YscO of Yersinia pestis Is a Mobile Core Component of the Yop Secretion System

The Yersinia pestis low-Ca²⁺ response stimulon is responsible for the temperature- and Ca²⁺-regulated expression and secretion of plasmid pCD1-encoded antihost proteins (V antigen and Yops). We have previously shown that lcrD, yscC, yscD, yscG, and yscR encode proteins that are essential for high-level expression and secretion of V antigen and Yops at 37°C in the absence of Ca²⁺. In this study, we characterized yscO of the Yop secretion (ysc) operon that contains yscN through yscU by determining the localization of its gene product and the phenotype of an in-frame deletion. The yscO mutant grew and expressed the same levels of Yops as the parent at 37°C in the presence of Ca²⁺. In the absence of Ca²⁺, the mutant grew independently of Ca²⁺, expressed only basal levels of V antigen and Yops, and failed to secrete these. These defects could be partially complemented by providing yscO in trans in the yscO mutant. Overexpression of YopM and V antigen in the mutant failed to restore the export of either protein, showing that the mutation had a direct effect on secretion. These results indicated that the yscO gene product is required for high-level expression and secretion of V antigen and Yops. YscO was found by immunoblot analysis in the soluble and membrane fractions of bacteria growing at 37°C irrespective of the presence of Ca²⁺ and in the culture medium in the absence of Ca²⁺. YscO is the only mobile protein identified so far in the Yersinia species that is required for secretion of V antigen and Yops.