Essential Role of Aspartokinase in L-Threonine Production by Escherichia coli W Mutants

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We previously constructed an L-threonine-producing strain of E. coli W, KY8280, which is an Ile⁺ revertant of KY8279 which requires L-methionine, α, ε-diaminopimelic acid and L-isoleucine [H. Kase et al., Agric. EM. Chem., 35, 2089 (1971)]. From KY8280, another L-threoninehyperproducing strain, KY8366, was obtained as an α-amino-β-hydroxyvaleric acid (AHV, a threonine analog)-resistant mutant. Enzymatic analysis revealed that KY8280 constitutively expressed 8-fold higher L-threonine-sensitive aspartokinase I activity than KY8279. In addition, KY8366 constitutively expressed 13-fold higher L-lysine-sensitive aspartokinase III activity than KY8280. Such elevated levels of aspartokinases may contribute to the hyperproduction of L-threonine by these mutant strains. KY8366 produced 28 mg/ml of L-threonine in a culture medium fed with 12% glucose.