Glutamate-based acid resistance in Escherichia coli: biochemical and regulatory aspects

Escherichia coli has the ability to resist severe acid stress, as that encountered during transit through the host stomach, and this is instrumental to host gut colonization. The glutamate-based acid resistance (AR) system plays a major role in the protection of the cell from the deleterious effects of a high-proton-concentration environment. Structural genes of this system are gadA, gadB and gadC, which encode two glutamate decarboxylase isoforms and a glutamate/γ-aminobutyrate (GABA) antiporter, respectively. Glutamate decarboxylation leads to both proton consumption and production of GABA, a neutral compound exported via the GadC antiporter. Even though the gadA and gadBC genes are 2,1 Mb apart, their transcription is under the control of the same regulators: GadE, GadX, GadW, H-NS and RpoS. These regulators also affect the expression of 12 genes located in the acid fitness island (AFI). We have identified the GadX (GadW) binding site, a 42 bp sequence, in the regulatory regions of gadA, gadBC, slp, hdeAB, gadE and gadY. All are AFI genes, but gadBC. In my talk I will show the most recent results from in vivo and in vitro analyses aimed at fully characterizing the GadX regulon. In addition I will show biochemical data on the decarboxylase’s intracellular activation/inactivation process and on additional effectors involved in GABA export.