Evidence that the asparagine, 322 mutant of the lactose permease transports protons and lactose with a normal stoichiometry and accumulates lactose against a concentration gradient

The single asparagine 322 mutant of the lactose permease was made by constructing a hybrid plasmid which contained the amino-terminal coding sequence from the wild-type permease gene and the carboxyl-terminal coding sequence from a previously characterized double mutant permease which contained an asparagine residue at position 322. Since histidine at position 322 has been postulated to be critically involved with H⁺ transport and the active accumulation of sugars, the ability of the Asn-322 mutant to couple H⁺ and sugar transport was carefully examined. Measurements of proton/lactose stoichiometries gave very similar values for the wild-type (0.78) and the Asn322 strain (0.82). Moreover, the Asn-322 mutant was able to effectively accumulate lactose against a concentration gradient although the levels of accumulation in the Asn-322 mutant (∼5-7-fold) were significantly less than that of the wild-type strain (∼30-40-fold). Over-all, these results are inconsistent with the notion that an ionizable histidine residue at position 322 is obligatorily required for H⁺ transport or the active accumulation of galactosides against a concentration gradient. The ability of the Asn-322 mutant to recognize a variety of sugars was compared with wild-type, Val177, and Val-177/Asn-322 strains. The Asn-322 mutant exhibited an ability to recognize and transport maltose (an α-glucoside) which was significantly better than the wild-type strain but not as good as either the single Val-177 mutant or the double Val-177/Asn-322 mutant. Both the Asn-322 and the Val-177/Asn-322 strain showed a relatively poor recognition for α-galactosides (i.e. melibiose), β-galactosides (lactose and thiodigalactoside), and β-glucosides (cellobiose). In contrast, the single Val-177 strain exhibited a normal recognition for these sugars.