The DNA Binding Domains of P1 ParB and the Architecture of the P1 Plasmid Partition Complex
Stable maintenance of P1 plasmids inEscherichia coliis mediated by a high affinity nucleoprotein complex called the partition complex, which consists of ParB and theE. coliintegration host factor (IHF) bound specifically to the P1parSsite. IHF strongly stimulates ParB binding toparS, and the minimal partition complex contains a single dimer of ParB. To examine the architecture of the partition complex, we have investigated the DNA binding activity of various ParB fragments. Gel mobility shift and DNase I protection assays showed that the first 141 residues of ParB are dispensable for the formation of the minimal, high affinity partition complex. A fragment missing only the last 16 amino acids of ParB bound specifically toparS, but binding was weak and was no longer stimulated by IHF. The ability of IHF to stimulate ParB binding toparScorrelated with the ability of ParB to dimerize via its C terminus. Using full and partialparSsites, we show that two regions of ParB, one in the center and the other near the C terminus of the protein, interact with distinct sequences withinparS. Based on these data, we have proposed a model of how the ParB dimer bindsparSto form the minimal partition complex.