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Recombinant F' Factors from Escherichia coli K-12 Strains Carrying recB or recC

Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520
Department of Radiology, Yale University, New Haven, Connecticut 06520

ABSTRACT

The frequency of genetic exchanges between F' factors and the bacterial chromosome was studied in recombination-deficient Escherichia coli mutants under conditions in which the recombinant F' factors were immediately transferred to new hosts. In a series of double matings, F101-1 thr⁺leu^– episomes were first transferred into each of four intermediate F^–thr^–leu⁺ strains carrying various rec alleles. After the original F' donors were killed with phage T6, the F101-1 episomes were then transferred from the intermediate cells to F^–thr^–leu^–Str^RrecA^– females. Recipients of nonrecombinant episomes formed Thr⁺ (Str^R) colonies, and recipients of recombinant episomes formed Leu⁺(Str^R) colonies. A comparison of the numbers of Leu⁺(Str^R) and Thr⁺(Str^R) colonies shows that recB^– males formed 18 to 21% and recC^– formed 47 to 60% of the wild-type level of recombinant episomes that could be detected after transfer. No recombinant episomes were detected using a recA^– intermediate strain. If the intermediate strains harboring the F101 episomes were purified, allowed to grow for 50 generations, and then mated with the recA^– recipient, recombinant episomes were transferred at 8% of the wild-type level for recB^– and 13% for recC^–. In contrast, only 0.4 and 0.6% of the normal number of recombinants were obtained from crosses between Hfr Cavalli donors and the same recB^– and recC^– strains. Recombinant episomes were detected with greater frequency among newly formed rec⁺, recB^–, and recC^– partial diploids than in those which were 50 generations old.

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