To overexpress these proteins, salicylate (SAL) can be used to block the activity of MarR (Martin & Rosner, 1995) and paraquat (PQ) can oxidize and hence activate SoxR (Demple, 1996). Alternatively, 2,2′- or 4,4′-dipyridyl (DIP) enhances post-translational activation of Rob (Rosner et al., 2002). As a result of the homology in their DNA binding domains, these proteins activate overlapping regulons leading to two major phenotypes: (1) the superoxide resistance phenotype, which depends upon increasing the expression of the sodA, fpr, acnA, zwf, and fumC genes,

among others; and (2) the multiple antibiotic or multidrug resistance (MDR) phenotype, which mostly depends on activation of the acrAB, tolC, and micF genes (Pomposiello et al., 2001; Martin & Rosner, 2002). However, these activators learn more differ in the extents to which they activate particular promoters, for example, SoxS activates fpr to a PARP inhibitor much greater extent than MarA does. According to these differences, overexpression of SoxS leads to greater superoxide resistance than overexpression of MarA. The primary basis of these effects is because of small differences in the binding affinities of the proteins to the DNA, particularly to the binding sequences termed

soxbox, when SoxS is the primary activator, or marbox, when all three activators can bind and activate the downstream genes (Fawcett & Wolf, 1995; Martin et al., 2000; Martin & Rosner, 2011). Mutations within marR (leading to a lack of repressor function) and soxR (leading to a constitutively active state) have been found to overexpress the corresponding activators, MarA and SoxS, and hence show an MDR phenotype in addition to organic solvent tolerance associated with the overexpression of the efflux pump AcrAB/TolC (Oethinger et al., 1998; Kern et al., 2000; Koutsolioutsou et al., 2005). In a previous study of our group (Fabrega et al., 2010), the stiripentol differences in gene expression between an MDR

E. coli selected in vitro and its susceptible parental clinical isolate were analyzed. Several genes were found to be up-regulated in the resistant mutant, for example, soxS, marA, acrAB, and ompN, and a mutation within soxR, leading to a truncated form of the protein and thus to a constitutively active state, was detected as the most likely explanation for the MDR phenotype. This work has focused on the study of the increased expression of the ompN gene and its possible link with the resistance phenotype. OmpN, like OmpX and OmpW, is one of the minor porins present in E. coli that are poorly expressed and it is closely related to other quiescent porins such as the OmpS1 of Salmonella Typhi and OmpK36 of Klebsiella pneumonia. Moreover, it displays functional properties (single-channel conductance) that closely resemble those of the OmpC porin (Prilipov et al., 1998). However, the physiological role of OmpN is yet to be determined. The bacterial strains and plasmids used in this study are listed in Table 1.