Regulation of conjugative transfer of plasmids and integrative conjugative elements

Horizontal gene transfer has been recognized as one of the principal contributors to bacterial evolution and diversification. One of the mechanisms involved in this process is conjugative transfer of plasmids and Integrative Conjugative Elements (ICEs). Plasmids and ICEs often encode traits beneficial for bacterial survival in specific environments, or for the establishment of symbiosis or pathogenesis, in addition to genes allowing conjugative transfer. In this review, we analyze the mechanisms that regulate the expression of conjugative transfer genes. For traits such as antibiotic or metal resistance, the compounds involved may induce conjugative transfer directly, while symbiosis and pathogenesis are modulated by quorum-sensing and/or signal molecules released by the host. However, multiple layers of regulation are usually involved in modulating transfer. In addition to the plasmid-encoded regulatory elements, conjugation seems to be regulated by what we have labeled as the "internal environment", defined by the interaction between the host chromosome and the plasmids or ICEs. Another regulatory level depends on the "external environment", which affects conjugative transfer due to the composition and conditions of the community.