Salmonella enterica serovar Typhimurium periplasmic superoxide dismutase SodCI is a member of the PhoPQ regulon and is induced in macrophages.

Salmonella enterica serovar Typhimurium replicates within host macrophages during the systemic stage of infection. In the macrophage, the bacteria must survive the respiratory burst that produces superoxide. Serovar Typhimurium strain 14028 produces two periplasmic superoxide dismutases, SodCI and SodCII, but only SodCI contributes to virulence. Although we have shown that this is primarily due to differences in the two proteins, evidence suggests differential regulation of the two genes. Using transcriptional sodCI- and sodCII-lac fusions, we show that sodCII is under the control of the RpoS sigma factor, as was known for the Escherichia coli ortholog, sodC. In contrast, we show that sodCI is transcriptionally controlled by the PhoPQ two-component regulatory system, which regulates an array of virulence genes required for macrophage survival. Introduction of a phoP-null mutation into the sodCI fusion strain resulted in a decrease in transcription and loss of regulation. The sodCI-lac fusion showed high-level expression in a background containing a phoQ constitutive allele. The sodCI gene is induced 15-fold in bacteria recovered from either the tissue culture macrophages or the spleens of infected mice. Induction in macrophages is dependent on PhoP. The sodCII fusion was induced three- to fourfold in macrophages and animals; this induction was unaffected by loss of PhoP. Thus, sodCI, which is horizontally transferred by the Gifsy-2 phage, is regulated by PhoPQ such that it is induced at the appropriate time and place to combat phagocytic superoxide.