Characterization and functional expression in Escherichia coli of the sodium/proton/glutamate symport proteins of Bacillus stearothermophilus and Bacillus caldotenax

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Abstract
The genes encoding the Na+/H+/L‐glutamate symport proteins of the thermophilic organisms Bacillus stearothermophilus (gltTBs) aid Bacillus caldotenax (gltTBc) were cloned by complementation of Escherichia coli JC5412 for growth on glutamate as sole source of carbon, energy and nitrogen. The nucleotide sequences of the gltTBs and gltTBc genes were determined. In both cases the translated sequences corresponded with proteins of 421 amino acid residues (96.7% amino acid identity between GltTeB and GltTBc)‐ Putative promoter, terminator and ribosome‐binding‐site sequences were found in the flanking regions. These expression signals were functional in E. coli. The hydropathy profiles indicate that the proteins are hydrophobic and could form 12 membrane‐spanning regions. The Na+/H+ coupled L‐glutamate symport proteins GltTBs and GltTBc are homologous to the strictly H+ coupled L‐glutamate transport protern of E. coli K‐12 (overall 57.2% identity). Functional expression of glutamate transport activity was demonstrated by uptake of glutamate in whole cells and membrane vesicles. In accordance with previous observations (de Vrij etal., 1989; Heyne et al., 1991), glutamate uptake was driven by the electrochemical gradients of sodium ions and protons.