Biochemical and genetic characterization of a mutant of Salmonella typhimurium defective in a locus for glutamate dehydrogenase activity

By two consecutive treatments with N-methyl-N′-nitro-N-nitrosoguanidine we obtained mutant SM151 of Salmonella typhimurium which differs from the parental LT2 strain in: a) is able to use l-glutamate as carbon source (first mutation), and b) requires that amino acid for growth (second mutation). It was found that the requirement of mutant SM151 for glutamate was due to a very low activity of glutamate dehydrogenase. Both glutamate-oxaloacetate transaminase and aspartase activities were present at normal levels. Glutamate dehydrogenase activity was strongly repressed by glutamate; aspartase activity was under severe catabolite (glucose) repression, while glutamate-oxaloacetate transaminase was partially repressed by glutamate. By conjugation and transduction the locus gdh, responsible for the low activity of the glutamate dehydrogenase of mutant SM151, was located at about minute 128 of the bacterial chromosome and found to be linked to the argC, argF, and metB loci.