초록
<P><B>ABSTRACT</B><P> Production of l -valine under oxygen deprivation conditions by Corynebacterium glutamicum lacking the lactate dehydrogenase gene <I>ldhA</I> and overexpressing the l -valine biosynthesis genes <I>ilvBNCDE</I> was repressed. This was attributed to imbalanced cofactor production and consumption in the overall l -valine synthesis pathway: two moles of NADH was generated and two moles of NADPH was consumed per mole of l -valine produced from one mole of glucose. In order to solve this cofactor imbalance, the coenzyme requirement for l -valine synthesis was converted from NADPH to NADH via modification of acetohydroxy acid isomeroreductase encoded by <I>ilvC</I> and introduction of Lysinibacillus sphaericus leucine dehydrogenase in place of endogenous transaminase B, encoded by <I>ilvE</I> . The intracellular NADH/NAD <SUP>+</SUP> ratio significantly decreased, and glucose consumption and l -valine production drastically improved. Moreover, l -valine yield increased and succinate formation decreased concomitantly with the decreased intracellular redox state. These observations suggest that the intracellular NADH/NAD <SUP>+</SUP> ratio, i.e., reoxidation of NADH, is the primary rate-limiting factor for l -valine production under oxygen deprivation conditions. The l -valine productivity and yield were even better and by-products derived from pyruvate further decreased as a result of a feedback resistance-inducing mutation in the acetohydroxy acid synthase encoded by <I>ilvBN</I> . The resultant strain produced 1,470 mM l -valine after 24 h with a yield of 0.63 mol mol of glucose <SUP>−1</SUP> , and the l -valine productivity reached 1,940 mM after 48 h. </P></P>