SORBITOL TRANSPORT BY STREPTOCOCCUS-SANGUIS-160

Sorbitol metabolism was examined with a sorbitol-fermenting strain (160) of Streptococcus sanguis isolated from the dental plaque of a subject using sorbitol-containing chewing-gum for 4 years. S. sanguis 160 was grown in continuous culture (pH, 7.0; dilution rate, 0.1 h-1) with glucose, sorbitol and nitrogen (sorbitol-excess) limitations. Cells grown with a glucose limitation exhibited low, but detectable, uptake of [C-14]-sorbitol and transition to medium limiting in sorbitol resulted in a 5-fold increase in sorbitol uptake. Kinetic data revealed that both glucose and sorbitol-limited cells possessed 2 transport systems for sorbitol (K(s) = 3.3-6.7 and 36-64-mu-M), but continued growth of the organism on limiting sorbitol resulted in the loss of the high-affinity system. Decryptified, sorbitol-limited cells phosphorylated sorbitol in the presence of phosphoenolypyruvate (PEP), but not with ATP, indicating sorbitol transport solely via the PEP phosphotransferase (PTS) system. PEP-dependent activity in glucose-limited and sorbitol-excess cells was 6- and 4-fold lower than that of the sorbitol-limited cells. Uptake of [C-14]-sorbitol and activity for Ell for sorbitol [Ell(sor0] of the PTS in cells in transition from a glucose to sorbitol limitation confirmed the induction of the sorbitol-PTS and the repression of the glucose-PTS in the presence of sorbitol. Cells grown with an excess of sorbitol exhibited very low Ell(sor) activity. A crossover experiment with membranes and soluble fractions from glucose-, sorbitol- and nitrogen-limited cells of S. sanguis 160 demonstrated the induction of a soluble PTS component in sorbitol-limited cells essential for sorbitol transport via the PTS. In addition, this factor, tentatively identified as lll(sor), was shown to stimulate 7-fold sorbitol phosphorylation by glucose-limited membranes, indicating the possibility that sorbitol can be transported via Ell(glc) in the presence of the sorbitol-specific, soluble component. This observation was confirmed by demonstrating that glucose and sorbitol compete for 1 of the 2 sorbitol transport systems in the organism. Our results indicate that transition of S. sanguis 160 from glucose to sorbitol-limited growth results in sorbitol transport initially via Ell(glc) and a sorbitol-specific soluble factor (lll(sor)) and this is followed by the relatively slow induction of Ell(sor) and the concomitant repression of Ell(glc).