The stability region of the large virulence plasmid of Shigella flexneri encodes an efficient postsegregational killing system

The large virulence plasmid pMYSH6000 of Shigella flexneri contains a deter minant that is highly effective in stabilizing otherwise unstable plasmids in Escherichia coli. Expression of two small contiguous genes, mvpA and mvp T (formerly termed STBORF1 and STBORF2), was shown to be sufficient for sta bility. Mutations in mvpT abolished plasmid stability, and plasm ids expres sing only mvpT kilted the cells unless mvpA was supplied from a separate pl asmid or from the host chromosome. When replication of a plasmid earning th e minimal mvp region was blocked, growth of the culture stopped after a sho rt lag and virtually all of the surviving cells retained the plasmid. Thus, the mvp system stabilizes by a highly efficient postsegregational killing (PSK) mechanism, with mvpT encoding a cell toxin and mvpA encoding an antid ote. The regions that surround the mvp genes in their original context have an inhibitory effect that attenuates plasmid stabilization and PSK. The re gion encompassing the mvp genes also appears to contain an additional eleme nt that can aid propagation of a pSC101-based plasmid under conditions wher e replication initiation is marginal. However, this appears to be a relativ ely nonspecific effect of DNA insertion into the plasmid vector.