Affinité pour la streptomycine des ribosomes d'Escherichia coli

Abstract

Ribosomes were isolated from four Escherichia coli K12 mutants, obtained by the action of ethyl methanesulfonate–with high (R) and low (r) resistance to streptomycin, dependent on high (D) and low (d) concentrations of streptomycin–and from the original sensitive strain (S). The ribosomal preparations were analyzed by sucrose‐gradient centrifugation, analytical ultra‐centrifugation, electron microscopy, determination of RNA to protein ratios, ability to synthesize polyphenylalanine in the presence of poly U, and the effect of streptomycin on this synthesis.The amounts of the antibiotic taken up by the ribosomes of the sensitive strain and of the mutants were determined for different concentrations of free [14C]streptomycin and the results plotted according to Scatchard's equation. The graphs show that each ribosome, in the mutants as well as in the sensitive strain, contains only one receptor site for one streptomycin molecule; as measured by the equilibrium constant of the reversible adsorption, the affinities of the binding site for the antibiotic are: S = 109 × 103; r = 13 × 103; R = 3.8 × 103; d = 42 × 103; D = 10 × 103.The observation that the ribosomes from the selected dependent mutants as well as those from the simply resistant ones have lower affinities for streptomycin than the ribosomes from the sensitive strain, could mean that the ribosomes from the resistant and the selected dependent mutants do not belong to different categories, and that E. coli dependence on streptomycin might have an extraribosomal origin.