37 ± 1.09). Transcript levels after treatment with H2O2 were similar as those observed in untreated cells (Figure 6B). One possibility for this result is that in the absence of ArcA, ArcB might phosphorylate (i.e ArcB-OmpR, [43]) one or more response regulators, either unspecifically or due to cross-talk, which could bind to the promoter region and therefore Ulixertinib supplier prevent binding of positive regulators like SoxS, which has been demonstrated to regulate ompW

and is up-regulated in response to HOCl [20, 44]. This could result in constant ompW transcript levels as shown in Figure 6A. On the other hand, in the absence of ArcB no phosphorylation occurs and SoxS or other positive regulator(s) might have free accessibility to the ompW promoter and therefore increase its expression (Figure 6B), although this possibility has not been evaluated in this study. Genetic complementation of ∆arcB restored the negative regulation

observed in wild type cells exposed to H2O2 and HOCl (0.19 ± 0.04 and 0.24 ± 0.11, respectively, Figure 6C). The ompD and ompC transcripts levels remained down-regulated after exposure ZD1839 datasheet to H2O2 and HOCl in the ∆arcB strain, while the negative control arcA remained unaltered (Figure 6B). The ArcA regulon in anaerobically grown S. Typhimurium was recently determined [27]. Interestingly, neither ompD nor ompW expression was down-regulated in an ArcA check details dependant manner, suggesting that the ArcA regulon under anaerobic and aerobic ROS conditions could be different. Even in E. coli ompW expression is suggested to be regulated by FNR in response to oxygen availability [39]. The difference between the ArcA regulons under aerobic and ROS conditions might be explained by studies

suggesting that the mechanism of ArcA activation under aerobic conditions is different from those classically described. E. coli mutant strains in residue H-717 of ArcB are able to phosphorylate and activate ArcA through the transfer of the phosphate group from residue His-292 under aerobic conditions [45] and Loui et al. (2009) suggested that H2O2 resistance is independent of ArcA phosphorylation at residue Asp-54. To the date, the detailed molecular mechanism of ArcAB activation in response to ROS remains unsolved. Therefore, further experiments to unveil the molecular mechanism by which Ixazomib the S. Typhimurium ArcAB two component system is activated are needed and under way in our laboratory. Conclusion We provide both genetic and biochemical evidence indicating that the OM porin OmpW mediates the influx of H2O2 and HOCl. The results revealed that the S. Typhimurium ompW gene is negatively regulated upon exposure to both toxic compounds. Furthermore, we demonstrate that the response regulator ArcA mediates ompW negative regulation in response to H2O2 and HOCl via a direct interaction with the upstream region of ompW.