ROS-sensing Transcription Factor YchJ Regulates RssB-RpoS Pathway to Protect Salmonella Against Oxidative Attack by Macrophages

Bacterial pathogens must overcome oxidative stress to survive within host phagocytes. Although canonical systems such as OxyR are well characterized, alternative pathways remain poorly understood. Here, we identified YchJ, a conserved yet uncharacterized protein, as a central redox-sensitive transcription factor that coordinates a major antioxidant defense system in Salmonella independently of OxyR. Deletion of ychJ severely impaired bacterial survival under H2O2 stress and within macrophages. Proteomic analysis revealed that YchJ represses rssB, leading to RpoS accumulation and upregulation of key antioxidant enzymes, including SodC and KatE. Our results show that YchJ directly binds the rssB promoter as a transcription factor. Structural analysis revealed that ROS sensing by YchJ is achieved through reversible dimerization mediated by an intermolecular disulfide bond. This conformational switch enables a C-terminal basic-rich region of the dimer to recognize a palindromic sequence in the rssB promoter and repress rssB transcription. Dual-transcriptome analysis further confirmed that YchJ directly activates antioxidant defenses in Salmonella and significantly disrupts host pathways during intramacrophage infection. Our findings elucidate a previously unrecognized redox-sensing pathway essential for bacterial virulence and uncover a novel transcriptional mechanism controlling RpoS stability, thereby expanding our understanding of the stress-response regulation system in pathogenic bacteria.