Comparison of the amino‐acid sequences of several methionyl‐tRNA synthetases indicates the occurrence of a few conserved motifs, having a possible functional significance. The role of one of these motifs, centered at position 300 in theE. coli enzyme sequence, was assayed by the use of site‐directed mutagenesis. Substitution of the His301 or Trp305 residues by Ala resulted in a large decrease in methionine affinity, whereas the change of Val298 into Ala had only a moderate effect. The catalytic rate of the enzyme was unimpaired by these substitutions. It is concluded that the above conserved amino‐acid region is located at or close to the amino‐acid binding pocket of methionyl‐tRNA synthetase.
