Isolation and characterization of thiodigalactoside-resistant mutants of the lactose permease which possess an enhanced recognition for maltose

In the current study, lactose permease mutants were isolated which exhibited an enhanced recognition for maltose (an α-glucoside) but a diminished recognition for thiodigalactoside, TDG (a β-galactoside). Maltose/TDG(R) mutants were obtained from four different parental strains encoding either a wild-type permease (pTE18), a mutant lactose permease which recognizes maltose (pB15) or mutant lactose permeases which recognize maltose but are resistant to inhibition by cellobiose (pTG and pBA). A total of 27 independent mutants were isolated: 12 from pTE18, 10 from pB15, 3 from pTG, and 2 from pBA. DNA sequencing of the 27 mutants revealed that the mutants contain single base pair substitutions within the lac Y gene which result in single amino acid substitutions within the lactose permease. All of the mutants obtained from pTE18, pTG, and pBA involved a change of Tyr-236 to histidine, phenylalanine, or asparagine. From pB15, three different types of mutants were obtained: Tyr-236 to histidine, Ile-303 to phenylalanine, or His-322 to asparagine. When assayed for [¹⁴C]maltose transport, the maltose/TDG(R) mutants were seen to transport maltose significantly faster than the wild type. Furthermore, although TDG was shown to inhibit the uptake of maltose in the four parental strains, all of the mutant strains exhibited a dramatic resistance to TDG inhibition. Most of the maltose/TDG(R) mutants were also shown to be very defective in the transport of lactose. However, certain mutants (i.e., Asn-322) exhibited moderate lactose transport activity. Finally, it was observed that all of the mutant strains were unable to facilitate the uphill accumulation of β-methylthiogalactopyranoside. The locations of the amino acid substitutions are discussed with regard to their possible role in sugar recognition.