Conjugation is mediated by cell-to-cell contact between the donor and recipient. Plasmid DNA is usually transferred through a tube-like structure known as a pilus, which is extruded by the donor and physically connects to the recipient cell. In the Gram-positive bacterium Enterococcus, this cell-to-cell contact is promoted by plas-mid-encoded aggregation substances that are induced in response to sex pheromones excreted by the recipient cell (70). As a large number of genes may be required for the conjugation process and these genes reside on the conjugative plasmid itself, small plasmids are usually not self-transmissible. Nevertheless, small plasmids that encode relaxase enzymes, which perform the initial nicking reactions at their cognate plasmid origins of transfer (oriT), can undergo conjugative mobilization if other conjugation functions are provided in trans by a helper plasmid within the cell (71).

Conjugative transfer of the F Plasmid is one of the best-characterized conjugation processes. In this system, the propilin protein encoded by the traA gene is processed by host-encoded leader peptidase into the pilin product