Factors enhancing l-valine production by the growth-limited l-isoleucine auxotrophic strain Corynebacterium glutamicum ΔilvA ΔpanB ilvNM13 (pECKAilvBNC)

Cell growth limitation is known to be an important condition that enhances l-valine synthesis in Corynebacterium glutamicum recombinant strains with l-isoleucine auxotrophy. To identify whether it is the limited availability of l-isoleucine itself or the l-isoleucine limitation-induced rel-dependent ppGpp-mediated stringent response that is essential for the enhancement of l-valine synthesis in growth-limited C. glutamicum cells, we deleted the rel gene, thereby constructing a relaxed (rel (-) ) C. glutamicum Delta ilvA Delta panB Delta rel ilvNM13 (pECKAilvBNC) strain. Variations in enzyme activity and l-valine synthesis in rel (+) and rel (-) strains under conditions of l-isoleucine excess and limitation were investigated. A sharp increase in acetohydroxy acid synthase (AHAS) activity, a slight increase in acetohydroxyacid isomeroreductase (AHAIR) activity, and a dramatic increase in l-valine synthesis were observed in both rel (+) and rel (-) cells exposed to l-isoleucine limitation. Although the positive effect of induction of the stringent response on AHAS and AHAIR upregulation in cells was not confirmed, we found the stringent response to be beneficial for maintaining increased AHAS, dihydroxyacid dehydratase, and transaminase B activity and l-valine synthesis in cells during the stationary growth phase.