STREPTOMYCIN CAUSES MISREADING OF NATURAL MESSENGER BY INTERACTING WITH RIBOSOMES AFTER INITIATION

The induction of misreading by streptomycin in vitro, previously observed with synthetic messengers, is now demonstrated with natural (endogenous or viral) messenger by the use of extracts of temperature-sensitive mutants lacking Glu-tRNA or Val-tRNA synthetase. With chain-elongating but noninitiating ribosomes (i.e., purified polysomes) deprived of an aminoacyl-tRNA, streptomycin and other aminoglycosides, over a wide range of concentrations, stimulate incorporation. With ribosomes initiating in the presence of streptomycin stimulation is also observed but it is restricted, just like phenotypic suppression in cells, to very low streptomycin concentrations which evidently allow some ribosomes to initiate and later encounter them in the course of chain elongation. The stimulation is accompanied by an increase in the size of the products; hence, it is evidently due to substitution of an incorrect aminoacyl-tRNA for a missing one. The test introduced here also has revealed a misreading effect of streptomycin on resistant ribosomes. In adition, significant intrinsic misreading was observed without streptomycin, indicating that under optimal conditions for in vitro protein synthesis an empty codon is frequently read by an incorrect aminoacyl-tRNA.