DksA-Dependent Resistance of Salmonella enterica Serovar Typhimurium against the Antimicrobial Activity of Inducible Nitric Oxide Synthase
ABSTRACTIn coordination with the ppGpp alarmone, the RNA polymerase regulatory protein DksA controls the stringent response of eubacteria, negatively regulating transcription of translational machinery and directly activating amino acid promoters andde novoamino acid biosynthesis. Given the effects of nitric oxide (NO) on amino acid biosynthetic pathways and the intimate relationship of DksA with amino acid synthesis and transport, we tested whether DksA contributes to the resistance ofSalmonellato reactive nitrogen species (RNS). Our studies show that the zinc finger predicted to position DksA in the secondary channel of the RNA polymerase is essential for the resistance ofSalmonella entericaserovar Typhimurium to RNS in a murine model of systemic salmonellosis. Despite exhibiting auxotrophies for various amino acids, ΔdksAmutantSalmonellastrains regain virulence in mice lacking inducible NO synthase (iNOS). DksA is also important for growth of this intracellular pathogen in the presence of NO congeners generated by iNOS during the innate response of murine macrophages. Accordingly,dksAmutantSalmonellastrains are hypersusceptible to chemically generated NO, a phenotype that can be prevented by adding amino acids. The DksA-dependent antinitrosative defenses do not rely on the Hmp flavohemoprotein that detoxifies NO to NO3−and appear to operate independently of the ppGpp alarmone. Our investigations are consistent with a model by which NO produced in the innate response toSalmonellaexerts considerable pressure on amino acid biosynthesis. The cytotoxicity of NO againstSalmonellaamino acid biosynthetic pathways is antagonized in great part by the DksA-dependent regulation of amino acid biosynthesis and transport.