pH-dependent modulation of cyclic AMP levels and GadW-dependent repression of RpoS affect synthesis of the GadX regulator and Escherichia coli acid resistance

Extreme acid resistance is a remarkable property of virulent and avirulent Escherichia coli. The ability to resist environments in which the pH is 2.5 and below is predicted to contribute significantly to the survival of E. coli during passage through the gastric acid barrier. One acid resistance system imports glutamate from acidic environments and uses it as a proton sink during an intracellular decarboxylation reaction. Transcription of the genes encoding the glutamate decarboxylases and the substrate-product antiporter required for this system is induced under a variety of conditions, including the stationary phase and a low pH. Acid induction during log-phase growth in minimal medium appears to occur through multiple pathways. We recently demonstrated that GadE, the essential activator of the genes, was itself acid induced. In this report we present evidence that there is a regulatory loop involving cross-repression of two AraC-like regulators, GadX and GadW, that can either assist or interfere with GadE activation of the gad decarboxylase and antiporter genes, depending on the culture conditions. Balancing cross-repression appears to be dependent on cAMP and the cAMP regulator protein (CRP). The control loop involves the GadX protein repressing the expression of gadW and the GadW protein repressing or inhibiting RpoS, which is the alternative sigma factor that drives transcription of gadX. CRP and cAMP appear to influence GadX-GadW cross-repression from outside the loop by inhibiting production of RpoS. We found that GadW represses the decarboxylase genes in minimal medium and that growth under acidic conditions lowers the intracellular cAMP levels. These results indicate that CRP and cAMP can mediate pH control over gadX expression and, indirectly, expression of the decarboxylase genes. Mutational or physiological lowering of cAMP levels increases the level of RpoS and thereby increases the production of GadX. Higher GadX levels, in turn, repress gadW and contribute to induction of the gad decarboxylase genes. The presence of multiple pH control pathways governing expression of this acid resistance system is thought to reflect different environmental routes to a low pH.