Prephenate dehydratase (PDT) is a key regulatory enzyme in l-phenylalanine biosynthesis. In Mycobacterium tuberculosis, expression of pheA, the gene encoding PDT, has been earlier reported to be iron-dependent (1, 2). We report that M. tuberculosis pheA is also regulated at the protein level by aromatic amino acids. All of the three aromatic amino acids (phenylalanine, tyrosine, and tryptophan) are potent allosteric activators of M. tuberculosis PDT. We also provide in vitro evidence that M. tuberculosis PDT does not possess any chorismate mutase activity, which suggests that, unlike many other enteric bacteria (where PDT exists as a fusion protein with chorismate mutase), M. tuberculosis PDT is a monofunctional and a non-fusion protein. Finally, the biochemical and biophysical properties of the catalytic and regulatory domains (ACT domain) of M. tuberculosis PDT were studied to observe that, in the absence of the ACT domain, the enzyme not only loses its regulatory activity but also its catalytic activity. These novel results provide evidence for a monofunctional prephenate dehydratase enzyme from a pathogenic bacterium that exhibits extensive allosteric activation by aromatic amino acids and is absolutely dependent upon the presence of catalytic as well as the regulatory domains for optimum enzyme activity.