Adaptation of Salmonella typhimurium mutants containing uncoupled enzyme IIGlc to glucose-limited conditions.

Uncoupled enzyme IIGlc of the phosphoenolpyruvate (PEP):glucose phosphotransferase system (PTS) in Salmonella typhimurium is able to catalyze glucose transport in the absence of PEP-dependent phosphorylation. As a result of the ptsG mutation, the apparent Km of the system for glucose transport is increased about 1,000-fold (approximately 18 mM) compared with wild-type PTS-mediated glucose transport. An S. typhimurium mutant containing uncoupled enzyme IIGlc as the sole system for glucose uptake was grown in glucose-limited chemostat cultures. Selective pressure during growth in the chemostat resulted in adaptation to the glucose-limiting conditions in two different ways. At first, mutations appeared that led to a decrease in Km value of uncoupled enzyme IIGlc. These results suggested that uncoupled enzyme IIGlc had significant control on the growth rate under glucose-limiting conditions. More efficient glucose uptake enabled a mutant to outgrow its parent and caused a decrease in the steady-state glucose concentration in the chemostat. At very low glucose concentrations (10 microM), mutants arose that contained a constitutively synthesized methyl-beta-galactoside permease. Apparently, further changes in the uncoupled enzyme IIGlc did not lead to a substantial increase in growth rate at very low glucose concentrations.