The DNA binding domains of P1 ParB and the architecture of the P1 plasmid partition complex

Stable maintenance of P1 plasmids in Escherichia coli is mediated by a high affinity nucleoprotein complex called the partition complex, which consist s of ParB and the E. coli integration host factor (IHF) bound specifically to the P1 parS site. IHF strongly stimulates ParB binding to parS, and the minimal partition complex contains a single dimer of ParB, To examine the a rchitecture of the partition complex, we have investigated the DNA binding activity of various ParB fragments. Gel mobility shift and DNase I protecti on assays showed that the first 141 residues of ParB are dispensable for th e formation of the minimal, high affinity partition complex. A fragment mis sing only the last 16 amino acids of ParB bound specifically to parS, but b inding was weak and was no longer stimulated by IHF. The ability of IHF to stimulate ParB binding to parS correlated with the ability of ParB to dimer ize via its C terminus. Using full and partial parS sites, we show that two regions of ParB, one in the center and the other near the C terminus of th e protein, interact with distinct sequences within parS. Based on these dat a, we have proposed a model of how the ParB dimer binds parS to form the mi nimal partition complex.