The high-temperature requirement (HtrA) family of stress response proteins are induced by different environmental stress conditions in a variety of bacteria and have been shown to contribute to the pathogenicity of some of these species. In this study, the htrA gene from Yersinia enterocolitica O:8 was amplified, cloned, and sequenced. Analysis of the deduced amino acid sequence predicted that the putative HtrA homolog contains a serine protease active site and a catalytic triad characteristic of trypsin-like serine proteases, structural features characteristic of previously described HtrA proteins. In order to evaluate the biological functions of Y. enterocolitica HtrA, an isogenic mutant was constructed by a reverse-genetics PCR-based approach. Characterization of the mutant provided evidence supporting a stress response function for the Y. enterocolitica htrA gene product. In contrast to the parent strain, the mutant showed increased sensitivity to killing by H2O2, O2- and temperature stress (50 degrees C). The mutant was avirulent in the murine yersiniosis injection model and offered partial protection to mice challenged with the parent strain. Further studies with the Y. enterocolitica htrA mutant should increase our knowledge of the host-pathogen interactions which occur during Yersinia infections.