Two kinds of activity promoted by culture filtrates of male strains of E. coli K-12: Inhibition of “Nitrogen mustard after-effect” and stimulation of genetic recombination

The protein factor present in culture filtrates of male strains of E. coli which inhibits the nitrogen mustard after-effect (NMAE) is produced only during the logarithmic phase of growth. Culture filtrates containing active NMAE inhibiting factor increase the yield of recombinants obtained in conjugation if the filtrates are present during incubation of the cross mixture, or if the recipient cells are pretreated with the active filtrate before crossing. The active filtrates increase the competence of recipient cells to accept and/or to integrate donor DNA.