In Escherichia coli, the phenylalanine-sensitive 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (DAHPS) AroG catalyzes the first committed step in the biosynthesis of aromatic compounds. To investigate the feedback inhibition site of AroG, mutated enzymes prepared with sequence-overlap extension PCR were expressed and purified. The enzymatic activity assay showed that the amino acid replacements at Phe144, Leu175, Leu179, Phe209, Trp215Ala and Val221 completely or partially relieved feedback inhibition of AroG addressed by the phenylalanine. Ile10Ala and Delta(1-15) desensitized feedback inhibition and caused a 70 approximately 90% loss of the specific catalytic activities. These results strongly suggest an involvement of the interior region and the N-terminus of the polypeptide chain of AroG in the formation of the feedback inhibition site of DAHPS.

The first regulatory step in the synthesis of aromatic amino acids is catalyzed by 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase (DAHPS). In Escherichia coli, the allosteric DAHPS exists as three isozymes, AroG, AroF and AroH, each independently feedback-inhibited by corresponding end product amino acids, phenylalanine, tyrosine and typtophan. Structural biological evidences have suggested that the N-terminus of AroG is involved in the formation of a putative inhibitor-binding site and feedback inhibition signal transmission. Our previous work showed that a single amino acid residue replacement Ile10Ala or deletion of 15 N-terminal amino acids could lead to a dramatic loss of AroG enzymatic activity (Hu et al. 2003). Here we demonstrate that the deletion of N-terminus prevents the enzyme from forming a dimeric structure, indicating that the N-terminus of AroG plays a critical role in the formation of the essential tight dimeric structure.

Tyrosine-inhibitable 3-deoxy-D-arabinoheptulosonic acid-7-phosphate (DAHP) synthase was purified to homogeneity without significant loss of sensitivity to inhibition by tyrosine from an operator-constitutive strain (tyrOc) of Salmonella. The enzyme had an apparent molecular weight of 76,000 by gel filtration and a subunit molecular weight of 40,000 by sodium dodecyl sulfate-gel electrophoresis and by reaction with dimethyl suberimidate. It had an isoelectric point of 4.68. Inhibition by L-tyrosine showed a Hill coefficient of 1.8 at pH 7.0, suggesting cooperative interaction between tyrosine-binding sites, and was competitive with phosphoenol pyruvate and noncompetitive with erythrose-4phosphate. Tyrosine-inhibitable 3-deoxy-n-arabinoheptulosonate-7-phosphate (DAHP) synthase (EC 4.1.2.15) has been extensively purified from an operator-constitutive (tyrO) strain of Salmonella typhimurium (13). Since gel electrophoresis of this preparation indicated minor impurities (10), we undertook a further purification of the enzyme and developed a procedure for preparing it in homogeneous form. We also studied its allosteric properties and subunit structure. (This report is in partial fulfillment of the requirements for the Ph.D. degree, submitted by C. Y. H., 1974.) MATERIALS AND METHODS Materials. Barium DAHP was prepared according to Sprinson et al. (24) and was converted to the potassium salt. Monocyclohexylammonium phosphoenolpyruvate (PEP) was prepared according to the method of Clark and Kirby (5) and used directly. Monocyclohexylammonium phosphoenol a-ketobutyrate was prepared according to Bondinell and Sprinson (3). Barium chorismate was prepared according to Gibson (12). Erythrose-4-phosphate (E4P) was prepared according to the method of Ballou and MacDonald (2). Hemoglobin was a gift from R. Benesch. Barium prephenate was isolated from accumulation media of strain tyrAl9 according to the method of Dayan and Sprinson (9). Dimethyl suberimidate was prepared according to the method of Davies and Stark (7). The following compounds were gifts: monocyclohexylammonium (Z)-phosphoenol-3-fluoropyruvate and a-(dihydroxyphosphinyl methyl)-acrylic acid from G. L. Kenyon (25); and fosfomycin from D. Hendlin (4, 15). All other materials used were obtained commercially and used without further purification.