Regulation of Aspartate Family Amino Acid Biosynthesis in Brevibacterium flavum

Abstract

The specific activities of aspartate kinase, aspartate-β-semialdehyde (ASA)* dehydrogenase, homoserine dehydrogenase, homoserine kinase and threonine synthetase, were measured for the cell free extracts of Brevibacterium flavum grown on glucose-salts medium containing aspartate family amino acid(s). Synthesis of both homoserine dehydrogenase and homoserine kinase was repressed by methionine. Any other enzymes except homoserine dehydrogenase and homoserine kinase were not repressed significantly by amino acids of aspartate family. Although an over-production of L-lysine by a homoserine-requiring mutant of B. flavum was strongly inhibited by the excess addition of L-threonine, specific activities of aspartate kinase and ASA dehydrogenase were almost independent of the amount of L-threonine added. When a threonine-requiring mutant derived from B. fiavum was cultured in a medium containing fixed amount of L-threonine, the addition of L-methionine, which repressed homoserine dehydrogenase, caused the increase of L-lysine produced but a decrease of L-homoserine production. Repression by L-methionine of homoserine dehydrogenase in the wild strain of B. flavum was accompanied by the accumulation of 5 g/liter of L-lysine monohydrochloride.