The Effect of Modification of Nucleotide-37 on the Interaction of Aminoacyl-tRNA with the A Site of the 70S Ribosome

To estimate the effect of modified nucleotide 37, the interaction of two yeast aminoacyl-tRNAs (Phe-tRNAPhe+Y and Phe-tRNAPhe–Y) with the A site of complex [70S · poly(U) · deacylated tRNAPhe in the P site] was assayed at 0–20°C. As comparisons with native Phe-tRNAPhe+Y showed, removal of the Y base decreased the association constant of Phe-tRNAPhe–Y and the complex by an order of magnitude at every temperature tested, and increased the enthalpy of their interaction by 23 kJ/mol. When the Y base was present in the anticodon loop of deacylated tRNAPhe bound to the P site of the 70S ribosome, twice higher affinity for the A site was observed for Phe-tRNAPhe–Y but not for Phe-tRNAPhe+Y. Thus, the modified nucleotide 3" of the Phe-tRNAPhe anticodon stabilized the codon–anticodon interaction both in the A and P sites of the 70S ribosome.