Genetic and biochemical studies of partially active tryptophan synthetase mutants of Saccharomyces cerevisiae.

Approximately 20% of the tryptophan synthetase mutants (tr(5)) of Saccharomyces cerevisiae retain activity in one of the half reactions catalyzed by this enzyme and have been identified as indole-accumulating or indole-utilizing tr(5) mutants by complementation tests. Ten indole-accumulating and six indole-utilizing mutants have been studied. For the half reactions they catalyze, these partially active mutants have from about one-half to twice the specific activities of the wild-type enzyme. Indole-accumulating mutant enzymes showed varying responses to pyridoxal phosphate and serine in the assay mixture. The partially active mutants were further characterized by their patterns of allelic complementation and their distribution on the fine-structure map of the locus. It was concluded that these mutants define two distinct functional regions of the tr(5) locus, corresponding to the two half reactions.