Kisumi M, Komatsubara S, Chibata I
J Biochem. 1977 Jul;82(1):95-103. doi: 10.1093/oxfordjournals.jbchem.a131698.
Leaky revertants isolated from isoleucine auxotrophs of Serratia marcescens mutant resistant to alpha-aminobutyric acid were previously reported to accumulate leucine in the medium, due to the absence of both feedback inhibition and repression of leucine biosynthesis. Growth of the revertant was accelerated by pyruvate, D(-)-citramalate, citraconate, and alpha-ketobutyrate, but not by threonine. Extracts of the revertant exhibited high activities of pyruvate-dependent coenzyme A liberation from acetyl-coenzyme A, hydration of citraconate, and conversion of citraconate to alpha-ketobutyrate, but showed no threonine-deaminating activity. In the leucine-accumulating revertants the above three activities were not affected by leucine, but in the wild strain and other revertants accumulating no leucine all or one of these activities was controlled by leucine. A leucine auxotroph isolated from the leucine-accumulating revertant showed isoleucine auxotrophy as well. From these data, it is concluded that, in leucine-accumulating revertants, of S. marcescent, isoleucine, is synthesized from alpha-ketobutyrate via citramalate formed from pyruvate annd acetyl-coenzyme A by leucine biosynthetic enzymes, as a result of desensitization of alpha-isopropylmalate synthetase to feedback inhibition.
先前有报道称，从对α-氨基丁酸具有抗性的粘质沙雷氏菌突变体的异亮氨酸营养缺陷型中分离出的渗漏回复突变体，由于缺乏亮氨酸生物合成的反馈抑制和阻遏作用，会在培养基中积累亮氨酸。丙酮酸、D(-)-柠苹酸、柠康酸和α-酮丁酸可加速回复突变体的生长，但苏氨酸则无此作用。回复突变体的提取物表现出较高的从乙酰辅酶A释放丙酮酸依赖性辅酶A的活性、柠康酸水合活性以及柠康酸转化为α-酮丁酸的活性，但未表现出苏氨酸脱氨基活性。在积累亮氨酸的回复突变体中，上述三种活性不受亮氨酸的影响，但在野生型菌株和其他不积累亮氨酸的回复突变体中，这些活性全部或其中之一受亮氨酸控制。从积累亮氨酸的回复突变体中分离出的亮氨酸营养缺陷型也表现出异亮氨酸营养缺陷型。根据这些数据可以得出结论，在粘质沙雷氏菌积累亮氨酸的回复突变体中，异亮氨酸是由α-酮丁酸经丙酮酸和乙酰辅酶A形成的柠苹酸，通过亮氨酸生物合成酶合成的，这是α-异丙基苹果酸合成酶对反馈抑制脱敏的结果。
