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The frequency of genetic exchanges between F′ factors and the bacterial chromosome was studied in recombination-deficientEscherichia coli mutants under conditions in which the recombinant F′ factors were immediately transferred to new hosts. In a series of double matings, F101-1thr + leu − episomes were first transferred into each of four intermediate F− thr − leu + strains carrying variousrec 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 − StrR recA − females. Recipients of nonrecombinant episomes formed Thr+ (StrR ) colonies, and recipients of recombinant episomes formed Leu+ (StrR ) colonies. A comparison of the numbers of Leu+ (StrR ) and Thr+ (StrR ) colonies shows thatrecB − males formed 18 to 21% andrecC − formed 47 to 60% of the wild-type level of recombinant episomes that could be detected after transfer. No recombinant episomes were detected using arecA − intermediate strain. If the intermediate strains harboring the F101 episomes were purified, allowed to grow for 50 generations, and then mated with therecA − recipient, recombinant episomes were transferred at 8% of the wild-type level forrecB − and 13% forrecC − . In contrast, only 0.4 and 0.6% of the normal number of recombinants were obtained from crosses between Hfr Cavalli donors and the samerecB − andrecC − strains. Recombinant episomes were detected with greater frequency among newly formedrec + ,recB − , andrecC − partial diploids than in those which were 50 generations old.

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