Restriction, de-restriction and mistranslation in missense suppression. Ribosomal discrimination of transfer RNA's

Abstract

The strA ribosomal mutations known to restrict the level of translational ambiguity and the efficiency of nonsense transfer RNA-suppressors, are shown also to restrict the efficiency of missense tRNA-suppressors; the efficiency of wildtype tRNA is not noticeably affected by these strA mutations and the extent by which the suppressor tRNA is restricted is shown to be dependent upon some structural aspect of the suppressor-tRNA molecule other than either the anticodon or the amino acid-accepting specificity. Ribosomal alteration mutants ( ram), known to reverse strA restriction of translational ambiguity, are shown to reverse also strA restriction of the efficiency of nonsense and missense tRNA-suppressors. Furthermore, introduction of the ram mutation into missense suppressor strains is shown greatly to increase the amount of mistranslation (a property peculiar to missense suppressors), while it has no significant effect in su − strains. The strA mutation is shown to reverse this ram effect on mistranslation. Presence of streptomycin, known to act on the ribosome, is shown to reverse the effect of strA on restriction and mistranslation in a manner similar and additive to that of the ram mutation. These observations suggest that: (a) the ribosome provides a recognition screen for tRNA's prior to, or simultaneous with, their interaction with messenger RNA; and (b) this postulated ribosomal screen discriminates normal from mutated tRNA.