A bacterial secreted translocator hijacks riboregulators to control type III secretion in response to host cell contact

Numerous Gram-negative pathogens use a Type III Secretion System (T3SS) to promote virulence by injecting effector proteins into targeted host cells, which subvert host cell processes. Expression of T3SS and the effectors is triggered upon host cell contact, but the underlying mechanism is poorly understood. Here, we report a novel strategy of Yersinia pseudotuberculosis in which this pathogen uses a secreted T3SS translocator protein (YopD) to control global RNA regulators. Secretion of the YopD translocator upon host cell contact increases the ratio of post-transcriptional regulator CsrA to its antagonistic small RNAs CsrB and CsrC and reduces the degradosome components PNPase and RNase E levels. This substantially elevates the amount of the common transcriptional activator (LcrF) of T3SS/Yop effector genes and triggers the synthesis of associated virulence-relevant traits. The observed hijacking of global riboregulators allows the pathogen to coordinate virulence factor expression and also readjusts its physiological response upon host cell contact. Author summary Many bacterial pathogens sense contact to host cells and respond by inducing expression of crucial virulence factors. This includes the type III secretion systems (T3SSs) and their substrates, which manipulate different host cell functions to promote colonization and survival of the pathogen within its host. In this study, we used enteropathogenic Yersinia pseudotuberculosis to elucidate the molecular mechanism of how cell contact is transmitted and translated to trigger this process. We found that multiple global riboregulators control the decay and/or translation of the major transcriptional activator of the T3SS. In the absence of cell contact, these important RNA regulators are coopted by one of the substrate proteins of the T3SS to repress expression of the secretion machinery. Translocation of the substrate protein upon cell contact relieves riboregulator-mediated repression. This leads to a strong induction of the master regulator of T3SS/effector gene expression promoting an increase of the virulence potential and provokes a fast adaptation of the pathogen's fitness, e.g. to compensate for the imposed energetic burden.