Electron microscope heteroduplex studies of sequence relations among plasmids of Escherichia coli: isolation of a new F-prime factor, F80, and its implication for the mechanism of F integration into the chromosome.

A new F-prime factor, F80, was isolated from an Escherichia coli strain harboring the F-prime factor F8 by selecting for transfer of the supE marker to a RecA- recipient. Genetic analysis shows that F80 carries a segment of the chromosomal DNA between lip and suc in addition to the tol-gal region normally in F8. Physical analysis by the electron microscope heteroduplex method suggests that the formation of F80 from F8 involves recombination between the alphabeta segment of F, which is present in F8, and the homologous sequence of F present in the E. coli chromosome at the site where F is supposed to integrate to form HfrP3. The implications of this result for the general mechanisms of F integration to form Hfr's are discussed.