Regulation of Aspartate Family Amino Acid Biosynthesis in Brevibacterium flavum

Homoserine O-transacetylase was purified about 10-fold from sonic extracts of a methionine-less mutant, strain M-116, derived from Brevibacterium flavum No. 2247 (wild type). This purification resulted in about an 80-fold increase in specific activity over that of the wild strain. Acetyl-CoA, but not succinyl-CoA, was effective as an acyl donor. The reaction mechanism is suggested to be “ping pong” type. Km values for homoserine and acetyl-CoA were 2.8 and 0.05 mM, respectively. The enzyme activity was not inhibited by methionine and/or S-adenosylmethionine. Homocysteine also gave no inhibitory effect. O-Acetylhomoserine, one of the reaction products, inhibited the enzyme activity in a mixed manner with respect to both substrates. The formation of homoserine O-transacetylase in B. flavum was strongly and almost completely repressed by methionine. The repression of homoserine dehydrogenase [EC 1. 1. 1. 3] by methionine might not be coordinate with that of homoserine O-transacetylase. Based on these results, the regulation of methionine biosynthesis in this bacterium is discussed.