Galactose transport systems in Streptococcus lactis

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Abstract
Galactose-grown cells of S. lactis ML3 have the capacity to transport the growth sugar by 2 separate systems: the phosphoenolpyruvate-dependent phosphotransferase system and an ATP-energized permease system. Proton-conducting uncouplers (tetrachlorosalicylanilide and carbonyl cyanide-m-chlorophenyl hydrazone) inhibited galactose uptake by the permease system, but had no effect on phosphotransferase activity. Inhibition and efflux experiments conducted using .beta.-galctoside analogs showed that the galactose permease had a high affinity for galactose, methyl-.beta.-D-thiogalactopyranoside and methyl-.beta.-D-galactopyranoside, but possessed little or no affinity for glucose and lactose. The spatial configurations of hydroxyl groups at C-2, C-4 and C-6 were structurally important in facilitating interaction between the carrier and the sugar analog. Iodoacetate had no inhibitory effect on accumulation of galactose, methyl-.beta.-D-thiogalactopyranoside, or lactose via the phosphotransferase system. After exposure of the cells to p-chloromercuribenzoate, phosphoenolpyruvate-dependent uptake of lactose and methyl-.beta.-D-thiogalactopyranoside were reduced by 75 and 100%, respectively, whereas galactose phosphotransferase activity remained unchanged. The independent kinetic analysis of each transport system was achieved by the selective generation of the appropriate energy source (ATP or phosphoenolpyruvate) in vivo. The maximum rates of galactose transport by the 2 systems were similar, but the permease system exhibited a 10-fold greater affinity for sugar than did the phosphotransferase system.