Dissociation of peptidyl-tRNA from ribosomes is perturbed by streptomycin and by strA mutations.

Peptidyl-tRNA may dissociate preferentially from ribosomes during protein synthesis when it is inappropriate to, does not correctly complement, the messenger RNA. To test this idea, growing cultures of Escherichia coli were treated with streptomycin to increase the frequency of errors during protein synthesis. Since the treated cells had a temperature-sensitive peptidyl-tRNA hydrolase and could not destroy dissociated peptidyl-tRNA, it was possible to measure the rate of its accumulation after raising the temperature to non-permissive conditions. Both low and high doses of streptomycin enhanced the rate of dissociation and accumulation of peptidyl-tRNA. The rank order of rates of dissociation/accumulation of various isoaccepting tRNA families was not significantly altered by the drug treatment. We concluded that streptomycin stimulated a normal pathway for dissociation of peptidyl-tRNA. Two streptomycin- resistant strains of E. coli had higher rates of dissociation of peptidyl-tRNA than did their sensitive parent strain. When treated with high doses of the drug, the resistant strains showed slightly reduced rates of dissociation of peptidyl-tRNA. These results were interpreted in terms of a two state, two site model for protein synthesis: streptomycin enhances the binding of aminoacyl-tRNA to a tight state of the ribosome A site; the strA mutation enhances translocation to a loose state of the ribosome P site.