The Salmonella enterica serovar Typhi tsx gene encodes a porin that facilitates the import of nucleosides. When serovar Typhi is grown under anaerobic conditions, Tsx is among the outer membrane proteins whose expression increases dramatically. This increase in expression is due, at least in part, to increased transcription and is dependent on Fnr but not on ArcA. A mutant derivative of serovar Typhi strain STH2370 with a deletion of the tsx gene is an auxotroph that requires either adenosine or thymidine for growth on minimal medium. In contrast, an otherwise isogenic nupG nupC double mutant, defective in the inner membrane nucleoside permeases, is a prototroph. Because anaerobic growth enhances the virulence of serovar Typhi in vitro, we assessed the role that the tsx gene plays in pathogenicity and found that the serovar Typhi STH2370 Δtsx mutant is defective in survival within human macrophage-like U937 cells. To understand why the Δtsx mutant is an auxotroph, we selected for insertions of minitransposon T-POP in the Δtsx genetic background that restored prototrophy. One T-POP insertion that suppressed the Δtsx mutation in the presence of the inducer tetracycline was located upstream of the pyrD gene. The results of reverse transcription-PCR analysis showed that addition of the inducer decreased the rate of pyrD transcription. These results suggest that the Tsx porin and the balance of products of the tsx and pyrD genes play critical roles in membrane assembly and integrity and thus in the virulence of serovar Typhi.
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