Restoration of phosphoribosyl transferase activity by partially deleting the trpB gene in the tryptophan operon of Salmonella typhimurium.

The amber mutant trpA28, which contains a mutation mapping within the so-called "unusual" region of the tryptophan (trp) operon of Salmonella typhimurium (between the genes trpA and trpB), lacks both components of the anthranilate synthetase (AS)-phosphoribosyl transferase (PRT) enzyme complex, the products of the genes trpA and trpB, respectively. Twenty-six revertants of this mutant selected on minimal medium supplemented with anthranilic acid, a substrate of PRT, contain deletions of various segments of the "unusual" region and make a species of PRT different in every respect from the wild-type, dissociated form of this enzyme. The results indicate that the unusual region corresponds to the operator proximal end of the trpB gene. Mutants in the unusual region, however, show unexpectedly low levels of AS activity and in two cases (trpA515 and trpA28) no detectable activity of this enzyme component.