The virulence protein Yop5 of Yersinia pseudotuberculosis is regulated at transcriptional level by plasmid-plB1 -encoded trans-acting elements controlled by temperature and calcium.

The DNA sequence of the structural gene (yopE) of one of the Yersinia pseudotuberculosis virulence plas-mid-plB1-encoded proteins, Yop5, is presented. The deduced protein showed a molecular weight of 22971 Daltons. A specific mutant, having a kanamycin-resistance fragment inserted within the yopE gene was no longer virulent for mice. The expression of the Yop5 protein is regulated at the level of transcription by temperature as well as by the Ca -concentration of the medium. A significant increase in the level of transcription was not detected until 45 min after a temperature shift from 26°C to 37°C in the absence of calcium; addition of Ca inhibited the expression. The yopE promoter is under positive, as well as negative, plB1-encoded control. The positive function is solely regulated by temperature, while the regulation of the negative function involves at least five different plasmid-encoded gene loci; one of these genes encodes the V-antigen.