Niiya S, Yamasaki K, Wilson T H, Tsuchiya T
J Biol Chem. 1982 Aug 10;257(15):8902-6.
The alpha-galactoside transport system of Escherichia coli utilizes either H+ of Na+ as a coupling cation for melibiose transport. Mutants were isolated which showed altered cation recognition for melibiose transport. The transport carrier of the mutants has lost the ability to accept H+ but can utilize either Na/ or Li+ for co-transport with melibiose. Such mutants might be predicted if the melibiose carrier were viewed as a descendant of an evolutionary transition between the H+-substrate co-transport systems of primitive cells and the Na+-substrate systems of more complex organisms. A second mutation was found in these mutants which involves the Na+ (Li+)-H+ exchange carrier. The maximal rate of Li+-H+ exchange in one of the mutants was 11 times higher than the parent and the Km for Li+ by this carrier was one-sixth that of the parent.
大肠杆菌的α-半乳糖苷转运系统利用H⁺或Na⁺作为与蜜二糖转运偶联的阳离子。分离出了一些突变体，它们对蜜二糖转运的阳离子识别发生了改变。这些突变体的转运载体失去了接受H⁺的能力，但可以利用Na⁺或Li⁺与蜜二糖进行共转运。如果将蜜二糖载体视为原始细胞的H⁺-底物共转运系统与更复杂生物体的Na⁺-底物系统之间进化转变的后代，那么可能会预测到这样的突变体。在这些突变体中发现了第二个突变，它涉及Na⁺（Li⁺）-H⁺交换载体。其中一个突变体中Li⁺-H⁺交换的最大速率比亲本高11倍，该载体对Li⁺的Km是亲本的六分之一。
