Certain Escherichia coli strains contain an F plasmid that has become permanently integrated into the cell’s chromosome. Such strains are called Hfr (high frequency recombination), not F+. F plasmid genes remain fully functional, so Hfr strains produce sex pili and can attach to F- cells. When plasmid gene transfer is initiated however, DNA from the donor cell chromosome (of which the F plasmid is now an integral part) is transferred as well. Mating pairs often break apart before there has been time to complete transfer of donor cell DNA, so recipient cells rarely receive the complete F plasmid, and remain F-. DNA from the donor cell may undergo homologous recombination with DNA in the recipient cell, resulting in recombinant cells with genetic properties unlike either parent cell. This process of gene transfer is call conjugation.

Students should compare this animation with the F+ x F- animation to observe similarities and differences.