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

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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−StrRrecA− 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 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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