Exposure of Escherichia coli to the antibiotic mitomycin C (MTC) at a concentration of 0.5 ,ug/ml caused cross-linkage between complementary strands of deoxyribonucleic acid (DNA). Derivatives of mitomycin, 7-methoxymitosene (7-MMT) and decarbamoyl mitomycin C (DCMTC), at a level as high as 20 ,ug/ml formed no cross-links between DNA strands. Ultraviolet light-sensitive mutants of E. coli K-12 bearing uvrA, uvrB, uvrC, or recA mutations were more sensitive to the lethal action of 7-MMT and of DCMTC than was the wild-type strain. Treatment of wild-type cells with these antibiotics resulted in the production of single-strand breaks in DNA, which were repaired upon incubation in a growth medium. Such breaks in DNA were not produced in the uvrA and the uvrB mutants. In the uvrC mutant, single-strand breaks were produced by 7-MMT or by DCMTC, but these breaks were not repaired upon incubation. These results are discussed in connection with the mechanism for removal of pyrimidine dimers in ultraviolet-irradiated bacteria.

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