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 utili zation, and both of these sugars were transported by the sugar phospho transferase system (PTS). Lactose catabolism was inducible, and S. bov is used the tagatose pathway to ferment lactose. Since a mutant that w as deficient in glucose PTS activity transported lactose as fast as th e wild-type, it appeared that S. bovis has separate enzyme Ils for glu cose and lactose. The nonmetabolizable glucose analogue 2-deoxyglucose (2-DG) was a noncompetitive inhibitor of methyl beta-D-thiogalactopyr anoside (TMC) transport, and cells that were provided with either gluc ose or 2-DG were unable to transport TMC or lactose. Because the gluco se-PTS-deficient mutant could ferment glucose, but could not exclude T MC, it appeared that enzyme IIGlc rather than glucose catabolism per s e was the critical feature of inducer exclusion. Cells that had accumu lated 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. Me mbrane vesicles also exhibited glucose-dependent TMC exclusion and TMG expulsion. Membrane vesicles that were electroporated with phosphoeno lpyruvate (PEP) and HPr retained TMG for more than 3 min, but vesicles that were electroporated with PEP plus HPr and fructose 1,6-diphospha te (FDP) (or glycerate 2-phosphate) lost their ability to retain TMG. Because FDP was able to trigger the ATP-dependent phosphorylation of H Pr, it appeared that inducer expulsion was mediated by an FDP-activate d protein kinase. This conclusion was further supported by the observa tion that mutant forms of HPr differed in their ability to faciliate i nducer 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 f orm with alanine substituted for serine at position 46. Based on these results, it appeared that glucose catabolism was needed for inducer e xpulsion, but not inducer exclusion.