The Amino Acid Sequence of the A Protein (α Subunit) of the Tryptophan Synthetase of Escherichia coli

Cyanogen bromide treatment of the tryptophan synthetase A protein (α subunit) of Escherichia coli gave five major fragments, as was expected from the number of methionine residues in the polypeptide chain. Three of the fragments were isolated by chromatography on Dowex 50; their combined amino acid content accounted for 34 residues. The other two fragments contained a total of 233 residues; they were obtained in pure form by preparative electrophoresis on polyacrylamide gel columns. The amino acid composition of each of the five fragments was in agreement with the composition of the tryptic peptides identified as components of each fragment. The order of the fragments in the A protein was determined, and definitive information was obtained on the order of several tryptic peptides. These results further establish that the tryptophan synthetase A protein is a single polypeptide chain containing 267 amino acid residues.