The relationship between external glucose concentration and cAMP levels inside Escherichia coli: implications for models of phosphotransferase-mediated regulation of adenylate cyclase

Abstract

The concentration of glucose in the medium influences the regulation of CAMP levels in Escherichia coli. Growth in minimal medium with micromolar glucose results in 8-to 10-fold higher intracellular cAMP concentrations than observed during growth with excess glucose. Current models would suggest that the difference in cAMP levels between glucose-rich and glucose-limited states is due to altered transport flux through the phosphoenolpyruvate :glucose phosphotransferase system (PTS), which 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 external glucose concentration and CAMP levels inside E. coli were investigated in detail, both through direct CAMP assay and indirectly through measurement of expression of CAMP-regulated genes. Responses were followed in batch, dialysis and glucose-limited continuous culture. A sharp rise in intracellular cAMP occurred when the nutrient concentration in minimal medium dropped to approximately 0 3 mM glucose. Likewise, addition of >03 mM glucose, but not c03 mM glucose, sharply reduced the intracellular CAMP level of starving bacteria. There was no striking shift in growth rate or ['4C]glucose assimilation in bacteria passing through the 0 5 to 0.3 mM concentration threshold influencing CAMP levels, suggesting that neither metabolic flux nor transporter saturation influenced the sensing of nutrient levels. The (IIA/IIBC)G'C PTS is 96-97% saturated a t 0.3 mM glucose so these results are not easily reconcilable with current models of cAMP regulation. Aside from the transition in CAMP levels initiated above 0 3 mM, a second shift occurred below 1 pM glucose. Approaching starvation, well below saturation of the PTS, CAMP levels either increased or decreased depending on unknown factors that differ between common E. coli K-12 strains.

Conservation of the metabolomic response to starvation across two divergent microbes

“…An important test of the validity of our approach is its ability to recapitulate literature findings on specific compounds. Among the best established of these is increased cAMP in E. coli upon glucose removal (21,22). Our data capture this rise in cAMP, which peaked during the first hour after glucose removal and fell off thereafter.…”

Identification and characterization of a second, inducible promoter of relA in Escherichia coli

“…5 and Fig. 6), which is activated by cAMP accumulated during glucose starvation (Notley-McRobb et al, 1997). In addition, the ppGpp synthetic activity of RelA is responsible for ppGpp accumulation in the early stage of glucose starvation (Chaloner-Larsson et al, 1978;Murray et al, 2003), and we therefore speculated that the accumulation of ppGpp might be stimulated by CRP activation of relAP2.…”

A Novel Metabolic Cycle Catalyzes Glucose Oxidation and Anaplerosis in Hungry Escherichia coli

“…Absence of Cra-mediated catabolite activation of phosphofructokinase, glyceraldehyde-3-P dehydrogenase, and pyruvate kinase (6, 7) would also be consistent with about 3-fold lower glycolytic fluxes in the slow growing wild-type chemostat culture and in batch cultures of the mutant. Strain-specific degrees of catabolite repression (10) may explain the apparent variations in glyoxylate shunt activity that was observed in batch cultures of an E. coli B strain (37) but was absent in other strains during slow, glucoselimited growth (12,13,38).…”