EFFECT OF MG-2+ CONCENTRATION, POLYAMINES, STREPTOMYCIN, AND MUTATIONS IN RIBOSOMAL-PROTEINS ON THE ACCURACY OF THE 2-STEP SELECTION OF AMINOACYL-TRANSFER RNAS IN PROTEIN-BIOSYNTHESIS

Abstract

Discrimination against the binding of noncognate aminoacyl(aa)-tRNA by mRNA-programmed ribosomes (in Escherichia coli) is the outcome of 2 selection steps, one involving an aa-tRNA.cntdot.EFTu[elongation factor Tu].cntdot.GTP complex, which occurs prior to and includes GTP hydrolysis, and the other involving the aa-tRNA alone, which follows GTP hydrolysis. Conditions which lead to errors in protein synthesis influenced the accuracy of one or both selection steps in a system measuring poly(U)-directed binding of Leu-tRNA2Leu. Streptomycin has a large effect only on the discrimination process following GTP hydrolysis, but the other perturbations of recognition studied, high [Mg2+], polyamines, the strA1 and ram1 mutations, affect both discrimination processes. Proofreading of aa-tRNA by ribosomes for the most part uses the same specificity determinants used in the initial selection of a ternary complex.