Metabolic interlock. The influence of histidine on tryptophan biosynthesis in Bacillus subtilis.

A regulatory relationship in Bacillus subtilis between histidine and the synthesis of tryptophan exemplifies “metabolic interlock,” a phenomenon of regulatory interactions exerted between biochemical pathways. The enzymatic basis for the histidine-tryptophan relationship was examined. Histidine activates anthranilate synthetase by increasing the apparent value of VmSx. Hence, histidine antagonizes growth inhibition caused by 5-methyltryptophan, a false feedback inhibitor of anthranilate synthetase, by increasing the velocity of the reaction. Mutant strain F-2, which synthesizes the tryptophan enzymes constitutively, excretes 4 times more tryptophan in minimal medium containing histidine than in unsupplemented minimal medium. This reflects the stimulation of anthranilate synthetase by histidine. In contrast to the situation in F-2, growth of wild type in the presence of histidine does not affect tryptophan excretion. Histidine supplementation indirectly leads to repression of the specific activity of at least one tryptophan enzyme, anthranilate synthetase, presumably as a consequence of the stimulation of tryptophan production. A histidine-excreting mutant, NP 187, exhibits the repressed level of anthranilate synthetase that is characteristic of wild type grown in the presence of histidine. This compensatory mechanism in which histidine plays a dual role, activating anthranilate synthetase activity but also indirectly repressing its biosynthesis, does not operate in F-2. The tryptophan enzymes of F-2 are not repressible and therefore lack the compensatory adjusting mechanism of wild type. Another mutant strain, NP 100, which is derepressed in the synthesis of tryptophan enzymes and which accumulates chorismate because of a block in chorismate mutase, excretes significantly more tryptophan than does F-2. This results from an increased level of chorismate available to anthranilate synthetase. Tryptophan excretion is not further elevated in this strain by histidine. To explain this, we propose that anthranilate-5-phosphoribosylpyrophosphate phosphoribosyltransferase (PR transferase), not anthranilate synthetase, is the rate-limiting reaction in the tryptophan pathway in B. subtilis. Although tryptophan and histidine do not allosterically influence PR transferase, both amino acids could effectively regulate PR transferase activity by controlling the availability of one of its substrates, anthranilate.