The role of P1 ParA in the partitioning of the P1 plasmid

The P1 plasmid partition system relies on the ATPase ParA, whose functions in autorepression and plasmid segregation depend on its nucleotide‑binding state. This study used structural and biochemical approaches to define how ParA’s conformations support its roles. Crystal structures showed that apo‑ParA forms a stable dimer through N‑terminal helix interactions, while an ADP‑bound structure revealed a basic region essential for DNA binding during autorepression. In contrast, ATP‑bound ParA assembled into filaments that recruited ParB and parS, indicating a role in plasmid segregation. Two ParB N‑terminal arginines were critical for stimulating ParA ATPase activity. ParA remains dimeric across states, with nucleotide‑dependent conformations directing function.