Ribosomes from a streptomycin-dependent strain of Escherichia coli

The ribosomes from a Sm-D strain of Escherichia coli have been compared in vitro with those from Sm-R and Sm-S bacteria. Maximum polyphenylalanine synthesis by Sm-D ribosomes is obtained in the presence of streptomycin; under the same conditions, Sm-S ribosomes are inhibited and Sm-R ribosomes are unaffected by streptomycin. Optimum rates of polyphenylalanine synthesis are obtained with all three classes of ribosomes at the same concentration of divalent cation (Ca²⁺), both in the presence and absence of streptomycin. Neomycin cannot replace streptomycin for maximum activity of Sm-D ribosomes. The requirement for streptomycin is a property of the 30 s subunit of the Sm-D ribosomes. It is concluded that the mutation leading to this Sm-D phenotype is an alteration of a genetic determinant of the structure of the 30 s ribosomal subunit.