The effect of different cellular free-energy states on the uptake of methyl G-D-glucopyranoSide, an analogue of glucose, by the Escherichia coli phosphoennlpyruvate :carbohydrate phosphotransferase system was investigated. The intracellular IATP]/[ADP] ratio was varied by changing the expression of the atp operon, which codes for the Hi-ATPase, or by adding an uncoupler of oxidative phosphorylation or an inhibitor of respiration. Corresponding initial phosphotransferase uptake rates were determined using an improved uptake assay that works with growing cells in steady state. Escheuichia coli can use any one of a large variety of compounds as the sole carbon source for growth. In the presence of a mixture of compounds, this bacterium will often show a preference for certain carbohydrates, such as glucose, and repress the expression of uptake systems for other compounds (inducer exclusion [I I). During growth on a different carbon source such as succinate, however, E. coli maintains a relatively high capacity for glucosc transport [I 1. The bacterial phosphoenolpyruvate : carbohydratc phosphotransferase system catalyses the uptake and concomitant phosphorylation of a variety of carbohydratcs, for example glucose 11 1. The phosphotransferase system is a group-transfer pathway consisting of two soluble phosphoproteins (Enzyme 1 and HPr) and, for each carbohydrate, a specil'ic protein or protein complex, the carbohydrate permease (for E. r d i in the case of glucose, soluble Enzyme IIA"" and membrane-bound Enzyme IICB""). During the uptake of a glucose moleculc, for example, a phosphoryl group from phosphoenol- Ahhvevi~ition.~. Phosphotransferase system, phosphoenolpyruvate :carbohydrate phosphotransferase system; HPr, histidine-containing protein; Enzyme I, phosphoenolpyruvate-protein phosphotransferase; Enryme 11, prolein-N"-phosphohistidine-sugar phosphotransferase; IIA"" and IICB"", EnLyines 11 of thc phosphoenolpyruvatc: glucose pliosphotransicrasc system; MeGlc, methyl cr-D-glucopyranoside; IPTG, isopropyl tliio-Pi,-galactoside. Enzsmc,.s. EnLyme 1 of the phosphotransferase system, phosphoenolpyruvate-protein phosphotransferase (EC 2.7.3.9); Enzyme I1 of the phosphotransferase system, protein-N"-phosphohistidine-sugar phosphotransferasc (EC 2.7.1.69).pyruvate is transferred sequentially along Enzyme 1, HPr, Enzymes IIAG1" and IICB"", to the glucose molecule.Metabolic activity and changes in the cellular energy state have been shown to affect phosphotransferase activity in a variety of ways: the addition of substrates to activate respiration reduced the steady-state accumulation of methyl am-glucopyranoside (MeGlc) 12, 31, a non-metabolisable analogue of the glucose phosphotransferase system; the inhibition of MeGlc uptake by respiration was more severe in atp mutants, which cannot use the protonmotive force to synthesise ATP 141 ; the inhibition by oxidisable substrates could be reversed by the addition of uncouplers of oxidative phosphorylation such as carbonyl cyanide rnchlorophenylhydrazone [2) or 2,4-dinitr...