UC Santa Cruz

Regulation of the Ysc type three secretion system (T3SS) of the human gut pathogen Yersinia pseudotuberculosis has been a widely explored field. Located on the Yersinia virulence plasmid, the Ysc T3SS has been shown to be regulated by environmental cues such as calcium, temperature, and host cell contact, and by bacterial factor such as AraC-like transcriptional regulators and histone-like proteins. These mechanisms allow for Yersinia to respond to environmental changes as well as entry into the host. Through a forward genetic screen to identify novel factors that regulate the T3SS, we discovered IscR, a global transcriptional regulator that coordinates an iron sulfur (Fe-S) cluster. No reports on Yersinia pseudotuberculosis IscR have been published, though Escherichia coli IscR is well studied. Here we describe the identification and initial characterization of this novel Yersinia transcriptional regulator. We constructed an in-frame IscR deletion mutant in Y. pseudotuberculosis (∆iscR) as well as a mutant expressing an iscR allele unable to coordinate an Fe-S cluster (apo-IscR). Interestingly, both the Y. pseudotuberculosis ∆iscR and apo-IscR mutants lacked the ability to secrete effector proteins and target macrophages through the T3SS. In contrast, the ∆iscR mutant displayed normal flagellar motility while the apo-IscR mutant had a severe motility defect. The flagellar basal body is a T3SS itself, indicating that the defect in the Ysc T3SS displayed by the ∆iscR mutant is not a result of gross abnormalities in secretion systems. Accordingly, the ∆iscR mutant showed normal growth in both rich and minimal media, while the apo-IscR mutant displayed a selective growth defect only in rich media. This suggests that the ratio between Fe-S-bound holo-IscR and apo-IscR may be important for balanced growth. Lastly, preliminary data suggest that the ΔiscR mutant has increased resistance to hydrogen peroxide in comparison to the parental strain, while apo-IscR was susceptible. Our findings suggest that Y. pseudotuberculosis IscR is an important regulator of the Ysc T3SS and plays a role in controlling resistance to ROS, motility, and balanced growth in vitro. As the iscR gene is almost identical in Y. pseudotuberculosis, Y. enterocolitica, and Y. pestis, we speculate that IscR may be important for the virulence of all three human pathogenic Yersinia.