The plasmid-encoded outer-membrane proteins of Yersinia pestis

The yersinial plasmid-encoded outer-membrane proteins (Yops) are encoded by the low-Ca++ response virulence plasmid present in the yersiniae pathogenic for humans. It has been shown that in Yersinia pestis KIM there are 11 of these proteins, which are expressed maximally during growth at 37 degrees C in the absence of Ca++. To recover these proteins in outer membranes, it was necessary to transfer the Y. pestis low-Ca++ response plasmid to Yersinia pseudotuberculosis. However, two of these proteins (Yops M and N) were present in the culture medium after growth of Y. pestis; no other Yops were released by the bacteria. This raises the possibility of multiple roles for Yops in the virulence of Y. pestis.