Galactoside-dependent proton transport by mutants of the Escherichia coli lactose carrier. Replacement of histidine 322 by tyrosine or phenylalanine.

Mutations have been introduced into the Escherichia coli lac Y gene by oligonucleotide-directed mutagenesis such that the lactose carrier contains either tyrosine or phenylalanine in place of histidine 322. These mutants did not carry out active accumulation of lactose, melibiose, or methyl-beta-D-galactopyranoside, but facilitated diffusion was still catalyzed. Galactoside-dependent H+ transport, measured with the pH electrode, was retained in both mutants. We conclude that although histidine 322 is important for energy transduction, neither an electronegative atom nor a dissociable proton is essential for proton cotransport with lactose or melibiose.