Isolation and Properties of α-Ketobutyrate-resistant Lysine-producing Mutants from Brevibacterium flavum

The growth of Brevibacterium flavum FA1-30, a L-lysine-producing mutant strain with an aspartokinase desensitized to feedback inhibition, was almost completely inhibited by a-ketobutyrate (αKB) at concentrations more than 4 mg/ml. Aspartic acid hydroxamate (Asphx) did not inhibit the growth at a concentration up to 6 mg/ml, but slightly stimulated the αKB inhibition. Mutants resistant to αKB in the presence and absence of Asphx (type-I and -II, respectively) were derived and further selceted by their lysine productivity. The best producers among the type-I and -II mutants produced 41.9 and 29.4 g/liter of L-lysine·HCl, 1.9- and 1.4-fold that by the parent, respectively. The type-II mutant was confirmed to be resistant to αKB alone, but was still sensitive to aKB in the presence of Asphx, similar to the parent. The type-I mutants were resistant to αKB in the presence and absence of Asphx. Pyruvate dehydrogenase (PD) activity or both PD and citrate synthase (CS) activities significantly decreased in the type-II or -I mutants, respectively. The apparent Km of PD with respect to pyruvate and that of CS with respect to oxaloacetate increased 1.7-fold higher than those of the parent, respectively.