soxR, a locus governing a superoxide response regulon in Escherichia coli K-12

The nfo (endonuclease IV) gene of Escherichia coli is induced by superoxide generators such as paraquat (methyl viologen). An nfo'-lacZ operon fusion was used to isolate extragenic mutations affecting its expression. The mutations also affected the expression of glucose 6-phosphate dehydrogenase, Mn2(+)-superoxide dismutase (sodA), and three lacZ fusions to soi (superoxide-inducible) genes of unknown function. The mutations were located 2 kilobases clockwise of ssb at 92 min on the current linkage map. One set of mutations, in a new gene designated soxR, caused constitutive overexpression of nfo and the other genes. It included insertions or deletions affecting the carboxyl end of a 17-kilodalton polypeptide. In a soxR mutant, the expression of sodA, unlike that of nfo, was also regulated independently by oxygen tension. Two other mutants were isolated in which the target genes were noninducible; they had an increased sensitivity to killing by superoxide-generating compounds. One had a Tn10 insertion in or near soxR; the other had a multigene deletion encompassing soxR. Therefore, the region functions as a positive regulator because it encodes one or more products needed for the induction of nfo. Regulation is likely to be at the level of transcription because the mutations were able to affect the expression of an nfo'-lac operon fusion that contained the ribosome-binding site for lacZ. Some mutant plasmids that failed to suppress (or complement) constitutivity in trans had insertion mutations several hundred nucleotides upstream of soxR in the general region of a gene for a 13-kilodalton protein encoded by the opposite strand, raising the possibility of a second regulatory gene in this region. The result define a new regulon, controlled by soxR, mediating at least part of the global response to superoxide in E. coli.