FINE-STRUCTURE ANALYSIS OF THE P7 PLASMID PARTITION SITE

The par region of bacteriophage P7 is responsible for active partition of the P7 plasmid prophage into daughter cells. The cis-acting partit ion site was defined precisely as a 75-bp sequence that was necessary and sufficient to promote correct segregation of an unstable vector pl asmid when the two P7 partition proteins, ParA and ParB, were supplied in trans. Roughly the same region was necessary to exert partition-me diated incompatibility. The minimal site contains an integration host factor (IHF) protein binding site bracketed by regions containing hept amer repeat sequences that individually bind ParB. An additional seque nce forms the left boundary of the site. Site-directed mutations in th e latter sequence, as well as the IHF motif and the rightmost ParB box , blocked site function. Although the P7 site shares 55% sequence iden tity with its counterpart in bacteriophage P1, functional interactions between the partition sites and the Par proteins of the two plasmids were entirely species specific in vivo. The P1 sequence has similar IH F and ParB binding motifs, but the left boundary sequence differs radi cally and may define a point of species-specific contact with the Par proteins. No evidence was found for the existence of a functional P7 a nalog of the P1 parS core, a small subregion of the P1 site that, in i solation, acts as an enfeebled partition site with modified incompatib ility properties.