Binding of Dihydrostreptomycin to Escherichia coli Ribosomes: Characteristics and Equilibrium of the Reaction

Abstract

The binding of dihydrostreptomycin to ribosomes and ribosomal subunits of a number of different Escherichia coli strains was studied, and the Mg 2+ and p H dependence, as well as the effect of salts and polynucleotides, was determined. The only requirement for binding with ribosomes and subunits from susceptible strains was 10 m m Mg 2+ . Monovalent salts weakened the binding in a manner similar to the effects on ribonucleic acid secondary structure, and this was antagonized to some extent by increased amounts of Mg 2+ . Bound dihydrostreptomycin could be readily exchanged by streptomycin and any antibiotically active derivative, but not by fragments of the antibiotic or any other aminoglycoside. With native (run-off) 70 S ribosomes from streptomycin-susceptible strains, the binding was rapid and relatively temperature independent over the range from 0 to 37 C. Polynucleotides did not stimulate the binding. With concentrations of dihydrostreptomycin up to 10 −5 m , greater than 95% of native 70 S ribosomes bound exactly 1 molecule of the antibiotic tightly, with a K diss for the bound complex at 25 C of 9.4 × 10 −8 m . The following thermodynamic parameters were found for the binding with 70 S ribosomes at 25 C:ΔG° = −9.6 kcal/mole, ΔH° = −6.2 kcal/mole, and ΔS° = +11.4 entropy units/mole. Differences in affinity for the antibiotic were found between ribosomes of K-12 strains and those of other E. coli strains. There was insignificant binding to 70 S ribosomes or subunits from streptomycin-resistant or -dependent strains, and to 50 S subunits from susceptible strains. The binding to 30 S subunits from susceptible strains was weaker by an order of magnitude than that to the 70 S particle, with a K diss at 25 C of 10 −6 m . Polyuridylic acid stimulated this binding slightly but did not influence the affinity of the bound molecule. At antibiotic concentrations above 10 −5 m , streptomycin-susceptible 70 S and 30 S particles bound additional molecules of the antibiotic, and binding also occurred to ribosomes from streptomycin-resistant and -dependent strains, as well as to 50 S subunits from all strains. K diss for all of these binding equilibria were [Formula: see text] 10 −4 m . This weaker non-specific binding coincided with the beginning of aggregation phenomena involving the particles, and occurred at sites distinct from the single site which binds the antibiotic tightly. This latter site was completely lost after the one-step mutation to high-level resistance or dependence.