GENERAL MICROBIOLOGY - T.P. Pyroh - 2004

19. BACTERIAL GENETICS: STABILITY, VARIATION, AND INHERITANCE OF TRAITS

19.3. INHERITANCE OF TRAITS AND GENETIC RECOMBINATION

19.3.1. Mechanisms of Genetic Recombination

Currently, three mechanisms of foreign DNA recombination with the bacterial chromosome (or plasmid) are known: general homologous, site-specific, and non-Homologous Recombination.

General homologous recombination. In this case, the recombination partners must share identical nucleotide sequences, meaning they must be as homologous as possible. Homologous recombination is controlled by the recA Gene. Mutants with a defect in this gene (recA) are incapable of homologous recombination.

Several models of this mechanism exist. It is believed that base pairing occurs between the unwound single-stranded regions of two DNA Double helices. The second strand is likely formed through Replication or repair.

Site-Specific Recombination. This process occurs independently of homologous recombination, meaning it is also possible in recA mutants. The Essence of the process is that a short double-stranded DNA molecule integrates at a specific site within a long double helix, causing the smaller partner to lose its autonomy. A typical example of site-specific recombination is the integration of bacteriophage lambda. Upon transitioning into a prophage state, the phage inserts into the host Cell chromosome at a specific locus—between the Operon and the biotin region. Phage integration is preceded by its attachment to a specific region of the bacterial chromosome. It was previously thought that this integration was driven by a high degree of nucleotide Structure/154.html">Sequence Homology, but this homology turned out to be negligible. Apparently, the decisive role in phage integration into the bacterial chromosome belongs to the phage-encoded integrase protein. Within a specific region of the phage DNA and the corresponding region of the bacterial DNA, this protein catalyzes the Cleavage and reciprocal joining of the phage and host cell genomes.

Non-homologous recombination. Recombination processes involving DNA segments that do not exhibit significant genetic homology are termed non-homologous recombination. Like site-specific recombination, it represents an integrative form of recombination, meaning it involves joining rather than exchanging DNA.

Non-homologous recombination is independent of the recA gene. Elements capable of this type of recombination include: insertion sequences (IS elements); Transposons (Tn); and bacteriophage Mu.

Insertion sequences (IS elements) are found in both bacterial Chromosomes and Plasmids. They consist of 800-1400 Base Pairs. They do not encode identifiable phenotypic traits. Their mutagenic effect is caused simply by the insertion of foreign DNA, which disrupts the METABOLISM/31.html">Transcription process.

Transposons are capable of "jumping" from one region of The Genome to another, particularly from the bacterial chromosome to a plasmid and vice versa. Transposons contain genes that encode traits such as Antibiotic Resistance, making them easier to detect than IS elements.

Bacteriophage Mu resembles IS elements and transposons due to its unusual behavior upon integration into the bacterial chromosome. It exhibits typical phage properties while simultaneously functioning as a giant transposon.