Cation specificity for sugar substrates of the melibiose carrier in Escherichia coli.
A study has been made of the sugar substrate specificities and the cation specificities of the melibiose transport system of Escherichia coli. The following beta-galactosides were found to be transported: lactose, L-arabinose-beta-D-galactoside, D-fructose-beta-D-galactoside, o- and p-nitrophenyl-beta-D-galactosides. These beta-galactosides were cotransported with Na+ but not with H+. The alpha-galactosides raffinose, melibiose and p-nitrophenyl-alpha-galactoside were transported with either H+ or Na+. Of the monosaccharides tested D-galactose could use either Na+ or H+ for cotransport whereas D-fucose, L-arabinose and D-galactosamine could use only Na+. The sugar specificity requirements for H+ cotransport are therefore more exacting than those for Na+ cotransport.