A Mutational Alteration of the Tryptophan Synthetase of Escherichia coli

Abstract

A tryptophan auxotroph of Escherichia coli produces an altered tryptophan synthetase which cannot convert indole to tryptophan, but can convert indole-3-glycerol phosphate to indole. As distinct from the normal tryptophan synthetase, which also catalyses this reaction, both pyridoxal phosphate and serine stimulate the activity of the mutant enzyme system. Fractionation and chromatography of the mutant tryptophan synthetase separated it into two protein components, A and B. Examinations of the separated components showed that the A protein was normal, while the B protein was altered. Studies of the effect of serine on the pH-activity response of mutant preparations in the indole-3-glycerol phosphate [forward arrow]indole reaction demonstrated that different responses are obtained, in the presence and absence of serine. The curves obtained were characteristic of the serine-requiring and serine-non-requiring reactions, respectively, of normal tryptophan synthetase. The saturation curves of the mutant component B by normal component A, with and without serine present, suggest that one role of serine and pyridoxal phosphate in the stimulation of the indole-3-glycerol phosphate[forward arrow]indole reaction is to bind together the A and B proteins in a catalytically effective complex.