Altered cation coupling to melibiose transport in mutants of Escherichia coli

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.