SUMMARY: A tryptophan auxotroph of produced an altered tryptophan synthetase which could not convert indole to tryptophan but converted indole-3-glycerol phosphate to indole. As distinct from the normal tryptophan synthetase, which also catalysed this reaction, both pyridoxal phosphate and serine stimulated the activity of the mutant enzyme system. Fractionation and chromatography of the mutant tryptophan synthetase separated it into its 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 → indole reaction demonstrated that different pH-activity responses were obtained, respectively, 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 added, suggest that one role of serine and pyridoxal phosphate in the stimulation of the indole-3-glycerol phosphate → indole reaction is to bind together the A and B proteins in a catalytically effective complex.


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