Features of Bacterial Recombination
Interaction between two genomes with different genotypes leads to the formation of daughter recombinant DNA. The main feature of this process in bacteria is that only a small DNA fragment, rather than the entire donor chromosome, enters the recipient cell.
As a result, a merozygote is formed inside the recipient—an incomplete zygote that represents a partially diploid cell. The outcome of this crossing is the formation of only a single recombinant. Its genotype is based on the recipient's original genotype with the incorporated donor fragment. No reciprocal exchange of segments occurs.
Three Pathways of Horizontal Gene Transfer
Genetic recombination is merely the final step. For the donor DNA to meet the recipient DNA, the material must be transferred via one of three fundamental pathways:
- Conjugation. Occurs through direct physical contact between two cells, connected by a specialized sex pilus (bridge). This process requires a conjugative plasmid. Directed transfer of genetic material proceeds from the donor (F+) to the recipient (F-), always initiating from the 5' end.
- Transformation. The uptake of high-molecular-weight free DNA. The source is lysed donor cells that release fragments of their genome into the environment. These fragments then penetrate a competent recipient cell from the surrounding medium.
- Transduction. A process mediated by a bacteriophage. During assembly within a lysed donor cell, the phage accidentally packages a segment of bacterial DNA. Upon release, it infects a new cell and introduces the genome fragment of the previous host.
Molecular Mechanisms of Recombination
Depending on molecular characteristics, there are three main mechanisms for integrating foreign DNA into the recipient genome.
- Homologous recombination. Requires a high degree of homology (similarity) between the exchanging segments. The mechanism involves the breakage and rejoining of strands to form a cruciform Holliday junction, where complementary base pairing occurs between single-stranded segments from different parental molecules. The process is tightly regulated by the REC system (recA, B, C, D genes). The products of these genes unwind DNA strands and orient (cleave) the Holliday junction to complete the exchange.
- Site-specific recombination. Occurs only at strictly defined sites in the genome and does not require a high degree of homology. It is independent of recA, B, C, D gene functions. Examples include plasmid integration into the bacterial chromosome (usually between identical IS elements), phage DNA integration into the E. coli chromosome, and gene regulation mechanisms such as phase variation of the flagellar H-antigen in Salmonella.
- Illegitimate (replicative) recombination. A non-homologous process that is also independent of the REC system. Its core mechanism is transposition (movement) of mobile genetic elements within a replicon or between different replicons. A critical feature is that this movement is always accompanied by duplication (replication) of the transferred DNA.