Integration of the plasmid encoding the synthesis of capsular antigen and murine toxin into Yersinia pestis chromosome.

Yersinia pestis, the causative agent of plague, usually carries three plasmids. The largest of them, a 60 megadalton (MDa) replicon designated pFra determines the synthesis of capsular antigen (fraction I) and murine toxin. Both products are involved in the expression of virulence. Previously, several cases of integration into the bacterial chromosome of the calcium dependence plasmid common to pathogenic Yersinia species have been described. In this study, using the Southern hybridization method, we have shown that the plasmid pFra of Y. pestis also integrates into the host chromosome in some clones of strain populations. The integration could be observed for both the intact plasmid and its mutant derivatives unable to express murine toxin or mediating a dramatically reduced level of capsular antigen synthesis.