GlnK Regulates the Type III Secretion System by Modulating NtrB-NtrC Homeostasis in Pseudomonas aeruginosa

Bacterial pathogens exploit host-derived nutrients to coordinate metabolism and virulence determinants to optimize fitness in vivo. In Pseudomonas aeruginosa, GlnK is a central regulator of nitrogen metabolism. It senses the intracellular nitrogen status by integrating 2-oxoglutarate (2-OG) and glutamine signals, which in turn triggers its uridylylation and conformational changes. This reversible post-translational modification modulates its interaction with target proteins, thereby precisely regulating carbon-nitrogen metabolic homeostasis and enabling adaptive nitrogen metabolism in response to host-derived nutrient cues. In this study, we found that glnK is upregulated during infection in a mouse pneumonia model. By growing bacteria in mouse bronchoalveolar lavage fluid (BALF), we demonstrated that the expression of glnK is activated by the NtrB-NtrC two-component regulatory system in response to the host nutrient environment. Mutation of glnK impairs bacterial virulence. Transcriptomic analysis revealed downregulation of the type III secretion system (T3SS) genes in the glnK mutant. Further studies revealed a role of GlnK in maintaining the homeostasis of the NtrB-NtrC system through a negative feedback mechanism, which is required for the expression of the T3SS genes. Collectively, these findings reveal a role of GlnK in interconnecting carbon–nitrogen balance and the T3SS in response to the host environment.