Codon–anticodon interaction at the ribosomal peptidyl-site

Abstract

VARIOUS models have been proposed to explain the interaction between an aminoacyl-tRNA molecule and the ribosomal A and P sites during one elongation cycle of protein biosynthesis^1–7. In general all these models agree as to the existence of at least two tRNA binding sites on the 50S subunit, the A and P sites. These models, however, do not agree on the important question of whether mRNA–tRNA (codon–anticodon) recognition occurs exclusively in the A site or in both A and P sites. Previous experimental evidence adduced in favour of either hypothesis has usually been based on analysis of the effects of various antibiotics on the partial reactions of protein synthesis. However, molecular mechanisms of action of such antibiotics are still poorly understood, and definite conclusions as to which type of model is correct cannot be based solely on such data. Here we show that factor-mediated binding of aminoacyl–tRNA (aa–tRNA) to the ribosomal A site is dependent on the presence of both an mRNA codon in the P site and of a tRNA which recognises that codon. Thus binding of a ternary complex EF-Tu · aa-tRNA · GTP to the ribosomal A site occurs only if the P site is occupied by a tRNA for which codon–anticodon interaction is maintained (for a preliminary report see ref. 8).