Control of Tryptophan Biosynthesis in Plant Tissue Cultures: Lack of Repression of Anthranilate and Tryptophan Synthetases by Tryptophan

Abstract Tobacco (cv. Xanthi and cv. Wisconsin 38), rice, carrot, tomato, and soybean tissue cultures were grown in liquid media containing L‐tryptophan. The addition of tryptophan increased the cellular tryptophan levels greatly (12–2500 fold), but did not lower appreciably the levels of two tryptophan biosynthetic enzymes, anthranilate synthetase and tryptophan synthetase. However, the addition of 50 μM tryptophan to the crude enzyme extract completely inhibited the anthranilate synthetase activity while 1 m M tryptophan inhibited the tryptophan synthetase activity by only 10–20°/o. This information indicates that tryptophan biosynthesis is controlled by the feedback inhibition of anthranilate synthetase by tryptophan and not by repression of enzyme synthesis. All of the species had significant enzyme levels. Anthranilate synthetase activity could not be detected in extracts from cells grown on tryptophan unless the extracts were first passed through two G‐25 Sephadex columns with a short 30 °C warming step in between, a procedure shown to remove an inhibitor of the enzyme.