Regulation of Sorbitol Metabolism by Glucose in Clostridium pasteurianum: a Role for Inducer Exclusion

SUMMARY: Cells of Clostridium pasteurianum grown on glucose lacked enzyme systems specific for sorbitol metabolism: the phosphoenolpyruvate (PEP)-dependent phosphotransferase (PTS) and sorbitol-6-phosphate dehydrogenase. These activities were induced by growth on sorbitol, but were partially repressed by glucose. During growth on excess glucose and sorbitol, the sorbitol PTS was absent until the growth rate declined at the onset of stationary phase. Synthesis of at least two PTS proteins, one soluble and one membrane-bound, was repressed in exponentially growing cells. In contrast to the PTS, sorbitol-6-phosphate dehydrogenase was present during the exponential phase.

Glucose and methyl α-glucoside inhibited the activity of the sorbitol PTS in permeabilized cells, and this inhibition was competitive at the level of PEP. This implies that in intact cells sorbitol is excluded by glucose, at least in part due to competition between the glucose and sorbitol phosphotransferases for a common pool of phosphate and energy. Exclusion of sorbitol may be an important factor in the repression of sorbitol metabolism by glucose.