Strains hypersensitive to mitomycin C (MC) were isolated from Escherichia coli K-12 after treatment with nitrosoguanidine. Of 43 MC-sensitive strains tested for their ultraviolet light (UV) sensitivity and for their ability to reactivate UV-inactivated λ phage, 38 were found to be insensitive to UV irradiation and to be able to reactivate UV-irradiated bacteriophage λ. Some properties of the MC-sensitive, uvr+ mutants were analyzed. Synthesis of deoxyribonucleic acid (DNA) in MC-sensitive, uvr+ mutants was inhibited at a lower concentration of MC than in the wild-type strain. Mutant cells, labeled with 3H-thymidine and then exposed to MC, released radioactivity as low molecular weight compounds. The amount of radioactivity released was the same as that from the wild-type strain. MC-sensitive, uvr+ mutants, as well as the corresponding wild-type strain, were equally susceptible to induction of prophage φ80 by UV irradiation. However, MC induction of prophage was achieved in MC-sensitive, uvr+ mutants at a lower concentration of the antibiotic than in the wild-type strain. Genetic experiments indicated that a gene controlling MC sensitivity is located close to that determining lactose fermentation of E. coli. It is situated on episome F′13, and the wild type is dominant to the MC-sensitive allele.
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