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The Yop virulon enables Yersinia spp. (Y. pestis, Y. pseudotuberculosis and Y. enterocolitica) to survive and multiply in the lymphoid tissues of their host. It is an integrated system allowing extracellular bacteria to communicate with the host cell's cytosol by the injection of effector proteins. It is composed of the following four elements. (i) A contact or type III secretion system called Ysc, which is devoted to the secretion of Yop proteins. This secretion apparatus, comprising some 22 proteins recognizes the Yops by a short N-terminal signal that is not cleaved off during secretion. (ii) A system designed to deliver bacterial proteins into eukaryotic target cells. This system is made of YopB, YopD and possibly other Yops such as LcrV. (iii) A control element (YopN). (iv) A set of effector Yop proteins designed to disarm these cells or disrupt their communications (YopE, YopH, YpkA/YopO, and YopM). The whole virulon is encoded by a 70 kb plasmid designated pYV. Transcription of the genes is controlled both by temperature and by contact with a eukaryotic cell.