THE RELATIONSHIP BETWEEN EXTERNAL GLUCOSE-CONCENTRATION AND CAMP LEVELS INSIDE ESCHERICHIA-COLI - IMPLICATIONS FOR MODELS OF PHOSPHOTRANSFERASE-MEDIATED REGULATION OF ADENYLATE-CYCLASE

The concentration of glucose in the medium influences the regulation o f cAMP levels in Escherichia coli. Growth in minimal medium with micro molar glucose results in 8- to 10-fold higher intracellular cAMP conce ntrations than observed during growth with excess glucose. Current mod els would suggest that the difference in cAMP levels between glucose-r ich and glucose-limited states is due to altered transport flux throug h the phosphoenolpyruvate :glucose phosphotransferase system (PTS), wh ich in turn controls adenylate cyclase, A consequence of this model is that cAMP levels should be inversely related to the saturation of the PTS transporter. To test this hypothesis, the relationship between ex ternal glucose concentration and cAMP levels inside E. coli were inves tigated in detail, both through direct cAMP assay and indirectly throu gh measurement of expression of cAMP-regulated genes, Responses were f ollowed in batch, dialysis and glucose-limited continuous culture. A s harp rise in intracellular cAMP occurred when the nutrient concentrati on in minimal medium dropped to approximately 0.3 mM glucose. Likewise , addition of >0.3 mM glucose, but not <0.3 mM glucose, sharply reduce d the intracellular cAMP level of starving bacteria. There was no stri king shift in growth rate or [C-14]glucose assimilation in bacteria pa ssing through the 0.5 to 0.3 mM concentration threshold influencing cA MP levels, suggesting that neither metabolic flux nor transporter satu ration influenced the sensing of nutrient levels. The (IIA/IIBC)(Glc) PTS is 96-97% saturated at 0.3 mM glucose so these results are not eas ily reconcilable with current models of cAMP regulation. Aside from th e transition in cAMP levels initiated above 0.3 mM, a second shift occ urred below 1 mu M glucose. Approaching starvation, well below saturat ion of the PTS, cAMP levels either increased or decreased depending on unknown factors that differ between common E. coli K-12 strains.