Altered sugar selection and transport conferred by spontaneous point and deletion mutations in the lactose carrier of Escherichia coli

Spontaneous mutants harboring the lacY gene on an F'-factor were isolated. Those mutants that failed to grow on 5 mM lactose minimal media plates were chosen for further study. The mutants showed striking mutations in the lactose carrier as well as in sugar selection properties during transport assays. DNA sequencing of the lacY gene of the mutants revealed the following mutations: M-1-I, R-144-W, G-370-C and a deletion of residues 387-392, located in helix 12 of the carrier. Transport studies indicated that ONPG transport ranged between 8 and 25% of normal for the M-1-I, G-370-C and D387-392 mutants and 51% of normal for the R-144-W mutant. The downhill transport of lactose was 2-fold greater than for melibiose in cells harboring the M-1-I mutation and 3-fold higher for cells with the G-370-C mutation. On the other hand, cells with the D387-392-deletion mutation showed no lactose downhill transport, but 47% melibiose transport. Accumulation of TMG, a lactose analog, was 3-fold higher than the accumulation of melibiose in cells with the G-370-C mutation. On the other hand, in cells with the D387-392 mutation, TMG accumulation was completely defective, whereas melibiose accumulation was 50-fold higher than that of TMG, indicating that one or more of these residues in helix 12 of the carrier play a role in the active transport of b-galactoside, but not a-galactoside sugars. Initial lactose downhill transport rates were too unreliable to obtain trustworthy kinetic data. TMG and melibiose accumulation activities were present, but severely reduced in the mutant containing the R144W mutation, confirming that Arg-144 is important for active transport. All transport data were normalized for expression levels. The results indicate that the affected residues play a role in dictating sugar specificity and transport in the lactose carrier. The results here are novel in that they represent mutations in unique locations along the lactose carrier protein. For example, the M-1-I mutation was located at the N-terminal cytoplasmic tail of the carrier. Furthermore, G-370-C was located in the periplasmic loop between helices 11 and 12, suggesting a role for residues in this loop in mediating sugar selection.