RfaH is essential for virulence and adaptive responses in <i>Yersinia pseudotuberculosis</i> infection

We previously suggested that increased expression of the gene encoding transcriptional antiterminator RfaH during <i>Yersinia pseudotuberculosis</i> transcriptional reprogramming is necessary for adapting to persistent infection. In this study, we examined the role of RfaH in virulence and bacterial physiology under infection-relevant stress conditions, and identified genes differentially regulated in the absence of RfaH in <i>Y. pseudotuberculosis</i>. We employed a mouse infection model and phenotypic assays to test RfaH's role in virulence and physiology, as well as RNA sequencing, including O-antigen biosynthesis-deficient strains. Our findings demonstrate that loss of RfaH significantly attenuates virulence, reducing the capacity of <i>Y. pseudotuberculosis</i> to establish persistent infection. RfaH expression is increased during the stationary growth phase and under various stress conditions, such as high osmolarity and bile salts, which are known to induce envelope stress. Functional assays revealed that the Δ<i>rfaH</i> strain displayed defects in motility and increased clumping, indicating altered surface properties affecting motility. Moreover, transcriptomic profiling of the Δ<i>rfaH</i> strain revealed a specific RfaH-dependent gene set after filtering out genes affected by O-antigen-related mutations, thereby minimizing confounding effects from surface structure alterations. These results suggest that RfaH influences a broader set of virulence and adaptation pathways beyond O-antigen regulation. Collectively, our findings suggest that RfaH is essential for the virulence and adaptive capacity of <i>Y. pseudotuberculosis</i> to colonize the host. This study provides insights into regulatory mechanisms that facilitate bacterial survival in hostile environments and highlights the importance of RfaH and its regulatory targets in the pathogenesis of <i>Y. pseudotuberculosis</i>.IMPORTANCEFor bacterial pathogens to establish infection and persist in the host, they must adapt to harsh environments and fine-tune gene expression accordingly. The transcriptional antiterminator RfaH plays a pivotal role in regulating key genes essential for adaptation and virulence in <i>Y. pseudotuberculosis</i>. In this study, we explored the function of RfaH in bacterial physiology, stress responses, and infection dynamics. Using a mouse infection model, we found that loss of RfaH significantly reduced virulence and impaired the pathogen's ability to establish persistent infection. Notably, RfaH expression increased under stress conditions, such as high osmolarity and temperature, underscoring its role in bacterial adaptation. On the other hand, the absence of RfaH led to motility defects and enhanced bacterial aggregation, suggesting alterations in surface properties. Transcriptomic analysis revealed that RfaH influences a broader set of genes beyond the O-antigen biosynthesis operon, including virulence factors critical for host adaptation. Overall, our findings establish RfaH as a key regulator of <i>Y. pseudotuberculosis</i> virulence, shedding light on the molecular mechanisms that enable bacterial survival in challenging environments.