On the basis of aminoglycoside-dependent growth of mutants of Escherichia coli: In vitro studies and the model

Summary

Ribosomes from an aminoglycoside-antibioticdependent mutant of Escherichia coli were analyzed for their antibiotic sensitivity and binding characteristics. Phage R17 RNA-dependent protein synthesis was highly sensitive to inhibition by dihydrostreptomycin when ribosomes were prepared from the mutant strain grown in the presence of paromomycin, but it was not inhibited when the strain was cultured for an at least sixfold mass increase without antibiotic or in the presence of ethanol. Measurement of dihydrostreptomycin binding by equilibrium dialysis corroborated the activity studies in that ribosomes from cells grown in the presence of paromomycin bound the antibiotic whereas those from cells grown in the presence of ethanol showed very low affinity toward the antibiotic. Deprivation of paromomycin and its readdition caused growth-dependent and time-dependent loss and acquisition of dihydrostreptomycin binding ability.

Cells of a streptomycin-dependent and of a resistant mutant showed a much lower rate of efflux of incorporated radioactive dihydrostreptomycin in comparison to those of a sensitive strain. It was found that in these strains dihydrostreptomycin was more firmly bound to ribosomal particles, and that this binding occurred only during growth, indicating that the binding proceeds only during assembly. Furthermore, at sublethal concentrations of streptomycin, the ribosomes of wild-type cells were found to bind the antibiotic in a fashion similar to those of resistant and dependent cells.

At higher concentrations the antibiotic inhibits ribosome synthesis and cellular growth and at the same time prevents its own binding to ribosomes. Based on these observations, a model is presented which predicts that the binding of aminoglycoside antibiotics to ribosomes occurs during assembly, thereby antagonizing the effect of the ribosomal protein mutation(s). As a consequence, functionally active ribosomes are synthesized. Growth in the absence of antibiotics leads to the synthesis of conformationally altered ribosomes with altered functional properties.