Figure 5

The glutaminase YbaS and the amino acid antiporter GadC together constitute an acid resistance system. (A) Simultaneous deletion of the Glu decarboxlyase genes gadA and gadB still allowed survival of E. coli under pH 2.5. Shown here are the results of acid resistance assays performed on E. coli of various genetic backgrounds. Deletion of gadA or gadB alone had little impact on the survival of E. coli in the presence of 10 mM Glu or Gln under acidic environment. Deletion of both gadA and gadB resulted in loss of acid resistance in the presence of Glu, but not Gln. (B) Simultaneous deletion of the gadA and gadB genes in E. coli still allowed time-dependent accumulation of ammonium in the medium. By contrast, deletion of the ybaS gene abrogated production of ammonium. The results shown in panels (A, B) reflect the average of three independent experiments, with the SDs indicated. (C) A summary of known ARs in E. coli. Although some features of AR1 are known, the full components remain to be identified. Both AR2 and AR3 contain two components. In AR2, GadC exchanges extracellular Glu and intracellular GABA, whereas GadA/GadB converts Glu to GABA by removing the α-carboxylate group of Glu. In AR3, AdiC uptakes extracellular Arg and expels intracellular agmatine (Agm), whereas AdiA decarboxylates Arg into Agm. The new AR system discovered by this study also contains two components: GadC and YbaS. GadC is an amino acid antiporter with specificity for Gln, Glu and GABA.