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.