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 8.6 mg/ml of l-lysine-HCl, and the homoserine dehydrogenase level was increased about 2-fold as compared with AJ 3585. On...