Screening ofl-Isoleucine Producers among Ethionine Resistant Mutants ofl-Threonine Producing Bacteria

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Abstract
Two types of L-isoleucine producing mutants were derived from L-threonine producers by supplementing resistance to ethionine. The main control site in the L-isoleucine biosynthetic pathway after threonine is threonine dehydratase. In the case of Brevibacterium flavum No. 14083, L-isoleucine production was based on the insensitivity of this key enzyme to feedback inhibition by L-isoleucine. In B. flavum No. 168, production was based on the increase in specific activity of this enzyme. The former produced 11.3 g/l of L-isoleucine and the latter produced 9.92 g/l from glucose. The former showed a vigorous ability of acetic acid assimilation; the latter did not.