Control of tryptophan biosynthesis by the methyltryptophan resistance gene in Bacillus subtilis.

5-Methyltryptophan-resistant mutants derived from Bacillus subtilis strain 168 synthesize all of the tryptophan biosynthetic enzymes constitutively and excrete tryptophan. These mutants can be divided into three classes: class 1, low enzyme level and low rate of tryptophan excretion; class 2, high enzyme level and intermediate rate of tryptophan excretion; and class 3, high enzyme level and high rate of tryptophan excretion. A bradytrophic requirement for phenylalanine is correlated with the rate of tryptophan excretion. The phenylalanine requirement is relieved when the rate of tryptophan excretion is reduced by either (i) lowering the level of the tryptophan enzymes, (ii) reducing the supply of a tryptophan precursor (chorismate), or (iii) stopping tryptophan synthesis by a mutational block in the pathway. All of the mutants map in a region of the chromosome previously reported as the mtr locus. Our data show that synthesis of the tryptophan enzymes is controlled through the mtr locus but not influenced by precursors of tryptophan.