Characterization of the stable maintenance of the Shigella flexneri plasmid pHS-2.

pHS-2 is a 3-kb plasmid originally isolated from Shigella flexneri infections associated with reactive arthritis in humans. This plasmid is stably maintained in many clinical isolates of Shigella flexneri. The nucleotide sequence of this plasmid displays two closely linked regions that may play a role in the maintenance of this plasmid. One region consists of a 250-bp locus showing a significant homology to the ColE1 cer site. The results indicate that the cer-like site of pHS-2, like the ColE1 cer site, acts as a recA-independent, site-specific recombination site involved in the resolution of multimers, requiring the presence of the host-encoded factors ArgR, PepA, XerC, and XerD. The second region consists of a 36-kDa open reading frame involved in generating resistance to the bactericidal effect of complement, which confers a selective advantage to cells containing this sequence. The results also indicate that pHS-2 can replicate in another species of Enterobacteriaceae (Escherichia coli) and is mobilized by the F plasmid.