GENE TRANSFER IN BACTERIA: Recombination

Gene Transfer in Bacteria

• Gene transfer in bacteria occurs through recombination, which incorporates extrachromosomal DNA into the chromosome.
• There are two types of recombination: homologous and nonhomologous.
• Homologous recombination occurs between closely related DNA sequences and requires a set of enzymes produced by the rec genes.
• Nonhomologous recombination occurs between dissimilar DNA sequences and produces insertions or deletions, requiring specialized recombination enzymes.

Recombination

• A part of or all of the genetic material in bacteria can be transferred from one cell to another.
• The process involves the uptake of DNA molecules from a donor cell by a recipient cell.
• The exogenous material from the donor cell is called the exogenote, and its response in the recipient cell is called the endogenote.
• The newly formed bacterium is recombinant, expressing the properties of both the donor and recipient cells.

Mechanisms of Genetic Transfer

• There are three main ways of genetic transfer between cells: transformation, transduction, and conjugation.

Transformation

• Transformation is the process by which bacteria take up fragments of naked DNA and incorporate them into their genomes.
• Competent cells are naturally capable of taking up exogenous DNA, including Haemophilus influenzae, Streptococcus pneumoniae, Bacillus spp., and Neisseria spp.

Transduction

• Transduction occurs only in similar bacterial types, such as Salmonella, Shigella, Escherichia, Pseudomonas, Vibrio, Proteus, Staphylococcus, and Bacillus.
• There are two types of transduction: localized transduction, which transfers a specific DNA region, and generalized transduction, which has an equal chance of transferring any single gene in the bacterial DNA.

Conjugation

• Conjugation is a form of horizontal gene transfer that requires cell-to-cell contact.
• It is a plasmid-encoded mechanism that can mediate DNA transfer between unrelated cells, even between different genera.
• Conjugative plasmids use this mechanism to transfer copies of themselves and the genes they encode, such as those for antibiotic resistance, to new host cells.

F Plasmid

• F plasmid (F stands for "fertility") is a circular DNA molecule that contains genes that regulate DNA replication and transposable elements that allow the plasmid to integrate into the host chromosome.
• The F plasmid has a large region of DNA, the tra region, containing genes that encode transfer functions.
• Only donor cells produce sex pilus (pili), which attach to female cells and form a bridge for conjugation.

Conjugation Process

• The transfer of the F plasmid converts an F- recipient cell into an F+ cell.
• The process involves the transfer of a single strand of F plasmid from the donor cell to the recipient cell via the bridge formed by the sex pilus.
• Both cells synthesize complementary strands, resulting in both cells becoming F+.

Mutation

• Mutation is a change in the nucleotide sequence of DNA that alters the structure and function of the protein to be synthesized.
• The organism that has undergone mutation is called a mutant.
• Mutation does not always result in a phenotypic change.

Mutation Mechanisms

• There are two main types of mutation mechanisms: point mutations and base-pair substitutions.
• Point mutations involve a single nucleotide change, which can result in a silent mutation, where the amino acid in the protein sequence remains the same.
• Base-pair substitutions involve the replacement of one nucleotide base pair with another, which can have various effects on the resulting protein and the organism's phenotype.

Plasmids

• Plasmids are small, circular, double-stranded DNA molecules that are physically separated from chromosomal DNA and can replicate independently.
• Plasmids can be transferred from one bacterium to another via conjugation, causing functional alterations in the recipient cells.
• Plasmids can carry genes that provide resistance to antibiotics, heavy metals, UV, and encode toxins, hemolysins, proteases, and pigment formation.

Main Plasmids

• F factor (F plasmid = fertility factor) carries genes for the formation of sex pili and for the initiation of conjugation.
• Col plasmid (colisinogenic factors) carries genes that encode colicins, which are proteins that kill other bacteria.
• Degradative plasmids carry genes for the breakdown of unusual organic compounds.
• Plasmids can provide a competitive advantage, facilitate the transfer of genetic material, or allow bacteria to utilize unusual substances as a source of carbon.