Dardonville B, Raibaud O
Unité de Génétique Moléculaire, Institut Pasteur, Paris, France.
J Bacteriol. 1990 Apr;172(4):1846-52. doi: 10.1128/jb.172.4.1846-1852.1990.
The expression of the maltose regulon of Escherichia coli is controlled by a transcriptional activator, the product of the malT gene, and is induced by the presence of maltose or maltodextrins in the growth medium. We isolated eight mutants with mutations in malT which lead to constitutive expression of the regulon. The nucleotide sequences of the mutated genes revealed that the eight mutations are clustered in two small regions in the first one-third of the malT gene. Two mutated MalT proteins (corresponding to a mutation in each cluster) were purified and examined for in vitro activation of the MalT-dependent malPp promoter. Whereas wild-type MalT activity was absolutely dependent upon the presence of maltotriose, even at high protein concentrations, both mutated proteins were partially active in the absence of this sugar. Indeed, while the activity of the mutated proteins was still increased by maltotriose at low protein concentrations, the proteins were fully active in the absence of maltotriose at high protein concentrations. Both proteins exhibited a fivefold-higher affinity for maltotriose than the wild-type protein did.
大肠杆菌麦芽糖操纵子的表达受转录激活因子（malT基因的产物）控制，且在生长培养基中存在麦芽糖或麦芽糊精时被诱导。我们分离出了八个malT基因发生突变的突变体，这些突变导致操纵子组成型表达。突变基因的核苷酸序列显示，这八个突变集中在malT基因前三分之一的两个小区域。纯化了两种突变的MalT蛋白（分别对应每个簇中的一个突变），并检测它们对依赖MalT的malPp启动子的体外激活作用。野生型MalT的活性绝对依赖于麦芽三糖的存在，即使在高蛋白浓度下也是如此，而两种突变蛋白在没有这种糖的情况下仍有部分活性。实际上，虽然在低蛋白浓度下麦芽三糖仍会增加突变蛋白的活性，但在高蛋白浓度下，这些蛋白在没有麦芽三糖时也具有完全活性。两种蛋白对麦芽三糖的亲和力都比野生型蛋白高五倍。
