Two Mechanisms of Allelic Complementation Among Tryptophan Synthetase Mutants of Saccharomyces cerevisiae

Two different types of allelic complementation were observed in tryptophan synthetase mutants of the yeast Saccharomyces cerevisiae. Each type is associated with a different mechanism for the enzymatic conversion of indole-3-glycerol phosphate (InGP) to tryptophan. Mechanism I is utilized by a hybrid tryptophan synthetase that resembles, but is not identical with, the wild-type enzyme. Mechanism II is due to a sequential conversion of InGP to free indole, and indole to tryptophan. Two partially active mutant enzymes rather than a single hybrid enzyme catalyze the sequential reaction steps. This is an example of intracellular cross-feeding. The quantitative evaluation of mechanism II leads to the conclusion that tryptophan synthetase in yeast is most likely a dimer of two identical subunits.