DNA Transfer by Bacterial Conjugation

Bacterial conjugation is defined as contact-dependent transmission of genetic information from a donor bacterium to a recipient cell (1 ). Transfer of DNA by conjugation is often termed lateral or horizontal gene transfer , as opposed to vertical transfer by which genetic information is transferred from mother to daughter cells. When genetic information is transferred by conjugation from the donor strain to the recipient strain, this population of recipient bacteria are called transconjugants. The genetic information is usually transferred to the recipient bacterium on a plasmid; however, conjugative transposons are also known. The mechanism and proteins involved in conjugative DNA transfer vary depending on the type of plasmid or transposon present in the donor strain. For example, RP4 and F (fertility) plasmids isolated from Escherichia coli are self-transmissible plasmids and thus contain genes that encode for all of the necessary proteins involved in the mobilization and transfer of the plasmid from one bacterium to another (2 ). Other plasmids that are conjugative, but non-self-transmissible, such as RSF1010 isolated from E. coli , can only be mobilized when the necessary functions are supplied in trans (3 ). The cointegration of a conjugative and nonconjugative circular plasmid can result in the transfer of both plasmids into a recipient strain. For example, in a h igh-f requency recombinant (Hfr ) E. coli K-12 strain, where the F plasmid has integrated into the bacterial chromosome, the whole chromosome can be conjugated to a recipient strain (4 ,5 ). Finally, conjugative transposons can be transferred from one bacterium to another.