Three ParA dimers cooperatively assemble on type Ia partition promoters
Accurate DNA segregation is essential for faithful inheritance of genetic material. In bacteria, this process is mainly ensured by a partition system (Par) composed of two proteins, ParA and ParB, and a centromere site. The auto-regulation of Par operon expression is important for efficient partitioning, and is primarily mediated by ParA for type Ia plasmid partition systems. For the plasmid F, four ParAF monomers were proposed to bind to four repeated sequences in the promoter region. By contrast, using quantitative surface plasmon resonance, we showed that three ParAF dimers bind to this region. We uncovered that one perfect inverted repeat (IR) motif, consisting of two hexamer sequences spaced by 28-bp, constitutes the primary ParAF DNA binding site. A similar but degenerated motif overlaps the former. ParAF binding to these motifs is well supported by biochemical and modeling analyses. In addition, molecular dynamics simulations predict that the winged-HTH domain displays high flexibility, which may favor the cooperative ParA binding to the promoter region. We propose that three ParAF dimers bind cooperatively to overlapping motifs thus covering the promoter region. A similar organization is found on both closely related and distant plasmid partition systems, suggesting that such promoter organization for auto-regulated Par operons is widespread and may have evolved from a common ancestor.