Streptomycin-induced conformational changes in the 70-S bacterial ribosome

Conformational alterations induced by streptomycin in the bacterial ribosome have been investigated using as probes, ethidium bromide, N-[14C]ethylmaleimide and a spin label nitroxide analog of N-ethylmaleimide. 1. The binding of the antibiotic to the ribosome does not affect the reactivity of sulfhydryl groups towards N-ethylmaleimide. 2. The motional freedom of spin labels bound to ribosomal proteins S1 and S18 is increased but it is hardly affected at other labeled sites. This observation suggests that the binding of streptomycin causes a local loosening of the ribosomal structure. 3. Ribosomes are found to bind less ethidium bromide in the presence of streptomycin, which suggests that the binding of streptomycin decreases the degree of organization of ribosomal RNA.