Heightened Virulence of Yersinia Is Associated with Decreased Function of the YopJ Protein

Despite the maintenance of YopP/J alleles throughout the human-pathogenic Yersinia lineage, the benefit of YopP/J-induced phagocyte death for Yersinia pathogenesis in animals is not obvious. To determine how the sequence divergence of YopP/J has impacted Yersinia virulence, we examined protein polymorphisms in this type III secreted effector protein across 17 Yersinia species and tested the consequences of polymorphism in a murine model of subacute systemic yersiniosis. Our evolutionary analysis revealed that codon 177 has been subjected to positive selection; the Yersinia enterocolitica residue had been altered from a leucine to a phenylalanine in nearly all Yersinia pseudotuberculosis and Yersinia pestis strains examined. Despite this change being minor, as both leucine and phenylalanine have hydrophobic side chains, reversion of YopJ^F177 to the ancestral YopJ^L177 variant yielded a Y. pseudotuberculosis strain with enhanced cytotoxicity toward macrophages, consistent with previous findings. Surprisingly, expression of YopJ^F177L in the mildly attenuated ksgA^- background rendered the strain completely avirulent in mice. Consistent with this hypothesis that YopJ activity relates indirectly to Yersinia pathogenesis in vivo, ksgA^- strains lacking functional YopJ failed to kill macrophages but actually regained virulence in animals. Also, treatment with the antiapoptosis drug suramin prevented YopJ-mediated macrophage cytotoxicity and enhanced Y. pseudotuberculosis virulence in vivo. Our results demonstrate that Yersinia-induced cell death is detrimental for bacterial pathogenesis in this animal model of illness and indicate that positive selection has driven YopJ/P and Yersinia evolution toward diminished cytotoxicity and increased virulence, respectively.