Biochemical and genetic characterization of L-glutamate transport and utilization in Salmonella typhimurium LT-2 mutants.

Two systems for L-glutamate transport were found in Salmonella typhimurium LT-2 GltU+ (glutamate utilization) mutants. The first one is similar to the glt system previously described in Escherichia coli; by transductional analysis the structural gene, gltS, coding for the transport protein was located at minute 80 of the chromosome as part of the operon gltC-gltS, and its regulator, the gltR gene, near minute 90; the gltS gene product transports both L-glutamate and L-aspartate, is sodium independent, and is beta-hydroxyaspartate sensitive. The second transport system, whose structural gene was called gltF and is located at minute 0, was L-glutamate specific, sodium independent, and alpha-methylglutamate sensitive. Two aspartase activities occurred in S. typhimurium LT-2: the first one was present only in the GltU+ mutants, had a pH 6.4 optimum, was essential for both L-glutamate and L-aspartate metabolism, and mapped at minute 94, close to the ampC gene. The second one had a pH 7.2 optimum, could be induced by several amino acids, and thus may have a general role in nitrogen metabolism.