Wild-type strains of Escherichia coli K-12 cannot grow in media with 'yaminobutyrate (GABA) as the sole source of carbon or nitrogen. Mutants were isolated which could utilize GABA as the sole source of nitrogen. These mutants were found to have sixto ninefold higher activities of 'y-aminobutyrate-a-835 on July 8, 2020 by guest

Halpern, Yeheskel S. (Hebrew University-Hadassah Medical School, Jerusalem, Israel), and Meir Lupo . Glutamate transport in wild-type and mutant strains of Escherichia coli . J. Bacteriol. 90: 1288–1295. 1965.—Mutants of Escherichia coli able to grow on glutamate as their source of carbon showed glutamate dehydrogenase and glutamate-oxaloacetate transaminase activities similar to those possessed by the parent strain. The mutants took up glutamate at a much faster rate and showed a several-fold greater capacity for concentrating the amino acid than did the corresponding parent strains. Curvilinear double reciprocal plots of velocity of uptake versus glutamate concentration were obtained with the E. coli H strains. A break in the curve of glutamate uptake was observed with the E. coli K-12 strains when incubated in a glucose medium. It is suggested that these findings may be due to allosteric activation of glutamate permease by its substrate.

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The pathway for the utilization of L-arginine, agmatine, L-ornithine, and putrescine as the sole nitrogen source by Escherichia coli K-12 has been elucidated. Mutants impaired in the utilization of one or more of the above compounds were isolated, and their growth on the different compounds as a sole source of nitrogen and the activities of enzymes of the putative pathway were examined. Our results show that L-arginine is first decarboxylated to agmatine, which is hydrolyzed to urea and putrescine. L-Ornithine is decarboxylated to putrescine. Putrescine is transaminated to y-aminobutyraldehyde, which is oxidized to y-aminobutyric acid.-y-Aminobutyric acid is degraded to succinate.

The properties of the glutamate transport system in two glutamate-utilizing mutants of Escherichia coli K-12 were investigated. Growth in the presence of glutamate enhanced the capacity of the bacteria for glutamate uptake. Accumulation of glutamate was found to be an energy-linked highly temperature-dependent process. Nonlinear double reciprocal plots of uptake were obtained in the absence of an exogenous energy source and in the presence of glucose or glycerol. Addition of-y-aminobutyrate, succinate, ketoglutarate, or aspartate accelerated glutamate uptake and brought about "normalization" of its kinetics. Straight-line kinetics of uptake was also observed when succinate served as the source of energy. Under these conditions, aspartate and a-ketoglutarate inhibited glutamate uptake in a noncompetitive fashion, whereas y-aminobutyrate activated the system. A number of other amino acids were found to act as "noncompetitive" inhibitors. D-glutamate and some derivatives of glutamate with an unsubstituted a-carboxylic and ca-amino group inhibited L-glutamate uptake in a strictly competitive fashion. An allosteric permease model, consistent with all of these findings, is proposed.