Physiological advantage of the mechanism of the tryptophan synthetase reaction.

Indole-accumulating and indole-utilizing tryptophan synthetase mutants of Saccharomyces cerevisiae were found to form complementing diploids with unusual growth properties. These strains grew at nearly the wild-type exponential rate, but only after an abnormally prolonged lag period of a duration that depended on the initial cell density. The exaggerated lag was reduced by adding low concentrations of indole in the medium. The growth properties were interpreted to be a consequence of the inability of these strains to synthesize tryptophan by the normal mechanism.