Disruption of the F plasmid partition complex in vivo by partition proteinSopA

The SopA protein plays an essential, though so far undefined, role in parti tion of the mini-F plasmid but, when overproduced, it causes loss of mini-F from growing cells. Our investigation of this phenomenon has revealed that excess SopA protein reduces the linking number of mini-F. It appears to do so by disturbing the partition complex, in which SopB normally introduces local positive supercoiling upon binding to the sopC centromere, as it occu rs only in plasmids carrying sopC and in the presence of SopB protein. SopA -induced reduction in linking number is not associated with altered sop pro moter activity or levels of SopB protein and occurs in the absence of chang es in overall supercoil density. SopA protein mutated in the ATPase nucleot ide-binding site (K120Q) or lacking the presumed SopB interaction domain do es not induce the reduction in linking number, suggesting that excess SopA disrupts the partition complex by interacting with SopB to remove positive supercoils in an ATP-dependent manner. Destabilization of mini-F also depen ds on sopC and SopB, but the K120Q mutant retains some capacity for destabi lizing mini-F. SopA-induced destabilization thus appears to be complex and may involve more than one SopA activity. The results are interpreted in ter ms of a regulatory role for SopA in the oligomerization of SopB dimers boun d to the centromere.