Fis suppresses late-stage virulence gene expression in Yersinia pseudotuberculosis at environmental temperatures

The nucleoid-associated protein Fis is a key transcriptional regulator in Gram-negative bacteria that supports rapid adaptation to environmental changes. In Yersinia pseudotuberculosis, Fis plays a critical, yet poorly understood role in virulence. Here, we present a comparative transcriptomic analysis of Y. pseudotuberculosis wild type and fis deletion mutant at environmental (25°C) and host-relevant (37°C) temperatures. Our data show that Fis modulates the expression of more than 600 genes across 16 functional categories. Notably, Fis exerts reciprocal, temperature-dependent control over virulence genes, including those encoding the type III secretion system (T3SS) and Yersinia effector proteins (Yops), flagella biosynthesis, and cell adherence/invasion factors. Functional assays revealed that fis deletion disrupts this regulatory balance, producing a host-defense-like state at 25°C characterized by complete loss of motility, upregulation of the virulence master regulator LcrF, aberrant Yop secretion, impaired phagocytosis by host cells, and increased pathogenicity in the Galleria mellonella infection model. These findings establish Fis as a central regulator that coordinates motility and early host-cell engagement while preventing premature activation of antiphagocytic defenses, thereby optimizing the initial stages of infection.