Newton, W. Austin (University of California, Berkeley), and Esmond E. Snell. Formation and interrelationships of tryptophanase and tryptophan synthetases in Escherichia coli. J. Bacteriol. 89:355-364. 1965.-In addition to the classical tryptophan-repressible tryptophan synthetase (TSase-tr), tryptophan auxotrophs of Escherichia coli contain another distinct tryptophan synthetase (TSase-ti) which is induced by tryptophan and is identical with tryptophanase (TPase). Escherichia coli B (wild type) forms only TSase-tr when the growth medium lacks tryptophan. When tryptophan is supplied, parallel induction of TPase and TSase-ti occurs while TSase-tr is repressed. Antiserum prepared against purified TPase neutralized TPase and TSase-ti equally, but not TSase-tr. TPase-negative strains of E. coli do not form TSase-ti. Unlike TSase-tr, TSase-ti is not readily detected by whole-cell assays. In the tryptophan auxotroph, E. coli B/1t7, a direct correlation exists between the effectiveness of 4-, 5-, and 6-methyl-tryptophan in inducing TPase and in promoting growth in the presence of indole. In a mutant of this organism, E. coli B/1t7-A, which is constitutive for TPase, 5-methyl-tryptophan and other substrates of TPase increased the rate of growth on limiting indole, a result ascribed to their ability to inhibit degradation of tryptophan and to supply the 3-carbon side chain for synthesis of tryptophan by TPase. This organism produced maximal amounts of TPase when inocula from log-phase cells grown in tryptophan-supplemented minimal medium were allowed to undergo two cell generations in an enriched broth medium.