Wasmuth J J, Umbarger H E
J Bacteriol. 1974 Jan;117(1):29-39. doi: 10.1128/jb.117.1.29-39.1974.
In Escherichia coli, the three branched-chain amino acid activating enzymes appear to be essential for multivalent repression of the isoleucine- and valine-forming enzymes. The results of experiments with a mutant, strain CU18, having an altered threonine deaminase, indicate that free isoleucine and some form of threonine deaminase (the product of the ilvA gene) are also involved in multivalent repression. This strain exhibits abnormally high derepressibility but normal repressibility of its ilv gene products, and its threonine deaminase is inhibited only by high concentrations of isoleucine. In strain CU18, the isoleucine analogue, thiaisoleucine, is incapable of replacing isoleucine in the multivalent repression of the ilv genes, whereas the analogue can fully replace the natural amino acid in repression in other strains examined. The dipeptide, glycyl-leucine, which, like isoleucine, is a heterotropic negative effector of threonine deaminase but is not a substrate for isoleucyl-transfer ribonucleic acid synthetase, can completely prevent the accumulation of threonine deaminase-forming potential during isoleucine starvation in strains with normal threonine deaminases. It can not, however, prevent such accumulation in strain CU18 or in other strains in which threonine deaminase is insensitive to any concentration of isoleucine.
在大肠杆菌中，三种支链氨基酸激活酶似乎对于异亮氨酸和缬氨酸合成酶的多价阻遏至关重要。对具有改变的苏氨酸脱氨酶的突变株CU18进行的实验结果表明，游离异亮氨酸和某种形式的苏氨酸脱氨酶（ilvA基因的产物）也参与多价阻遏。该菌株表现出异常高的去阻遏性，但其ilv基因产物的阻遏性正常，并且其苏氨酸脱氨酶仅受到高浓度异亮氨酸的抑制。在菌株CU18中，异亮氨酸类似物硫代异亮氨酸在ilv基因的多价阻遏中无法替代异亮氨酸，而在其他检测的菌株中，该类似物在阻遏过程中可以完全替代天然氨基酸。二肽甘氨酰 - 亮氨酸，与异亮氨酸一样，是苏氨酸脱氨酶的异向负效应物，但不是异亮氨酰 - 转移核糖核酸合成酶的底物，在具有正常苏氨酸脱氨酶的菌株中，它可以在异亮氨酸饥饿期间完全阻止苏氨酸脱氨酶形成潜力的积累。然而，它不能阻止菌株CU18或其他苏氨酸脱氨酶对任何浓度异亮氨酸均不敏感的菌株中出现这种积累。
