Cellular levels, excretion, and synthesis rates of cyclic AMP in Escherichia coli grown in continuous culture

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Abstract
Changes in dilution rate did not elicit large and systematic changes in cellular cAMP levels in E. coli grown in a chemostat under C or phosphate limitation. However, the technical difficulties of measuring low levels of cellular cAMP in the presence of a large background of extracellular cAMP precluded firm conclusions on this point. The net rate of cAMP synthesis increased exponentially with increasing dilution rate through either the entire range of dilution rates examined (phosphate limitation) or a substantial part of the range (lactose and glucose limitations). It is probable that growth rate regulates the synthesis of adenylate cyclase. The maximum rate of net cAMP synthesis was greater under lactose than under glucose limitation, which is consistent with the notion that the uptake of phosphotransferase sugars is more inhibitory to adenylate cyclase than the uptake of other C substrates. Phosphate-limited cultures exhibited the lowest rate of net cAMP synthesis, which could be due to the role of phosphorylated metabolites in the regulation of adenylate cyclase activity. Under all growth conditions examined, > 99.9% of the cAMP synthesized was found in the culture medium. The function of this excretion, which consumed up to 9% of the total energy available to the cell and which evidently resulted from elaborate regulatory mechanisms, remains entirely unknown.