Acetohydroxy acid synthase (AHAS) and acetohydroxy acid isomeroreductase (AHAIR) encoded by ilvBN and ilvC are two key enzymes which play important roles in the biosynthetic pathway of L-valine. Brevibacterium flavum MD515 was used as the origin strain and site-specific mutagenesis was performed in its ilvN gene which coded for the regulatory subunit of AHAS

resulting in the obtainment of an anti-feedback inhibition gene

named ilvBN'C. Then

the ilvBNrC gene was ligased to plasmid pZ8-1 for construction of the recombinant plasmid pZ8-1-ilvBNrC

which was subsequently transfored into B. flavum MD515. With this method

the targeted transformant B. flavum MD515/pZ8-1-ilvBNrC showed better L-valine producing capacity of 29.5 g·L-1

27.7% increase than that of original strain when it was cultured in 250 ml shake flasks. Meanwhile

the yield of leucine and isoleucine also increased while the alanine decreased. The biomass and growth rate were also increased. Moreover

fermentation experiments was performed in a 30 L fermentor and the results indicated that the L-valine productivity of B. flavum MD515/pZ8-1-ilvBNrC reached to 61.7 g·L-1 while the conversion rate of glucose/valine was up to 39.2%. The work finally made some simple investigations about the purification of L-valine in B. flavum MD515/ pZ8-1-ilvBNrC and B. flavum MD515. The results showed that the light transmittance of strain B. flavum MD515/pZ8-1-ilvBNrC was higher while the protein content was lower than that of B. flavum MD515

which was beneficial for subsequent separation process.