Prephenate dehydratase is a key regulatory enzyme in the phenylalanine-specific pathway of Corynebacterium glutamicum. PCR-based random mutagenesis and functional complementation were used to screen for m-fluorophenylalanine ( mFP)-resistant mutants. Comparison of the amino acid sequence of the mutant prephenate dehydratases indicated that Ser-99 plays a role in the feedback regulation of the enzyme. When Ser-99 of the wild-type enzyme was replaced by Met, the specific activity of the mutant enzyme was 30% lower than that of the wild-type. The Ser99Met mutant was active in the presence of 50 microM phenylalanine, whereas the wild-type enzyme was not. The functional roles of the eight conserved residues of prephenate dehydratase were investigated by site-directed mutagenesis. Glu64Asp substitution reduced enzyme activity by 15%, with a 4.5- and 1.7-fold increase in Km and kcat values, respectively. Replacement of Thr-183 by either Ala or Tyr resulted in a complete loss of enzyme activity. Substitution of Arg-184 with Leu resulted in a 50% decrease of enzyme activity. The specific activity for Phe185Tyr was more than 96% lower than that of the wild-type, and the Km value was 26-fold higher. Alterations in the conserved Asp-76, Glu-89, His-115, and Arg-236 residues did not cause a significant change in the Km and kcat values. These results indicated that Glu-64, Thr-183, Arg-184, and Phe-185 residues might be involved in substrate binding and/or catalytic activity.