Enzymes of Common Pathway for Aromatic Amino Acid Biosynthesis in Brevibacterium flavum and Its Tryptophan-producing Mutants

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Five enzymes after 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (DS) in the common pathway for aromatic amino acid biosynthesis in Brevibacterium flavum were examined for general enzymatic and regulatory properties. These enzyme activities were not feedback-inhibited by the end products, phenylalanine, tyrosine, and tryptophan. On the other hand, syntheses of dehydroquinate synthase (DQS), shikimate dehydrogenase (SD), shikimate kinase (SK) and 5-enolpyruvylshikimate-3-phosphate synthase (ESPS) were derepressed 2- to 8-fold by tyrosine limitation as is DS. The azaserine-resistant tryptophan-producing mutant A-100 had 1.6- to 2.9-fold higher levels of DS, DQS, SD, SK, and chorismate synthase than those of the parent. Moreover, further increases of DS and DQS activities were found in sulfaguanidine-resistant mutants derived from strain A-100. These enzymes in the azaserine- and sulfaguanidine-resistant mutants were still repressed by tyrosine but showed higher activities than did the respective parents at any tyrosine concentration or any growth phase.
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