Amino acid substitutions and alteration in cation specificity in the melibiose carrier of Escherichia coli.

We isolated mutants of Escherichia coli which showed Li+-resistant growth on melibiose. The melibiose carrier of the mutants lost the ability to couple to H+, whereas it retained the ability to couple to Na+. The mutated gene, melB, of the mutants was cloned, and the nucleotide sequence was determined. The nucleotide replacements caused the following substitutions of amino acid residues in the melibiose carrier: Pro-142 with Ser, Leu-232 with Phe, or Ala-236 with Thr or Val. These amino acid residues are located in slightly hydrophobic regions of the melibiose carrier. The results provide strong support for the idea that such regions or their vicinities which contain those amino acid residues play an important role in H+ (or Li+) recognition or H+ (or Li+) transport by the melibiose carrier.