A 30 amino acid helix-loop of known structure on the surface of the C-terminal domain of the class I Escherichia coli methionine tRNA synthetase is essential for methionine tRNA anticodon discrimination. Replacing this 30 amino acid peptide with a previously described sequence containing residues from the wild-type protein imbedded in a sequence matrix of mostly alanines and serines, we used a combinatorial mutagenesis and selection strategy to define residual wild-type residues that are not replaceable with alanine or serine, because they are needed for specific recognition of methionine tRNA. Four were identified, of which three have functional side chains (Asn, Arg, Lys). These four and a fifth (Trp) that was previously identified are located at the end of the helix and within the loop, lie on the same side of the structure, and span a distance of about 20 A. We conclude that, within the alanine, serine sequence matrix, only a few non-alanine, non-serine residues in the specificity-determining part of the structure are essential.