Plasmid Partition System of the P1 par Family from the pWR100 Virulence Plasmid of Shigella flexneri

Abstract

ABSTRACT P1 par family members promote the active segregation of a variety of plasmids and plasmid prophages in gram-negative bacteria. Each has genes for ParA and ParB proteins, followed by a parS partition site. The large virulence plasmid pWR100 of Shigella flexneri contains a new P1 par family member: pWR100 par . Although typical parA and parB genes are present, the putative pWR100 parS site is atypical in sequence and organization. However, pWR100 parS promoted accurate plasmid partition in Escherichia coli when the pWR100 Par proteins were supplied. Unique BoxB hexamer motifs within parS define species specificities among previously described family members. Although substantially different from P1 parS from the P1 plasmid prophage of E. coli , pWR100 parS has the same BoxB sequence. As predicted, the species specificity of the two types proved identical. They also shared partition-mediated incompatibility, consistent with the proposed mechanistic link between incompatibility and species specificity. Among several informative sequence differences between pWR100 parS and P1 parS is the presence of a 21-bp insert at the center of the pWR100 parS site. Deletion of this insert left much of the parS activity intact. Tolerance of central inserts with integral numbers of helical DNA turns reflects the critical topology of these sites, which are bent by binding the host IHF protein.