Melibiose transport of Escherichia coli

Transport of [3H]melibiose, prepared from [3H]raffinose, was investigated in Escherichia coli. Na+ stimulated the transport of melibiose via the melibiose system, whereas Li+ inhibited it. Kinetic parameters of melibiose transport were determined. The Kt values were 0.57 mM in the absence of Na+ or Li+, 0.27 mM in the presence of 10 mM NaCl, and 0.29 mM in the presence of 10 mM LiCl. The Vmax values were 40 and 46 nmol/min per mg of protein in the absence and in the presence of NaCl and 18 nmol/min per mg of protein in the presence of LiCl. Melibiose transport via the melibiose system was temperature sensitive in a wild-type strain of Escherichia coli and was not inhibited by lactose. On the other hand, melibiose uptake via the lactose system was not temperature sensitive, was inhibited by lactose, and was not affected by Na+ and Li+. Methyl-beta-D-thiogalactoside, a substrate for both systems, inhibited the transport of melibiose via both systems.