⚡ Core Concept: Mechanisms of Horizontal Gene Transfer (HGT)
Horizontal Gene Transfer (HGT) drives lateral genetic diversification in bacteria without sexual reproduction. The three classical mechanisms—Transformation (uptake of naked exogenous DNA), Conjugation (cell-to-cell contact via F-pilus and relaxosome), and Transduction (bacteriophage-mediated DNA transfer)—mediate the rapid dissemination of antibiotic resistance genes and pathogenicity islands.
1. Bacterial Transformation & Natural Competence
First discovered by Frederick Griffith (1928) in Streptococcus pneumoniae, transformation requires a physiological state known as natural competence. Competence is regulated by quorum sensing (e.g., ComP-ComA two-component system in Bacillus subtilis sensing ComX pheromone). Double-stranded DNA binds the surface receptor (ComEA); an endonuclease cleaves it, and the ComEC membrane channel translocates a single strand into the cytoplasm while the other strand is degraded. Intracellularly, RecA polymerizes on the ssDNA to catalyze homologous recombination into the bacterial chromosome via displacement-loop (D-loop) formation.
2. Conjugation & The F-Factor Transfer Machinery
Conjugation is mediated by conjugative plasmids (e.g., Fertility factor F). In an F+ × F– mating, a type IV secretion system (T4SS) elaborates the sex pilus, establishing contact and retracting to bring donor and recipient into close proximity. The relaxosome (featuring the TraI relaxase) nicks the plasmid at the origin of transfer (oriT). TraI covalently links to the 5′ end of the nicked strand and guides it through the T4SS pore into the recipient, accompanied by rolling-circle replication in the donor to reconstitute the dsDNA plasmid.
3. Generalized vs. Specialized Transduction
In generalized transduction (e.g., phage P22 or P1), bacterial host DNA is accidentally packaged into the phage capsid during the lytic cycle due to pseudo-pac sites. Any bacterial chromosomal marker can be transferred. In specialized transduction (e.g., temperate λ phage), imprecise excision of the prophage during lysogenic induction incorporates adjacent host genes (e.g., gal or bio operons) into the phage genome at high frequency.