Graves L M, Switzer R L
Department of Biochemistry, University of Illinois, Urbana 61801.
J Biol Chem. 1990 Sep 5;265(25):14947-55.
Aspartokinase II from Bacillus subtilis was shown by immunochemical methods to be regulated by degradation in response to starvation of cells for various nutrients. Ammonium starvation induced the fastest aspartokinase II decline (t1/2 = 65 min), followed by amino acid starvation (t1/2 = 80 min) and glucose limitation (t1/2 = 120 min). Loss of enzyme activity was closely correlated with the disappearance of the alpha subunit; degradation of the beta subunit was somewhat delayed or slower under some conditions. Pulse-chase experiments demonstrated that aspartokinase II was stable during exponential growth; the synthesis of the enzyme rapidly declined in response to nutrient exhaustion. The degradation of aspartokinase II was interrupted by inhibitors of energy production and protein synthesis but was not changed in a mutant lacking a major intracellular protease. Mutants lacking a normal stringent response displayed only a slight decrease in the rate of aspartokinase II degradation, even though aspartate transcarbamylase was degraded more slowly in the same mutant cells. These results indicate that although energy-dependent degradation of biosynthetic enzymes is a general phenomenon in nutrient-starved B. subtilis cells, the degradation of specific enzymes probably involves different pathways.
免疫化学方法显示，枯草芽孢杆菌中的天冬氨酸激酶II受细胞对各种营养物质饥饿反应的降解调控。铵饥饿诱导天冬氨酸激酶II下降最快（半衰期 = 65分钟），其次是氨基酸饥饿（半衰期 = 80分钟）和葡萄糖限制（半衰期 = 120分钟）。酶活性的丧失与α亚基的消失密切相关；在某些条件下，β亚基的降解有所延迟或较慢。脉冲追踪实验表明，天冬氨酸激酶II在指数生长期是稳定的；随着营养物质耗尽，该酶的合成迅速下降。天冬氨酸激酶II的降解被能量产生和蛋白质合成的抑制剂打断，但在缺乏主要细胞内蛋白酶的突变体中没有变化。缺乏正常严谨反应的突变体中天冬氨酸激酶II的降解速率仅略有下降，尽管在相同的突变体细胞中天冬氨酸转氨甲酰酶降解得更慢。这些结果表明，尽管生物合成酶的能量依赖性降解在营养缺乏的枯草芽孢杆菌细胞中是一种普遍现象，但特定酶的降解可能涉及不同的途径。
