Interactions of tryptophan synthetase subunits in Escherichia coli containing mutationally altered beta2 subunits.

The (Y and /I2 subunits of tryptophan synthetase are specified by the A and B genes, respectively, of the trp operon in Escherichia coli. Missense mutations in trpB result in catalytically inactive /I2 subunits. In some cases, these mutations also diminish the catalytic and /I2 subunit stimulating activities of the (Y subunit in cell extracts. Analysis of repressor-negative (trpR) strains having either the trpB9578 or trpB9611 mutation reveals that the mutants contain normal levels of LY subunit protein and experiments with prototrophic recombinants derived from the mutants show that the (Y subunit is not genetically altered. From studies of the (Y subunit binding properties and LY subunit stimulating activities of the mutant pz subunits, we conclude that the diminished cx subunit activities observed in cell extracts result from binding of the (Y subunit in inactive or partially active mutant complexes. Experiments in which CY subunit is preincubated with a partially purified preparation of one of the mutant p2 subunits and then assayed in the presence of excess normal & subunit are consistent with this conclusion. Genetic mapping experiments demonstrate that the trpB9578 and trpB9611 mutations alter the /I2 subunit structure at sites 100 to 125 amino acid residues apart.