T6SS4 is heterogeneously expressed in Y. pseudotuberculosis and is a target for transcriptional and post-transcriptional regulation

The type VI secretion system (T6SS) is a complex secretion system encoded by many Gram-negative bacteria to translocate effector proteins directly into target cells. Due to its high complexity and energy-intensive firing process, regulation of the T6SS is tightly controlled in many organisms. Y. pseudotuberculosis encodes four complete T6SS clusters but lacks genes implicated in T6SS gene regulation in other microorganisms, indicating a distinct control mechanism. Here, we could show that the T6SS4 of Y. pseudotuberculosis is heterogeneously expressed within a population, which is determined by the transcriptional T6SS4 activator RovC. Moreover, the T6SS4 and RovC are embedded in a complex and global regulatory network, including the global post-transcriptional regulator CsrA, the Yersinia modulator A (YmoA), the global protease Lon, and RNases (PNP and RNase III). Post-transcriptional processing of the T6SS4 polycistron and different transcript stability within the operon also achieve a higher regulatory complexity. In summary, our work provides new insights into the sophisticated and complex regulatory network of the T6SS4 of Y. pseudotuberculosis, which clearly differs from regulation in other organisms. Authors summaryBacteria use a specialized multi-protein complex called the Type VI secretion system (T6SS) to inject toxic proteins into other cells to compete with target microorganisms or to infect host organisms. While the T6SS has been extensively studied in some model organisms, much less is known about the function and regulation of the four T6SS clusters of the food-borne human pathogen Yersinia pseudotuberculosis. In this study, we found that the T6SS4 of Y. pseudotuberculosis is only expressed in a small subpopulation in vitro. This suggests that its regulation is fundamentally different from what is known in other organisms. We show that a complex regulatory network regulates T6SS4 gene expression, and the T6SS4 transcript is post-transcriptionally processed, resulting in different mRNA levels of the individual T6SS components. These findings contribute to a deeper understanding of how bacteria, especially Y. pseudotuberculosis, regulate complex secretion systems at multiple levels.