MECHANISM OF ACTION OF NEOTHRAMYCIN

I. THE EFFECT ON MACROMOLECULAR SYNTHESES

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Neothramycin was observed to prevent growth of mouse lymphoblastoma L5178Y and HeLa cells at the concentration of 0.5-1.0μg/ml and exhibited a lethal effect at 5.0μg/ml. Approximately 50% growth inhibition of E. coli was found at the concentration of 37 μg/ml. The effect of neothramycin on macromolecular syntheses in the mammalian and bacterial cells was investigated. The antibiotic produced a preferential inhibition of RNA over DNA synthesis in the intact cells of lymphoblastoma L5178Y: i.e. approximately 50% inhibition of the former was observed at the antibiotic concentration of 1.4 μg/ml, and that of the latter at 12.0μg/ml. Protein synthesis was not significantly affected.
Contrary to the effect in the mammalian cells, thymine uptake into DNA was more markedly blocked than uridine incorporation into RNA by the antibiotic in the intact cells of E. coli. However, neothramycin was observed to cause a significant degradation of DNA, and the apparent inhibition of thymine uptake seemed to be due to degradation of DNA but not to the inhibition of net DNA synthesis. In E. coli the antibiotic prevented RNA synthesis more profoundly than DNA synthesis as in the mammalian cells. DNA synthesis with toluene-treated cells of E. coli polA^- was not significantly affected by neothramycin, while RNA synthesis with the same system was markedly blocked by the antibiotic.
Neothramycin was demonstrated to prevent DNA-dependent RNA and DNA polymerase reactions, using E. coli enzymes and calf thymus DNA as a template. RNA polymerase reaction was more profoundly inhibited than DNA polymerase I: i.e. approximately 50% inhibition of RNA polymerase reaction was observed at the antibiotic concentration of 11 μg/ml, and that of DNA polymerase I at 100 μg/ml. The inhibition degree of DNA polymerase I was reversed by the increasing concentration of template DNA but not by that of the enzyme, suggesting the direct interaction of the antibiotic with DNA. The degree of inhibition of the polymerase reactions depended upon the period of preincubation of template DNA and neothramycin, increasing gradually until the preincubation reached 60 minutes. It suggested that the interaction of DNA and the antibiotic might need a certain time.