Salmonella entericaSerovar TyphimuriumrdoAIs Growth Phase Regulated and Involved in Relaying Cpx-Induced Signals

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Abstract
The disulfide oxidoreductase, DsbA, mediates disulfide bond formation in proteins as they enter or pass through the periplasm of gram-negative bacteria. Although DsbA function has been well characterized, less is known about the factors that control its expression. Previous studies withEscherichia colidemonstrated thatdsbAis part of a two-gene operon that includes an uncharacterized, upstream gene,yihE, that is positively regulated via the Cpx stress response pathway. To clarify the role of theyihEhomologue ondsbAexpression inSalmonella entericaserovar Typhimurium, the effect of this gene (termedrdoA) on the regulation ofdsbAexpression was investigated. Transcriptional assays assessingrdoApromoter activity showed growth phase-dependent expression with maximal activity in stationary phase. Significant quantities ofrdoAanddsbAtranscripts exist in serovar Typhimurium, but only extremely low levels ofrdoA-dsbAcotranscript were detected. Activation of the Cpx system in serovar Typhimurium increased synthesis of bothrdoA-anddsbA-specific transcripts but did not significantly alter the levels of detectable cotranscript. These results indicate that Cpx-mediated induction ofdsbAtranscription in serovar Typhimurium does not occur through anrdoA-dsbAcotranscript. A deletion of therdoAcoding region was constructed to definitively test the relevance of therdoA-dsbAcotranscript todsbAexpression. The absence of RdoA affects DsbA expression levels when the Cpx system is activated, and providingrdoAintranscomplements this phenotype, supporting the hypothesis that a bicistronic mechanism is not involved in serovar TyphimuriumdsbAregulation. TherdoAnull strain was also shown to be altered in flagellar phase variation. First it was found that induction of the Cpx stress response pathway switched flagellar synthesis to primarily phase 2 flagellin, and this effect was then found to be abrogated in therdoAnull strain, suggesting the involvement of RdoA in mediating Cpx-related signaling.