Soj (ParA) DNA binding is mediated by conserved arginines and is essential for plasmid segregation

Soj is a member of the ParA family of ATPases involved in plasmid and chromosomal segregation. It binds nonspecifically and cooperatively to DNA although the function of this binding is unknown. Here, we show that mutation of conserved arginine residues that map to the surface of Bacillus subtilis Soj caused only minimal effects on nucleotide-dependent dimerization but had dramatic effects on DNA binding. Using a model plasmid partitioning system in Escherichia coli, we find that Soj DNA-binding mutants are deficient in plasmid segregation. The location of the arginines on the Soj structure explains why DNA binding depends on dimerization and was used to orient the Soj dimer on the DNA, revealing the axis of Soj polymerization. The arginine residues are conserved among other chromosomal homologues, including the ParAs from Caulobacter crescentus, Pseudomonas aeruginosa, Pseudomonas putida, Streptomyces coelicolor, and chromosome I of Vibrio cholerae indicating that DNA binding is a common feature of members of this family.