Inhibition of Replication of an F′lacEpisome in Hfr Cells ofEscherichia coli

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Abstract
Hfr strains ofEscherichia coliK-12 were found capable of accepting a F′lacepisome during mating, with a frequency approximating that of F−strains. However, the F′lacepisome was unable to replicate in the Hfr cells, and was diluted out during the growth of the culture. Thelac+gene of the episome can be “rescued” by recombination into the host chromosome, as shown by the appearance of variegated recombinant colonies on a lactose-fermentation indicator medium. InrecAHfr strains, however, nolac+offspring were obtained in crosses with F′lacdonors. The induced synthesis of β-galactosidase in F′lac+× Hfr zygotes was studied. Rates of enzyme synthesis were approximately constant with respect to time as expected from unilinear inheritance of the F′lacepisome. However, the rate of synthesis eventually increased, presumably due to integration of thelac+gene in some of the zygotes. In F′lac+×recAHfr zygotes the rate of β-galactosidase synthesis remained constant with respect to time, as expected.