The glutamate (AR and arginine-inducible amino acid decorboxylases (AR were associated with the of a protein or proteins (extracellular sensors) the growth Rowbury and others 1999). These amino acid decarboxylatior systems appeared to act as inducible pH homeostasis systems, and played. an important role in pHin (internal pH) maintenance inress response of Escherichia coli. enterobacteria (Bearson and others 1997). They elevated pHin by consuming a proton during decarboxylation and exchanging the decarboxylation end product for a new substrate via a bound antiporter (Figure 2). The glutamate and arginine decarboxylase systems in E. coli are considered distinct systems. The GAD system encompassed three genes that are essential components of the glutamate-induced AR 2. Two of the three genes, gadA and gadB, encode highly homologous glutamate decarboxlyase isoforms (a protein that has the same function as an other protein. but which is encoded by a different gene) in E. coli (Smith and others 1992) and the third gene gadC encoded a putative glutamate y amino butyric acid antiporter (Hersh and others 1996; Castanie Cornet and others 1999; Hovde. and others 1999). Arginine decarboxylase, encoded by adiA was responsible for AR3-based acid survival in E. coli (Auger and others 1989; Lin and others 1995).