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The Transport of l-Glutamate by Rhizobium leguminosarum Involves a Common Amino Acid Carrier
Abstract
Rhizobium leguminosarum MNF3841 grown on glucose/NH4Cl constitutively transported l-asparagine, l-aspartate, l-glutamate, l-glutamine, glycine, l-leucine, l-methionine and l-phenylalanine. Transport rates were increased 1.5-4-fold by growth on glucose/l-glutamate. Uptake of L-glutamate, l-glutamine, l-asparagine and l-leucine was inhibited to varying extents by a broad range of l-amino acids. Analogues of l-glutamate in which the amino group or α-hydrogen was methylated inhibited L-glutamate transport much less effectively. Also while 2-and 3-amino acids interfered with l-glutamate uptake, d-glutamate did not. Inhibition by 2,4-dinitrophenol, carbonyl cyanide m-chlorophenylhydrazone and cyanide indicated that amino acid transport was active. The ratio of the intracellular to extracellular concentration of l-leucine after 5 min accumulation was 768. Cells loaded with l-[14C]leucine exhibited exchange not only with external l-leucine but also with l-glutamate. The apparent K m for l-glutamate transport was 0·081 μm. Both l-aspartate and l-alanine were competitive inhibitors of l-glutamate uptake with apparent K i values of 0·164 μm and 2·3 μm, respectively. These results suggest that there is an extremely high affinity carrier for l-glutamate that is not only very sensitive to inhibition by l-aspartate but also capable of being inhibited by a broad range of amino acids at an order of magnitude higher concentration.
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© Society for General Microbiology 1985