Instability and stabilization of an L-leucine producing strain of Corynebacterium glutamicum. Part I. Properties of revertants appearing in L-leucine fermentation culture broth.

The L-leucine productivity of an L-leucine producing strain, H-1204, of Corynebacterium glutamicum substantially decreased during a large-scale culture or repetitive subculturing. This instability was found to be due to the appearance of revertants with lower or no L-leucine productivity. Strains in the culture broth could be roughly classified into three types on the basis of their phcnotypes: L-type, original L-leucine producing strain, ValL Leu+ (valine leaky); M-type, Val+ Leu+ (prototroph); V-type, Val+ Leu- (leucine auxotroph). The appearance of these revertants was determined to be caused by the distribution imbalance of α-ketoisovaleric acid, the common precursor for L-leucine and L-valine biosynthesis.