strain, of a plasmid that carries the genes required for promoting DNA transfer E. coli and other Gram-negative bacteria RP4

Transfer of DNA The transfer of plasmid DNA from the donor to the recipient (Figure 6.3) is initiated by a protein which makes a single-strand break (nick) at a specific site in the DNA, known as the origin of transfer (oriT) The transfer of plasmid DNA from the donor to the recipient (Figure 6.3) is initiated by a protein which makes a single-strand break (nick) at a specific site in the DNA, known as the origin of transfer (oriT)

Mobilization and chromosomal transfer Not all plasmids are capable of achieving this transfer to another cell unaided; those that can are known as conjugative plasmids In order to understand mobilization the plasmid ColEI can be taken as an Example bom site (oriT, the origin of transfer): where the Mobnuclease makes a nick in the DNA Not all plasmids are capable of achieving this transfer to another cell unaided; those that can are known as conjugative plasmids In order to understand mobilization the plasmid ColEI can be taken as an Example bom site (oriT, the origin of transfer): where the Mobnuclease makes a nick in the DNA

Note: This is an important factor in genetic modification as removal of the bom site from a plasmid vector ensures that the modified plasmids cannot be transferred to other bacterial strains This is an important factor in genetic modification as removal of the bom site from a plasmid vector ensures that the modified plasmids cannot be transferred to other bacterial strains

The F plasmid

Hfr strains

Integration and excision of F: formation of F0 plasmids Integration of the F plasmid occurs by recombination between a sequence on the plasmid and a chromosomal site Integration of the F plasmid occurs by recombination between a sequence on the plasmid and a chromosomal site

Conjugation in other bacteria