Lactose metabolism in Streptococcus lactis: studies with a mutant lacking glucokinase and mannose-phosphotransferase activities.
A mutant of Streptococcus lactis 133 has been isolated that lacks both glucokinase and phosphoenolpyruvate-dependent mannose-phosphotransferase (mannose-PTS) activities. The double mutant S. lactis 133 mannose-PTSd GK- is unable to utilize either exogenously supplied or intracellularly generated glucose for growth. Fluorographic analyses of metabolites formed during the metabolism of [14C]lactose labeled specifically in the glucose or galactosyl moiety established that the cells were unable to phosphorylate intracellular glucose. However, cells of S. lactis 133 mannose-PTSd GK- readily metabolized intracellular glucose 6-phosphate, and the growth rates and cell yield of the mutant and parental strains on sucrose were the same. During growth on lactose, S. lactis 133 mannose-PTSd GK- fermented only the galactose moiety of the disaccharide, and 1 mol of glucose was generated per mol of lactose consumed. For an equivalent concentration of lactose, the cell yield of the mutant was 50% that of the wild type. The specific rate of lactose utilization by growing cells of S. lactis 133 mannose-PTSd GK- was ca. 50% greater than that of the wild type, but the cell doubling times were 70 and 47 min, respectively. High-resolution 31P nuclear magnetic resonance studies of lactose transport by starved cells of S. lactis 133 and S. lactis 133 mannose-PTSd GK- showed that the latter cells contained elevated lactose-PTS activity. Throughout exponential growth on lactose, the mutant maintained an intracellular steady-state glucose concentration of 100 mM. We conclude from our data that phosphorylation of glucose by S. lactis 133 can be mediated by only two mechanisms: (i) via ATP-dependent glucokinase, and (ii) by the phosphoenolpyruvate-dependent mannose-PTS system.
D001692:Biological Transport; D002250:Carbon Radioisotopes; D005285:Fermentation; D005941:Glucokinase / Q000172:deficiency*; D005947:Glucose / Q000378:metabolism; D013294:Lactococcus lactis / Q000254:growth & development / Q000378:metabolism*; D007785:Lactose / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D009154:Mutation; D010731:Phosphoenolpyruvate Sugar Phosphotransferase System / Q000172:deficiency*; D010766:Phosphorylation
D002250:Carbon Radioisotopes; D010731:Phosphoenolpyruvate Sugar Phosphotransferase System; C037992:phosphoenolpyruvate-mannose phosphotransferase; D005941:Glucokinase; D005947:Glucose; D007785:Lactose