Regulation of Synthesis of Aspartate Kinase by Methionine, Threonine, and Lysine in Escherichia coli Strain B

Abstract

Abstract Evidence has been presented in this investigation to show that the synthesis of aspartate kinase in Escherichia coli B is regulated by the ratio of the intracellular concentrations of the end products, methionine, threonine, and lysine. l-Lysine and l-threonine at a concentration of 5 mm repress the enzyme to the extent of 60 to 70% and 15 to 25%, respectively. On the other hand, methionine at a concentration of 5 mm stimulates the synthesis of aspartate kinase to the extent of 2- to 4-fold. The increased aspartate kinase activity observed when the bacteria is grown in the presence of methionine is not due to the presence of an activator of the enzyme in the cell-free extract. The increased enzyme activity is sensitive to the presence of chloramphenicol in the growth media, along with methionine. This suggests that the observed increased aspartate kinase activity in extracts obtained from cells grown in the presence of methionine is due to an additional synthesis of enzyme protein. It also appears that, under conditions in which protein synthesis is completely abolished by chloramphenicol, there is a methionine-dependent formation of a substance which subsequently promotes the synthesis of aspartate kinase upon removal of both chloramphenicol and methionine. From the heat stability data and from the results with respect to sensitivity toward lysine, it seems that the newly synthesized enzyme is a lysine-sensitive aspartate kinase. When added in vitro, methionine does not inhibit or stimulate aspartate kinase activity in the cell-free extract obtained from E. coli B grown either in minimal media or in the presence of methionine. When amino acids are used in combinations in the growth media, they seem to have an effect on the synthesis of aspartate kinase which depends upon the ratio of the intracellular concentration of the amino acids. Increased specific activity of aspartate kinase in the cell-free extract has been observed when the ratio of methionine to lysine and threonine concentrations increases beyond a certain value; decreased specific activity of the enzyme has been observed when this ratio goes below a certain value.