2 UPTAKE SYSTEMS FOR FRUCTOSE IN LACTOCOCCUS-LACTIS SUBSP CREMORIS FD1 PRODUCE GLYCOLYTIC AND GLUCONEOGENIC FRUCTOSE PHOSPHATES AND INDUCE OSCILLATIONS IN GROWTH AND LACTIC-ACID FORMATION

Fructose transport in lactococci is mediated by two phosphotransferase systems (PTS). The constitutive mannose PTS has a broad specificity a nd may be used for uptake of fructose with a fructose saturation const ant (K(Fru)) of 0.89 mM, giving intracellular fructose 6-phosphate. Th e inducible fructose PTS has a very small saturation constant (K(Fru), < 17 muM), and the fructose 1-phosphate produced enters the Embden-Me yerhof-Parnas (EMP) pathway as fructose 1,6-diphosphate. Growth in bat ch cultures of Lactococcus lactis subsp. cremoris FD1 in a yeast extra ct medium with fructose as the only sugar is poor both with respect to specific growth rate and biomass yield, whereas the specific lactic a cid production rate is higher than those in similar fermentations on o ther sugars metabolized via the EMP pathway, e.g., glucose. In fructos e-limited chemostat cultures, the biomass concentration exhibits a str ong correlation with the dilution rate, and starting a continuous cult ure at the end of a batch fermentation leads to large and persistent o scillations in the biomass concentration and specific lactic acid prod uction rate. Two proposed mechanisms underlying this strange growth pa ttern follow. (i) Fructose transported via the fructose PTS cannot be converted into essential biomass precursors (glucose 6-phosphate or fr uctose 6-phosphate), because L. lactis subsp. cremoris FD1 is devoid o f fructose 1,6-diphosphatase activity. (ii) The fructose PTS apparentl y produces a metabolite (presumably fructose 1-phosphate) which exerts catabolite repression of both mannose PTS and lactose PTS. Since the repressed mannose PTS and lactose PTS are shown to have identical maxi mum molar transport rates, the results indicate that it is the general PTS proteins which are repressed.