Fumarase C, the stable fumarase of Escherichia coli, is controlled by the soxRS regulon.

Abstract

Fumarase C was strongly induced by paraquat in a parental strain of Escherichia coli but was not induced in a strain lacking the soxRS response. Moreover, a strain that constitutively expresses the soxRS regulon contained more fumarase C than did the parental strain. The Mn-containing superoxide dismutase and glucose-6-phosphate dehydrogenase, members of the soxRS regulon, were similarly induced by paraquat. Mutational defects in glucose-6-phosphate dehydrogenase increased the induction of fumarase C by paraquat. For Mn-containing superoxide dismutase, responsiveness to paraquat was also enhanced in the glucose-6-phosphate dehydrogenase-defective strains. Overproduction of the Mn-containing superoxide dismutase, elicited by isopropyl beta-D-thiogalactoside in a tac-sodA fusion strain, did not diminish induction of fumarase C or of glucose-6-phosphate dehydrogenase by paraquat, and induction of these enzymes was more sensitive to paraquat when the cells were growing on succinate rather than on LB medium. These results indicate that fumarase C is a member of the soxRS regulon and that this regulon does not respond to changes in O2- concentration but perhaps does respond to some consequence of a decrease in the ratio of NADPH to NADP+.

Regulation of the soxRS Oxidative Stress Regulon

“…Recently, we demonstrated that protein can be reversibly inactivated in vitro by reduction of its Fe-S centers with dithionite under conditions that should not have resulted in dissociation of the Fe-S centers (3). This finding was consistent with an observation that the regulon's induction may be sensitive to the redox balance of the cell (11). However, what we can demonstrate in vitro is not necessarily related to what happens in vivo.…”

“…The model also postulated that there must be an efficient redox pathway for maintaining the autooxidizable protein in a reduced form during aerobic growth (11,30). The present work demonstrates that pathway.…”

“…It is enzymatically reduced in the cell with electrons derived from NADPH (11), and reduced paraquat will directly reduce SoxR (3). Therefore, we must explain its activity as an inducer on the basis of its ability to deplete the cell of NADPH by redox cycling, as suggested previously (11,30).…”

“…(iii) Nitric oxide (or perhaps a more reactive derivative thereof) could induce the regulon under anaerobic conditions (29). Therefore, the following theory was proposed (11,30 generators accomplish this by depleting the cell of NAD(P)H through redox cycling. Although many of the inducing agents are small molecules that should be able to oxidize or reduce SoxR directly, this ability cannot explain their effectiveness as inducers because the midpoint redox potential (E h ) of an inducer can either be higher or lower than that of SoxR (E h ϭ Ϫ0.28 V) (3).…”

Genetic definition of the Escherichia coli zwf "soxbox," the DNA binding site for SoxS-mediated induction of glucose 6-phosphate dehydrogenase in response to superoxide

“…Thus, unlike gnd, zwf expression is transcriptionally activated by SoxS during episodes of oxidative stress induced by exposure of E. coli to superoxide-generating agents such as paraquat (17,18,39). Indeed, E. coli strains deleted for zwf are hypersensitive to oxidative stress, thus making zwf a pivotal member of the soxRS regulon (18,21). In addition, treatment of cells with several antibiotics and aromatic weak acids appears to activate zwf transcription through the action of marA, thus making zwf a member of the multiple antibiotic resistance (mar) regulon (2,6,7,14,16).…”