Kinetics of recA function in conjugational recombinant formation.

Summary Genetic recombination was studied in F^- strains of E. coli carrying a mutation (recA200) that confers a thermosensitive Rec^- phenotype. Recombination during Hfr matings at 35C was monitored by raising the temperature of incubation to 42C at various intervals so that only merozygotes that had completed those functions dependent on the activity of the recA gene product could form recombinant progeny. The results indicated that no more than 1–2% of the merozygotes present while mating was in progress were able to form recombinant colonies at 42C. Separation of mating pairs reduced the yield of recombinants obtained at 35C by 50 to 200-fold if plating on agar medium was delayed for 15–30min by continuing incubation in broth medium. recA200 merozygotes that were also recB21 sbcB15 proved relatively stable when plating was delayed in this manner, which suggested that Hfr DNA is prone to exonuclease inactivation in recA200 merozygotes after mating pairs have separated. Post-mating incubation in high salt medium or on agar plates promoted the recovery of recombinants at 35C. However, the majority of recA200 merozygotes did not acquire the ability to form recombinant colonies at 42C under these more stable conditions until mating pairs had been separated and incubation continued at 35C for 40–60 min. It was concluded that recA200 strains are partially defective for recombination even at low temperature but that terminating mating promotes the recovery of recombinants. A mechanism involving the stimulation of RecA activity by mating pair separation is postulated to account for the efficient recovery of recombinants from HfrxF^-recA200 crosses at 35C.