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Studies in the mechanism of action of streptomycin Autor, Dorothy Anne Pomeroy


Using three different streptomycin variants of Escherichia coli the mechanism of action of streptomycin, as dihydrostreptomycin, has been studied. The variants used were either susceptible to, resistant to or dependent upon, the presence of dihydrostreptomycin during growth. It is a well-known fact that streptomycin, in sufficient quantities, will form a precipitate with nucleic acids, thus the precipitate formed from dihydrostreptomycin and bacterial cell contents was studied. Streptomycin also inhibits the formation of proteins, as exemplified by adaptive enzyme production, when it is added, during induction, to susceptible organisms. Corresponding effects correlating growth and synthesis have been noted, also, with the dependent and resistant variants. Using these facts and the fact that adaptive enzymes can be removed by the precipitation of cell contents with dihydrostreptomycin, regeneration of enzyme activity from the precipitate was attempted with polymethacrylic acid. This compound will dissociate protamine-nucleic acid complexes but proved ineffective in regenerating enzyme activity. By using dihydrostreptomycin precipitates of disrupted susceptible E. coli the observation was confirmed that DNase is ineffective in depolymerizing these precipitates. Dihydrostreptomycin precipitates of susceptible E. coli are partially depolymerized by RNase, however. Analysis in the ultra-centrifuge was carried out on cell-free material from susceptible E. coli on the addition of RNase, DNase and low, biologically active concentrations of dihydrostreptomycin. RNase and dihydrostreptomycin both removed two of the three typical components shown to be present in susceptible E. coli cells. DNase had no effect on these components. Purine and pyrimidine analysis of these dihydrostreptomycin precipitates from E. coli showed an absence of uracil, as such, but a relatively unaffected thymine content which indicates that it is mainly DNA rather than RNA affected by the dihydrostreptomycin. From these and previously observed facts the following theory is advanced. Streptomycin reacts with DNA to form a complex which in susceptible E. coli is not depolymerized by DNase. This complex formation between dihydrostreptomycin and DNA prevents protein synthesis as exemplified by adaptive enzyme formation. Because depolymerization of DNA by DNase is prevented, cell division cannot take place in the presence of dihydrostreptomycin. And because the directing effect of DNA on enzyme production is blocked, this process cannot occur. It must then follow that in the streptomycin-dependent E. coli, the antibiotic is required for the depolymerization of DNA and for the directing effect of DNA on protein synthesis.

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