Characterization and sequencing of an uncoupled lactose carrier mutant of Escherichia coli.

A lactose carrier mutant of Escherichia coli (ML308-22) showed a severe defect in thiomethylgalactoside accumulation but a faster than normal entry of o-nitrophenyl-galactoside. Sequencing of the mutant lacY gene revealed a point mutation resulting in the substitution of glycine-159 by a cysteine residue. The mutant showed an increased sensitivity to sulfhydryl reagents, a property that is consistent with the view that the Cysteine-159 is in or near the sugar recognition site and the energy coupling region of the carrier.