- Thirteen different yeast tRNAPhe variants with single nucleotide changes in positions 34-37 in the anticodon region were prepared by an enzymatic procedure described previously. Aminoacylation kinetics using purified yeast phenylalanyl-tRNA synthetase revealed that the level of aminoacylation was very different for different sequences inserted. The low level of aminoacylation was the result of a steady state between a slow forward reaction rate and spontaneous deacylation of the product. Aminoacylation kinetics performed at higher synthetase concentrations revealed that substitution at position 34 in tRNAPhe decreased the Km nearly 10-fold but only had a small effect on Vmax. Similar substitutions at positions 35, 36, and 37 had a lesser effect. These data suggest a sequence-specific contact between the anticodon of yeast tRNAPhe and the cognate synthetase.