Abstract
The regulatory mechanism of homoserine dehydrogenase and dihydrodipicolinate synthetase was investigated in Brevibacterium lactofermentum. Brevibacterium lactofermentum AJ 3585, which is a mutant resistant to S-(2-aminoethyl) L-cysteine (AEC) and α-amino-β-hydroxyvaleric acid (AHV), simultaneously produced 13 mg/ml each of L-threonine and L-lysine•HCl. Homoserine dehydrogenase of AJ 3585 was genetically desensitized to feedback inhibition by L-threonine and L-isoleucine, and its affinity for asparto β-semialdehyde was increased about 8-fold compared with the parental strain. However, L-threonine production and homoserine dehydrogenase formation of AJ 3585 were inhibited by L-methionine. We tried to derive the mutant resistant to S-methylcysteine sulfoxide from AJ 3585 to release the repression of homoserine dehydrogenase. Derepressed mutant, M-15, produced 17.4 mg/ml of L-threonine and 8.6mg/ml of L-lysine•HCl, and the homoserine dehydrogenase level was increased about 2-fold as compared with AJ 3585. On the other hand, L-lysine production and dihydrodipicolinate synthetase formation were inhibited by L-leucine. On the contrary, L-threonine accumulation increased remarkably in the presence of excess L-leucine. The data provides significant evidence that the level of homoserine dehydrogenase and dihydrodipicolinate synthetase in Brevibacterium lactofermentum affects the production of L-threonine and L-lysine.
