Direct evidence for a constitutive internal promoter in the tryptophan operon of Corynebacterium glutamicum.

Insertional inactivation of the trpE gene in Corynebacterium glutamicum AS019 blocked expression of the trp operon from the primary promoter and resulted in tryptophan auxotrophy. The presence of indole, the substrate for the terminal trp pathway enzyme tryptophan synthase, reversed this auxotrophy thereby providing evidence for the existence of an internal promoter independently expressing the tryptophan synthase genes (trpB and trpA). The toxic tryptophan analogue 6-fluorotryptophan failed to inhibit the growth permitted by indole thus suggesting that expression of tryptophan synthase from the internal promoter is constitutive.