GLUCOSE-TRANSPORT BY A MUTANT OF STREPTOCOCCUS-MUTANS UNABLE TO ACCUMULATE SUGARS VIA THE PHOSPHOENOLPYRUVATE PHOSPHOTRANSFERASE SYSTEM

Streptococcus mutans transports glucose via the phosphoenolpyruvate (P EP)-dependent sugar phosphotransferase system (PTS). Earlier studies i ndicated that an alternate glucose transport system functions in this organism under conditions of high growth rates, low pH, or excess gluc ose. To identify this system, S. mutans BM71 was transformed with inte gration vector pDC-5 to generate a mutant, DC10, defective in the gene ral PTS protein enzyme I (EI). This mutant expressed a defective EI th at had been truncated by approximately 150 amino acids at the carboxyl terminus as revealed by Western blot (immunoblot) analysis with anti- EI antibody and Southern hybridizations with a fragment;of the wild-ty pe EI gene as a probe. Phosphotransfer assays utilizing P-32-PEP indic ated that DC10 was incapable df phosphorylating HPr and EIIA(Man), ind icating a nonfunctional PTS. This was confirmed by the fact that DC10 was able to ferment glucose but not a variety of other PTS substrates and phosphorylated glucose with ATP and not PEP. Kinetic assays indica ted that the non-PTS system exhibited an apparent K-s of 125 mu M for glucose and a V-max of 0.87 nmol mg (dry weight) of cells(-1) min(-1), Sugar competition experiments with DC10 indicated that the non-PTS tr ansport system had high specificity for glucose since glucose transpor t was not significantly inhibited by a 100-fold molar excess of severa l competing sugar substrates, including 2-deoxyglucose and alpha-methy lglucoside. These results demonstrate that S. mutans possesses a gluco se transport system that can function independently of the PEP PTS.