The role of a phosphoenolpyruvate-dependent kinase system in beta-glucoside catabolism in Escherichia coli.

Two distinct mechanisms catalyzing glucoside uptake have been detected in E. coli K12. One of these has high stereospecificity for glucose and alkyl-aor alkyl-,3-glucosides,l and the second, for alkyl-jBand aryl-,3-glucosides.2' 3 Lin and his collaborators4 5 have concluded that aMG is not actively transported, but is trapped intracellularly as the result of a phosphorylation reaction catalyzed by enzymes of the kinase system of Kundig, Ghosh, and Roseman.' Schaefler reported that ethyl-1-thio-f3-glucoside (TEG) was present intracellularly as a phosphorylated derivative when uptake was mediated by the mechanism responsible for aMG accumulation, but as the free glycoside when accumulated by the mechanism eliciting high specificity for aryl-f3-glucosides in a mutant which did not accumulate aMG.2 He concluded that the accumulation of TEG by cells in which both mechanisms for glucoside uptake were operative was mediated primarily by the one which catalyzed uptake of TEG in nonphosphorylated form. This conclusion implies that uptake by the phosphorylating mechanism does not involve equilibration of unphosphorylated substrates across the cell membrane, for such equilibration would allow the actively transported glucoside to exit. In contrast to Schaefler's observations, we find that 13-glucosides are accumulated solely as phosphorylated derivatives and present evidence that the sequence of reactions leading to 83-glucoside catabolism is as