Identification of SoxS-regulated genes in Salmonella enterica serovar Typhimurium

Salmonella enterica serovar Typhimurium responds to superoxide-generating agents through soxR-mediated activation of the soxS gene, whose product, SoxS, is necessary for resistance to oxidative stress. The S. enterica serovar Typhimurium soxRS system also mediates redox-inducible resistance to diverse antibiotics, which may be relevant to clinical infections. In order to identify SoxS-regulated genes in S. enterica serovar Typhimurium, a lacI-regulated expression system for the S. enterica serovar Typhimurium soxS gene was developed. This system was used to demonstrate that soxS expression is sufficient for the induction of resistance to the superoxide-generating drug paraquat and for the transcriptional activation of the sodA and micF genes. In addition, a library of random lacZ insertions was generated and screened for clones displaying differential beta-galactosidase activity in the presence or absence of SoxS. This selection yielded six independent chromosomal lacZ transcriptional fusions that were activated by either artificial expression of SoxS or exposure of wild-type cells to micromolar concentrations of paraquat. Moreover, disruption of the inducible genes by the insertions rendered S. enterica serovar Typhimurium hypersensitive to millimolar concentrations of paraquat. Nucleotide sequence determination identified the disrupted genes as sodA (Mn-containing superoxide dismutase), fpr (NADPH:ferredoxin oxidoreductase), and ydbK (a putative Fe-S-containing reductase).