Regulation of gamma-aminobutyric acid degradation in Escherichia coli by nitrogen metabolism enzymes

The possible role of glutamate dehydrogenase, glutamate synthase, and glutamine synthetase in the regulation of enzyme formation in the gamma-aminobutyric acid (GABA) catabolic pathway of Escherichia coli K-12 was investigated. Evidence is presented indicating that glutamine synthetase acts as a positive regulator in the E. coli GABA control system. Mutations impairing glutamate synthase activity prevent the depression of the enzymes of the GABA pathway in ammonia-limited glucose media. However, mutations resulting in constitutive synthesis of glutamine synthetase (GlnC) restore the ability of the glutamate synthase-less mutants to grow in glucose-GABA media and result in depressed synthesis of the GABA enzymes. It is suggested that the loss of glutamate synthesis activity affects the GABA control system indirectly by lowering glutamine synthetase levels.