Redox sensor SsrB Cys²⁰³ enhances Salmonella fitness against nitric oxide generated in the host immune response to oral infection

We show herein that the Salmonella pathogenicity island 2 (SPI2) response regulator SsrB undergoes S-nitrosylation upon exposure of Salmonella to acidified nitrite, a signal encountered by this enteropathogen in phagosomes of macrophages. Mutational analysis has identified Cys²⁰³ in the C-terminal dimerization domain of SsrB as the redox-active residue responding to nitric oxide (NO) congeners generated in the acidification of nitrite. Peroxynitrite and products of the autooxidation of NO in the presence of oxygen, but not hydrogen peroxide, inhibit the DNA-binding capacity of SsrB, demonstrating the selectivity of the reaction of Cys²⁰³ with reactive nitrogen species (RNS). These findings identify the two-component response regulator SsrB Cys²⁰³ as a thiol-based redox sensor. A C203S substitution protects SsrB against the attack of RNS while preserving its DNA-binding capacity. When exposed to SPI2-inducing conditions, Salmonella expressing the wild-type ssrB allele or the ssrB C203S variant sustain transcription of the sifA, sspH2, and srfJ effector genes. Nonetheless, compared with the strain expressing a redox-resistant SsrB C203S variant, wild-type Salmonella bearing the NO-responsive allele exhibit increased fitness when exposed to RNS in an NRAMP^R, C3H/HeN murine model of acute oral infection. Given the widespread occurrence of the wild-type allele in Salmonella enterica, these findings indicate that SsrB Cys²⁰³ increases Salmonella virulence by serving as a redox sensor of NO resulting from the host immune response to oral infection.