REGULATIONS OF ASPARTOKINASE IN BACILLUS LICHENIFORMIS

The specific activity of aspartokinase (ATP:l-aspartate 4-phosphotransferase, EC 2.7.2.4) decreases rapidly to a 10 to 20-fold lower level at the end of the growth phase in Bacillus licheniformis grown on a minimal glucose-salts medium. Concurrent with this decrease in specific activity is a loss of sensitivity to feedback inhibition by l-lysine and a slower decrease in the concerted inhibition caused by l-lysine and l-threonine. Growth of the cells in the minimal medium plus l-lysine and l-threonine caused the production of aspartokinase with characteristics similar to those of enzyme from cells harvested 4 h after the end of growth. Growth in the presence of either l-lysine or l-threonine did not affect the specific activity of the enzyme but did alter its inhibition sensitivity. The addition of l-lysine to the medium during growth on the minimal medium caused a rapid loss of sensitivity to lysine inhibition, 50% desensitization occurring in 15 min. These latter conditions did not produce an alteration in the specific activity of the enzyme or in the concerted inhibition by lysine and threonine. Despite the marked alteration in feedback-inhibition properties, changes in physical or kinetic properties of purified aspartokinase were not observed. Throughout the purification steps there was no evidence that more than one aspartokinase was present in B. licheniformis cells. It is concluded that this enzyme is unusually plastic and assumes alternate forms depending on the physiology of the cell. © 1969.