The ATP‐dependent ClpXP and Lon proteases regulate expression of the Yersinia pestis type III secretion system via regulated proteolysis of YmoA, a small histone‐like protein

Summary The Yersinia pestis plasmid pCD1‐encoded type III secretion system (T3SS) is essential for the pathogenicity of Y. pestis in mammalian hosts. T3SS‐associated genes are maximally expressed at 37°C in the absence of extracellular calcium. Expression of T3SS genes requires LcrF, an AraC‐like transcriptional activator, and is repressed by YmoA, a small histone‐like protein. The mechanism by which temperature regulates T3SS gene expression has not been determined; however, changes in DNA topology have been implicated in this process. We report here that a Y. pestis strain deficient in production of the ClpXP and Lon proteases does not express a functional T3SS partly because of high cytosolic levels of YmoA. YmoA is rapidly degraded at 37°C in wild‐type Y. pestis, but remains stable in a clpXPlon deletion mutant. The stability of YmoA in wild‐type Y. pestis increased as the growth temperature of the culture decreased; in contrast, YmoA was stable at all temperatures examined in the clpXPlon deletion mutant. These results indicate that the ClpXP and Lon proteases contribute to the environmental regulation of the Y. pestis T3SS system through regulated proteolysis of YmoA.