Effects of Coumarin, Thiopurines, and Pyronin Y on Amplification of Phleomycin-Induced Death and Deoxyribonucleic Acid Breakdown in Escherichia coli

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Phleomycin (≤2 μg/ml) induces neither deoxyribonucleic acid (DNA) breakdown nor cell death in stationary-phase Escherichia coli B cells, but the addition of 8 mm caffeine immediately initiates these changes in the same way as increasing the phleomycin concentration 10-fold. This phenomenon is termed “amplification” (6). Pyronin Y, a number of nontoxic thio- and mercaptopurines (of which the most active were 6,7- and 6,9-dimethyl-2-methylthiopurine), and coumarin have been found to be considerably more efficient amplifiers of phleomycin activity than caffeine. Thus 2 mm 6,7- and 6,9-dimethyl-2-methylthiopurine, 0.16 mm pyronin, and 4 mm coumarin killed 10 to 100 times more phleomycin-treated bacteria within 2 hr than 8 mm caffeine. As with caffeine, amplification of cell death by these compounds was accompanied by degradation of DNA to acid-soluble fragments. A number of compounds including 2,6-dichloropurine, 6-hydroxy-2-methylthiopurine, α-naphthol, β-naphthol, naphthionic acid, and α-naphthol-4,8-disulphonic acid inhibited the action of phleomycin, if they were present in the cell suspension during phleomycin treatment, but some caused amplification if added subsequent to the phleomycin. Although no mutants resistant to ≥10 μg of phleomycin per ml were observed among 1011E. coli B cells screened, such mutants occurred with a frequency of 10−6 to 10−7 among cultures resistant to 1 to 2 μg of phleomycin per ml. These double mutants were cross-resistant to phleomycin plus caffeine. The amplifying compounds, though structurally dissimilar, shared the common characteristic of binding selectively to denatured DNA as measured by equilibrium dialysis methods. The implications of these observations in supporting a model of phleomycin amplification proposed previously (6) and their utility in providing a logic for developing a new class of antibiotics are discussed.

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