RcsB positively regulates the Yersinia Ysc-Yop type III secretion system by activating expression of the master transcriptional regulator LcrF

The Rcs phosphorelay is a complex signaling pathway used by the family Enterobacteriaceae to sense, respond and adapt to environmental changes during free-living or host-associated lifestyles. In this study, we show that the Rcs phosphorelay pathway positively regulates the virulence plasmid encoded Ysc-Yop type III secretion system (T3SS) in the enteropathogen Yesinia pseudotuberculosis. Both the overexpression of the wild-type Rcs regulator RcsB or the constitutive active RscB(D56E) variant triggered more abundant Ysc-Yop synthesis and secretion, whereas the non-phosphorylatable mutant RcsB(D56Q) negated this. Congruently, enhanced Yops expression and secretion occurred in an in cis rscB(D56E) mutant but not in an isogenic rscB(D56Q) mutant. Screening for regulatory targets of RcsB identified the virG-lcrF operon that encodes for LcrF, the Ysc-Yop T3SS master regulator. Protein-DNA binding assays confirmed that RcsB directly bound to this operon promoter, which subsequently caused stimulated lcrF transcription. Moreover, active RcsB enhanced the ability of bacteria to deliver Yop effectors into immune cells during cell contact, and this promoted an increase in bacterial viability. Taken together, our study demonstrates the role of the Rcs system in regulating the Ysc-Yop T3SS in Yersinia and reports on RcsB being the first transcriptional activator known to directly control lcrF transcription.