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Rifamycin-resistant mutation in E. coli: effect of rifamycin on DNA-dependent RNA polymerase and on R17 phage growth.

Rifamycin-resistant mutants in E. coli, K12, Hfr strain, were isolated without mutagenic treatment. The frequency of natural mutation of this type appeared to be approximately 10−9. Selection of resistant strains was effected by plating 109 cells on agar plates containing 50 μg of rifamycin per milliliter. A total of 11 strains resistant to rifamycin thus isolated remained Hfr and, in the presence of rifamycin, permitted the growth of the RNA-containing, male specific coliphage R17, which is sensitive to rifamycin in a wild-type E. coli. DNA-dependent RNA polymerase was prepared from these resistant mutants and examined for sensitivity toward rifamycin and other antibiotics. It was found that the RNA polymerase of the resistant strain is indeed resistant to rifamycin. However, both rifamycin-resistant and -sensitive enzymes are inhibited by the other three antibiotics (acridine orange, ethidium bromide, actinomycin D), which are known to interact with the template molecule.