Maintenance of nitric oxide and redox homeostasis by the Salmonella flavohemoglobin Hmp

Intracellular pathogens must resist the antimicrobial actions of nitric oxide (NO.) produced by host cells. To this end pathogens possess several NO.-metabolizing enzymes. Here we show that the flavohemoglobin Hmp is the principal enzyme responsible for aerobic NO. metabolism by Salmonella enterica serovar typhimurium. We further show that Hmp is required for Salmonella virulence in mice, in contrast to S-nitrosoglutathione reductase, flavorubredoxin, or cytochrome c nitrite reductase. Abrogation of murine-inducible NO. synthase restores virulence to hmp mutant bacteria. In the presence of nitrosative stress, Hmp-deficient Salmonella exhibits reduced NO. consumption, impaired growth, increased protein S-nitrosylation, and filamentous morphology. However, under aerobic conditions in the absence of nitrosative stress, elevated hmp expression increases S. typhimurium susceptibility to hydrogen peroxide. Both the heme binding and flavoreductase domains are required for resistance to NO., whereas the flavoreductase domain is responsible for iron-dependent susceptibility to oxidative stress. This provides a rationale for the regulation of hmp expression by the transcriptional repressor NsrR in response to both nitrosative stress and intracellular free iron concentration. The Hmp flavohemoglobin plays a central role in the response of Salmonella to nitrosative stress but requires precise regulation to avoid the exacerbation of oxidative stress that can result if electrons are shuttled to extraneous iron.