9.A.72. The Putative Nucleoside Efflux Transporter (NET) Family
The Salmonella enterica tsx gene encodes a nucleoside-specific outer membrane channel. The Tsx porin is essential for the prototrophic growth of S. enterica sv. Typhi in the absence of nucleosides. RT-PCR analysis showed that the tsx gene is cotranscribed with an open reading frame unique to S. enterica, impX (STY0450), which encodes an inner membrane protein 108 amino acids in length, with two transmembrane α-helices. Transcription of both genes is induced in the presence of adenosine. A null mutation in the S. Typhi impX gene suppresses the induced auxotrophy for adenosine or thymidine, resulting from a tsx mutation and confers sensitivity to high concentrations of adenosine or thymidine. impX mutants are defective in the export of 3H-radiolabeled thymidine. The authors suggested that the S. Typhi Tsx porin and the ImpX inner membrane protein facilitate competing mechanisms of thymidine influx and efflux, respectively, to maintain the steady-state levels of internal nucleoside pools (Bucarey et al. 2006).