Depression of by-product formation during L: -isoleucine production by a living-cell reaction process.

Two unnatural and unwanted amino acids, norvaline (Nva) and O-ethylhomoserine (O-EH) are formed as by-products in L: -isoleucine production by Brevibacterium flavum AB-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 of L: -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 of L: -methionine. Furthermore, the O-EH-forming enzyme, which converts O-acetylhomoserine to O-EH, was speculated to be repressed by L: -methionine.