Two unnatural and unwanted amino acids, norvaline (Nva) andO-ethylhomoserine (O-EH) are formed as by-products inl-isoleucine production byBrevibacterium flavumAB-07 using a new process named the living cell reaction process. Nva formation was depressed by using a leucine auxotrophic mutant (AB-07-Leu-2) derived from strain AB-07. It was found that Nva formation was closely related to leucine biosynthesis.O-EH formation was repressed by addition ofl-methionine to the reaction mixture. However, the homoserine-O-acetyltransferase of AB-07-Leu-2 was not subject to either inhibition or repression by addition ofl-methionine. Furthermore, theO-EH-forming enzyme, which convertsO-acetylhomoserine toO-EH, was speculated to be repressed byl-methionine.
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