Mechanisms of Horizontal Gene Transfer and DNA Recombination

Comparative genomics is revealing extensive diversity within many bacterial species. The pan-genome of a species is composed of core genes present in all strains and dispensable genes that provide a selective advantage under specific conditions. Movement of these dispensable genes between species, genera and kingdoms is known as horizontal gene transfer (HGT). There are three primary mechanisms of HGT in bacteria. Transformation: uptake of naked DNA from the environment by naturally competent cells. Transduction: transfer of bacterial DNA between cells using bacteriophages as vectors. Conjugation: intimate cell-to-cell contact with transfer of single-stranded DNA by a type-IV-like secretion system. Horizontally acquired DNA that cannot replicate autonomously must be integrated into the genome of the recipient if it is to be maintained. Incoming DNA with significant similarity to the recipient genome can integrate by homologous recombination. Mobile genetic elements, such as integrative and conjugative elements, that have limited homology to the host genome use site-specific recombination to integrate at target sequences. Understanding these processes provides insight into the evolution of bacteria and emerging pathogens.