Coarse and fine control of citrate synthase from Bacillus subtilis

Abstract

The synthesis of citrate synthase (citrate oxaloacetate-lyase (CoA-acetylating), EC 4.1.3.7) from Bacillus subtilis was found to respond to a dual control: A specific feedback repression caused by glutamate (or α-ketoglutarate), the biosynthetic end product of the first half of the tricarboxylic acid cycle, and a more general catabolite repression resulting from the accumulation of catabolic intermediates when cells were grown on glucose, a readily utilizable carbon source. The synthesis of aconitate hydratase (citrate (isocitrate) hydro-lyase, EC 4.2.1.3, formerly known as aconitase) from this organism is sensitive to similar metabolic signals 1. Because the kinetic patterns of repression and depression of the two enzymes are similar, and since a plot of the specific activities of aconitate hydratase vs. those of citrate synthase from B. subtilis cultures grown under differing nutritional conditions results in a straight line, the suggestion is made that citrate synthase and aconitate hydratase may be subject to coordinate regulation. Kinetic studies on a partially purified enzyme preparation show ATP to be an inhibitor of citrate synthase. ATP inhibition is competitive with CoASAc but not oxaloacetate. The results of this investigation lead to the conclusion that while the synthesis of citrate synthase is sensitive to both anabolic and catabolic signals, once the enzyme is formed, its activity is regulated primarily by catabolic effectors.