Escherichia coli mutant trpA34 has an Asp----Asn change at active site residue 60 of the tryptophan synthetase alpha chain.

Asp-60 is believed to be a catalytically essential residue of the tryptophan synthetase alpha chain of Escherichia coli (Nagata, S., Hyde, C.C., and Miles, E.W. (1989) J. Biol. Chem. 264, 6288-6296). Surprisingly, mutations altering Asp-60 were not observed in the many trpA missense mutants characterized in the 1960s. However, there was one genetic class of trpA missense mutants, represented by trpA34, for which protein structure analyses failed to detect an amino acid substitution. DNA sequence analyses have now shown that the trpA34 mutation was in codon 60 and that it resulted in replacement of Asp-60 by Asn. This finding provides additional support for the conclusion that the tryptophan synthetase alpha chain contains only a small number of absolutely essential residues.