SUMMARY: An Hfr strain G11 and an F strain D1 of K 12 both containing the R-factor R 1a were treated with ethyl methane sulphonate (EMS). Mutation of the bacteria to high ampicillin resistance occurred at a frequency of 10 to 10 and it was coupled with a simultaneous increase in resistance to chloramphenicol, streptomycin and sulphanilamide. Resistance to all the antibiotics was infective. Increased ampicillin resistance was due to increased penicillinase activity of the bacteria. In a strain containing a mutant R-factor (R1 B1) this was due to an increased quantity of an enzyme that seemed to be identical to that of a strain containing the unmutated R 1a. Metabolism of chloramphenicol in strains containing R1 B1 was greater than in those with R 1a. Mating experiments with an Hfr strain carrying R 1a or R1 B1 revealed that the two R-factors reduced fertility to about the same extent. Pair formation was two to three times greater in the presence of R1 B1 than in the presence of R 1a, and R-factor transfer was increased more than chromosome transfer. It is likely that the R 1a genome is read two to three times more efficiently in strains containing the mutated R1 B1, perhaps due to an increased number of copies of the R-factor genome per bacterium.


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