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Originally published as Genetics Published Articles Ahead of Print on February 19, 2006.
Genetics, Vol. 173, 25-34, May 2006, Copyright © 2006
doi:10.1534/genetics.105.054700
The Cotranscribed Salmonella enterica sv. Typhi tsx and impX Genes Encode Opposing Nucleoside-Specific Import and Export Proteins
Sergio A. Bucarey*,
Nicolas A. Villagra
,
Juan A. Fuentes* and
Guido C. Mora,1
* Programa Doctorado de Genética Molecular y Microbiología, Pontificia Universidad Católica de Chile and Departamento de Ciencias Biológicas, Universidad Andrés Bello, Santiago, Chile
1 Corresponding author: Laboratorio de Microbiología, Departamento de Ciencias Biológicas, Universidad Andrés Bello, República 217, Santiago, Chile.
E-mail: gmora{at}unab.cl
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 shows 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, which is predicted to have only two transmembrane 
-helices. Fusions of the lacZ gene to both tsx and impX reveal that the 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. The ImpX protein, when tagged with a 3xFLAG epitope, is functional and associates with the inner membrane; impX mutants are defective in the export of 3H-radiolabeled thymidine. Taken together, these and other results suggest that the S. Typhi Tsx porin and ImpX inner membrane protein facilitate competing mechanisms of thymidine influx and efflux, respectively, to maintain the steady-state levels of internal nucleoside pools.
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