Salmonella typhimurium Mutants with Altered Glutamate Dehydrogenase and Glutamate Synthase Activities

Although glutamate is a key compound in nitrogen metabolism, little is known about the function or regulation of its two biosynthetic enzymes, glutamate dehydrogenase and glutamate synthase. To begin the characterization of glutamate formation in Salmonella typhimurium, we isolated mutants having altered glutamate dehydrogenase and glutamate synthase activities. Mutants which failed to grow on media with glucose as the carbon source and less than 1 mM (NH₄)₂SO₄ as the nitrogen source (Asm⁻) had about one-fourth the normal glutamate synthase activity and one-half the glutamine synthetase activity. The asm mutations also prevented growth with alanine, arginine, or proline as nitrogen sources and conferred resistance to methionine sulfoximine. When a mutation (gdh-51) causing the loss of glutamate dehydrogenase activity was transferred into a strain with an asm-102 mutation, the resulting asm-102 gdh-51 mutant had a partial requirement for glutamate. A strain isolated as a complete glutamate auxotroph had a third mutation, in addition to the asm-102 gdh-51 lesions, that further decreased the glutamate synthase activities to 1/20 the normal level. Both the asm-102 and gdh-51 mutations were located on the S. typhimurium linkage map at sites distinct from those found for mutations causing similar phenotypes in Klebsiella aerogenes and Escherichia coli.