The mechanism of codon-anticodon interaction in ribosomes. Quantitative study of codon-dependent binding of tRNA to the 30-S ribosomal subunits of Escherichia coli

Abstract

The formation of a ternary complex 30-S-subunit . poly(U) . tRNAPhe is discussed and the conditions for its correct description by Langmuir's isotherm are deduced. The affinity constant of the binary complex 30-S-subunit . poly(U) is measured. The reversibility of binding of tRNAPhe to the complex 30-S-subunit . poly(U) is proved in a direct way. The main reason for the heterogeneity of ternary complexes was found to be due to the ability of high-molecular-weight poly(U) to form complicated aggregates with 30-S subunits. If a fraction of poly(U) of moderate molecular weight (30 000) is used, then the ternary complexes are homogeneous in stability and yield the same affinity constants for deacylated, aminoacylated and peptidyl-tRNAPhe (1 X 10(8) M-1 at 20 mM Mg2+, 200 mM NH+4 and 0 degrees C). Ribosomal protein S1 increases the binding constant of poly(U) with 30-S subunits but does not change the binding constant of tRNAPhe with the 30-S-subunit . poly(U) complex. All 30-S subunits, even partially stripped of S1 protein, are active in the binding of both poly(U) and tRNAPhe.