SopA belongs to a large family of bacterial partition protein ATPases. It helps stabilize the F plasmid by acting as the primary repressor of transcription of the sopAB operon, preventing the destabilizing effects of Sop protein excess. It is also thought to act directly in the F partition mechanism. We have examined the role of SopA in partition and repression by observing the consequences of replacing an invariant ATP-binding site lysine, K120, by glutamine or arginine. Circular dichroism studies of the purified mutant proteins revealed no major differences from wild-type, but in the presence of ADP or ATP each protein showed a characteristic spectrum which suggested a distinct conformational change. The K120Q mutant retained most of the wild-type ATPase activity while the K120R mutant lost it. In neither case was the residual activity stimulated by SopB, as occurs for wild-type SopA. The strength of sop promoter repression by the mutant SopA proteins alone was comparable to that resulting from SopB-enhancement of wild-type SopA, but SopB enhanced repression by the mutant SopA proteins either slightly (K120R) or not at all (K120Q). Mini-Fs in which the sop operon was controlled by a constitutive promoter were destabilized by the mutations, demonstrating the need for SopA and its ATP-binding site in the partition process. The K120R mini-F was lost at the same rate as a mini-F lacking the sopC centromere, the K120Q mutant was lost faster. SopAK120R at high levels was more effective than SopA(+) in disrupting the partition complex, whereas SopAK120Q did not disrupt it at all. These results suggest that one function of SopA in the partition mechanism is to break the paired plasmid structure to allow F molecules to segregate to daughter cells.