Galactose transport systems in Streptococcus lactis

Abstract

Galactose-grown cells of Streptococcus lactis ML3 have the capacity to transport the growth sugar by two separate systems: (i) the phosphoenolpyruvate-dependent phosphotransferase system and (ii) an adenosine 5'-triphosphate-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-galactoside 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. However, 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 (adenosine 5'-triphosphate or phosphoenolpyruvate) in vivo. The maximum rates of galactose transport by the two systems were similar, but the permease system exhibited a 10-fold greater affinity for sugar than did the phosphotransferase system.

Genetics of lactose utilization in lactic acid bacteria

“…Strains of Lactococcus lactis are known to transport and catabolize galactose via two different systems [37,60]. When transported via a galactose permease the internalized galactose is metabolized via the Leloir pathway (see below).…”

“…In the latter case tagatose 6-phosphate is formed, which explains the induction of the lac operon during growth on galactose [35]. Thc galactose permease system is highly specific for galactose and has little or no affinity for lactose [60,61]. In addition, the affinity of this permease for galactose is ten-fold higher than that of the galactose PTS, which has a K m of 1 mM [60].…”

Energy transduction in lactic acid bacteria

“…The system is highly specific for galactose, TMG and other galactose analogs, but exhibits poor affinity for lactose. A similar system has been described for L. lactis ML3 [26]. Lactose transporters driven by the electrochemical proton gradient have been described for S. thermophilu,~ and L. hulgaricus [27].…”

Regulation of sugar transport and metabolism in lactic acid bacteria

“…3,A and B) results from the inactivation of pyruvate kinase by: (a) depletion of positive effectors of the allosteric enzyme (e.g., FDP, [29][30][31][32]), and (b) marked increase in intracellular Pi concentration, a potent inhibitor of PK in vitro [29,31]. The presence of the PEP-potential in starved organisms is of experimental and physiological importance because: (i) it permits the characterization of PTS functions and isolation of PTS products in intact cells [20,32,33]; (ii) the slow utilization of PEP by PK may provide the necessary maintenance energy for the organism during starvation [20,26]; (iii) starved cells are 'primed' for the immediate transport of lactose and other PTS sugars when these compounds are once more available [ Fig. 3 C]; and (iv) the PEP-potential permits preloading of cells with non-metabolizable sugar (phosphate) analogs for investigations concerned with intracellular dephosphorylation and expulsion of sugars [22,34].…”

“…TMG is often used to monitor lactose-PTS activity [12,20,33,42], but it is also known that glucose can prevent accumulation of the /3galactoside by many lactic acid bacteria including S. lactis [34,43], S. pyogenes [22,44], S. faecalis [24] and L. casei [23]. This exclusion of TMG can be attributed to the preferential utilization of HPr-(his)-P during translocation of glucose by the mannose-PTS [43].…”