Characterization of lactose carrier mutants which transport maltose

Brooker and Wilson (Brooker, R. J., and Wilson, T. H. (1985) Proc. Natl. Acad. Sci. U. S. A. 82, 3959-3963) previously isolated lactose carrier mutants which were able to transport maltose. All of the mutants were found to be single amino acid substitutions for alanine 177 or for tyrosine 236. In the present study, we have examined the ability of these mutants to transport maltose, lactose, o-nitrophenyl-beta-D-galactopyranoside, methyl-beta-D-thiogalactopyranoside, and H+. Both the position 177 and 236 mutants have enhanced rates of maltose transport and exhibit apparent Km values for maltose which are substantially less than that of the wild-type strain. The position 177 mutants transport lactose and other galactosides at a normal rate and with normal affinity during downhill transport and show counterflow transport rates which are faster than the wild-type strain. Interestingly, these mutants are markedly defective in accumulating substrates against a concentration gradient, yet retain a normal H+:galactoside stoichiometry. The position 236 mutants appear to be defective in the downhill, uphill, and counterflow transport of galactosides but exhibit a normal H+:galactoside stoichiometry.