Characterization of a functional bacterial homologue of sodium-dependent neurotransmitter transporters

The tnaT gene of Symbiobacterium thermophilum encodes a protein homologous to sodium-dependent neurotransmitter transporters. Expression of the tnaT gene product in Escherichia coli conferred the ability to accumulate tryptophan from the medium and the ability to grow on tryptophan as a sole source of carbon. Transport was Na+-dependent and highly selective. The K-m for tryptophan was similar to145 nM, and tryptophan transport was unchanged in the presence of 100 muM concentrations of other amino acids. Tryptamine and serotonin were weak inhibitors with K-I values of 200 and 440 muM, respectively. By using a T7 promoter-based system, TnaT with an N-terminal His(6) tag was expressed at high levels in the membrane and was purified to near-homogeneity in high yield.