IscR-mediated sensing of iron-sulfur cluster demand coordinates virulence gene expression in Yersinia pseudotuberculosis

The type III secretion system (T3SS) is a needle-like appendage that translocates effector proteins into host cells to disrupt host defenses. Strict control of T3SS expression is critical for facultative pathogens such as Yersinia pseudotuberculosis , as the T3SS is indispensable for virulence but is metabolically costly. We previously showed that the iron-sulfur (Fe-S) cluster-coordinating transcription factor IscR controls expression of LcrF, the master regulator of the Yersinia T3SS and the YadA adhesin. Clusterless apo-IscR, the predominant IscR form during high cellular Fe-S cluster demand (aerobic, low iron conditions), promotes LcrF, T3SS, and YadA expression. Importantly, binding of apo-IscR to the lcrF promoter at a type II IscR binding site facilitates Yersinia disseminated infection. Here, we show that mutating the lcrF promoter to allow only [2Fe-2S]-IscR binding ( lcrF ^pTypeI ) results in hyperexpression of LcrF, the T3SS, and YadA during low Fe-S cluster demand (anaerobic, iron replete conditions). These data suggest that switching the form of IscR that can bind to the lcrF promoter reversed how iron and oxygen regulate Yersinia virulence factors. We used barcoded Y. pseudotuberculosis to probe how control of the lcrF promoter in response to iron and oxygen modifies infection dynamics. We found that the lcrF ^pTypeI mutant experiences a tighter bottleneck in the cecum, where Yersinia is expected to experience a low oxygen, iron replete environment. Taken together, these findings suggest that by tying T3SS and YadA expression to cellular Fe–S cluster demand, Yersinia can fine-tune its virulence repertoire to the host tissue microenvironment.