J. Biol. Chem., Vol. 265, Issue 12, 6624-6625, 04, 1990
Escherichia coli mutant trpA34 has an Asp----Asn change at active site residue 60 of the tryptophan synthetase alpha chain
L Shirvanee, V Horn and C Yanofsky
Department of Biological Sciences, Stanford University, California 94305-5020.
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.