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SUMMARY A streptomycin-sensitive strain and six streptomycin-resistant mutants of pneumococcus have been studied. These strains differed in resistance to streptomycin over a 5000-fold range, and the resistance mutations occurred at recombinationally distinct sites of a single genetic locus (the str locus). The effects of streptomycin on cell-free amino acid incorporating systems prepared from each of the strains were studied. Both polyuridylic acid (poly U)- and endogenous mRNA-directed systems were employed. As in Escherichia coli, sensitivity to streptomycin was found to reside in the ribosomal fraction of the amino acid incorporating system. Streptomycin caused both inhibition of l4CC-phenylalanine incorporation and stimulation of 3H-isoleucine incorporation in poly U-directed experi- ments. The resistance of the amino acid incorporating systems to these effects of streptomycin paralleled the streptomycin resistance of the strain from which the system was derived. In endogenous mRNA-directed systems the in- corporation of 14C-valine and 3H-isoleucine was followed simultaneously. Streptomycin caused an inhibition of valine incorporation and a lesser inhibi- tion of isoleucine incorporation. At a higher magnesium concentration streptomycin caused a stimulation of isoleucine incorporation while still inhibiting valine incorporation. The disparate behaviour of isoleucine and valine incorporation in the presence of streptomycin may be due to strepto- mycin-induced misreading of endogenous mRNA. As in the poly U-directed experiments, the magnitude of the effects of streptomycin on endogenous mRNA directed amino acid incorporation correlated in an inverse fashion with the resistance of the strain from which the system was prepared. These results lead us to conclude that the ribosome is the primary target for streptomycin in pneumococcus. We propose that the affinity of the ribosome for streptomycin at a critical site or sites determines the level of resistance of the bacterium, and that this affinity is affected by mutations in the str locus.

Levels of Resistance in ribosomes from Genetically Linked, Streptomycin-resistant Mutants of Pneumococcus