Activation of oxidative stress genes by mutations at the soxQ/cfxB/marA locus of Escherichia coli.

Exposure of Escherichia coli to superoxide-generating drugs, such as menadione or paraquat, uniquely induces approximately 40 proteins, nine of which are under the positive control of the soxR locus (at min 92). We report here that certain mutations at a separate locus that we have named soxQ (at min 34) confer some of the phenotypes seen in soxR-constitutive strains, including resistance to menadione. A previously known mutation called cfxB, identified through antibiotic resistance, is likely an allele of soxQ. The soxQ1 and cfxB mutations cause transcriptional activation of the genes that encode Mn-containing superoxide dismutase, glucose 6-phosphate dehydrogenase, and the soi-17/19::lac and soi-28::lac fusions. These genes are also activated by soxR, but the soxQ1 and cfxB mutations increase the synthesis of seven other proteins not influenced by soxR. Moreover, the soxQ1- and cfxB-dependent phenotypes do not depend on the soxR gene, and gene induction by soxR in response to redox stress does not depend on the soxQ locus. As well as increasing cellular resistance to some oxidants, the soxQ1 and cfxB mutations confer elevated resistance to various antibiotics, probably via diminished expression of outer membrane protein OmpF. The marA1 multiple-antibiotic resistance mutation (also at min 34) behaves like a weak allele of soxQ but probably resides in a nearby gene that, with soxQ, is part of a regulatory complex. We propose that soxQ helps control some oxidative stress proteins as part of another regulon that responds to an unknown environmental signal.