Abstract
OmpW is a minor porin whose biological function has not been clearly defined. Evidence obtained in our laboratory indicates that in Salmonella enterica serovar Typhimurium the expression of OmpW is activated by SoxS upon exposure to paraquat and it is required for resistance. SoxS belongs to the AraC family of transcriptional regulators, like MarA and Rob. Due to their high structural similarity, the genes under their control have been grouped in the mar/sox/rob regulon, which presents a DNA-binding consensus sequence denominated the marsox box. In this work, we evaluated the role of the transcription factors MarA, SoxS and Rob of S. enterica serovar Typhimurium in regulating ompW expression in response to menadione. We determined the transcript and protein levels of OmpW in different genetic backgrounds; in the wild-type and Δrob strains ompW was upregulated in response to menadione, while in the ΔmarA and ΔsoxS strains the induction was abolished. In a double marA soxS mutant, ompW transcript levels were lowered after exposure to menadione, and only complementation in trans with both genes restored the positive regulation. Using transcriptional fusions and electrophoretic mobility shift assays with mutant versions of the promoter region we demonstrated that two of the predicted sites were functional. Additionally, we demonstrated that MarA increases the affinity of SoxS for the ompW promoter region. In conclusion, our study shows that ompW is upregulated in response to menadione in a cooperative manner by MarA and SoxS through a direct interaction with the promoter region.
Highlights
The deleted region in E. coli is highly conserved in Salmonella enterica serovar Typhimurium and contains the ompW gene (Gil et al, 2007), which encodes a minor porin that has been well studied and its structure in E. coli and Vibrio cholerae has
A S. enterica serovar Typhimurium ceftriaxone-resistant strain showed decreased expression of ompW, suggesting that it might be involved in the uptake of this antibiotic (Hu et al, 2005)
Evidence obtained in our laboratory indicates that in S. enterica serovar Typhimurium 14028s OmpW expression is increased in the presence of paraquat and it mediates resistance (Gil et al, 2007)
Summary
Porins are aqueous channels that allow the passive diffusion of hydrophilic solutes, nutrients or toxic compounds through the bacterial outer membrane and participate, at least in part, in the ability of bacteria to adapt to diverse environments, in drug resistance mechanisms and in bacterial pathogenesis (Benz & Bauer, 1988; Chatfield et al, 1991; Jeanteur et al, 1991; Weiss et al, 1991; Groisman & Ochman, 1994; Nikaido, 1996; Zgurskaya & Nikaido, 2000; Koebnik et al, 2000; Rodrıguez-Morales et al, 2006). Upon exposure to O22 and/or ammonium quaternary compounds, SoxR is oxidized and converted to an active form that induces the transcription of soxS, which binds to the promoter regions of several genes whose products are involved in the response to oxidative damage (Storz & Imlay, 1999; Scandalios, 2002; Imlay, 2008; Gu & Imlay, 2011). In this context, the evidence supports a model in which ammonium quaternary compounds are responsible for SoxR activation (Krapp et al, 2011; Gu & Imlay, 2011); it has been recently confirmed that
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