REGULATION OF THE LACTOSE PHOSPHOTRANSFERASE SYSTEM OF STREPTOCOCCUS-BOVIS BY GLUCOSE - INDEPENDENCE OF INDUCER EXCLUSION AND EXPULSION MECHANISMS

Streptococcus bovis had a diauxic pattern of glucose and lactose utilization, and both of these sugars were transported by the sugar phosphotransferase system (PTS). Lactose catabolism was inducible, and S. bovis used the tagatose pathway to ferment lactose. Since a mutant that was deficient in glucose PTS activity transported lactose as fast as the wild-type, it appeared that S. bovis has separate enzyme Ils for glucose and lactose. The nonmetabolizable glucose analogue 2-deoxyglucose (2-DG) was a noncompetitive inhibitor of methyl beta-D-thiogalactopyranoside (TMC) transport, and cells that were provided with either glucose or 2-DG were unable to transport TMC or lactose. Because the glucose-PTS-deficient mutant could ferment glucose, but could not exclude TMC, it appeared that enzyme IIGlc rather than glucose catabolism per se was the critical feature of inducer exclusion. Cells that had accumulated TMG as TMG 6-phosphate expelled free TMC when glucose was added, but 2-DG was unable to cause TMG expulsion. The glucose-PTS-deficient mutant could still expel TMG in the presence of exogenous glucose. Membrane vesicles also exhibited glucose-dependent TMC exclusion and TMG expulsion. Membrane vesicles that were electroporated with phosphoenolpyruvate (PEP) and HPr retained TMG for more than 3 min, but vesicles that were electroporated with PEP plus HPr and fructose 1,6-diphosphate (FDP) (or glycerate 2-phosphate) lost their ability to retain TMG. Because FDP was able to trigger the ATP-dependent phosphorylation of HPr, it appeared that inducer expulsion was mediated by an FDP-activated protein kinase. This conclusion was further supported by the observation that mutant forms of HPr differed in their ability to faciliate inducer expulsion. S46DHPr, a mutant HPr with aspartate substituted for serine at position 46, promoted TMG expulsion from membrane vesicles in the absence of FDP better than wild-type HPr or S46AHPr, a mutant form with alanine substituted for serine at position 46. Based on these results, it appeared that glucose catabolism was needed for inducer expulsion, but not inducer exclusion.