We examined the effects of
sugar concentration in the medium on
sugar uptake and
phosphoenolpyruvate-dependent
sugar phosphotransferase system (PTS) activities in Streptococcus mutants GS-5. Kinetic analyses of
sucrose uptake in cells harvested under conditions of
sucrose excess or
sucrose limitation showed that increased uptake under the latter condition was almost completely due to an increase in the Vmax of the high-affinity PTS. In a series of experiments in which cells growing under conditions of
sucrose or
glucose excess were shifted to a medium lacking
sugar,
starvation resulted in a stimulation of
sugar uptake and a parallel increase in PTS activity. These
starvation-induced increases in PTS-mediated uptake were not affected by the presence of either
chloramphenicol or
rifampin during the
starvation period, indicating that neither
protein nor
RNA synthesis was necessary for the stimulation. In vivo labeling experiments with 32Pi revealed that uptake stimulation during
starvation was accompanied by a loss of
acid-stable
phosphate covalently bound to the
phosphocarrier protein HPr of the PTS. We conclude, therefore, that stimulation of PTS-mediated uptake of
sucrose and
glucose during
sugar limitation in S. mutans GS-5 is at least partially the result of increased activities of preexisting PTS
proteins and that this may be due, at least in part, to dephosphorylation of a previously identified site in S. mutans HPr that can be phosphorylated by an
ATP-dependent
kinase.