Monomer-dimer equilibrium of the pSC101 RepA protein

The pSC101 RepA protein, which is required for plasmid DNA replication, but is inhibitory to replication at high concentration, has been found in both monomeric and dimeric forms. While RepA monomers bind to direct repeat iterons near the pSC101 replication origin, dimers bind to sequences that autoregulate RepA synthesis. We investigated the solution properties of purified RepA protein by analytical ultracentrifugation analysis, and found that RepA exists in Escherichia coli cells in a monomer-dimer equilibrium (Kd = 4 microM), and, moreover, that RepA is primarily in the monomeric form at the concentration (500 molecules per cell; 2 microM) we found by Western blot analysis to occur in cells carrying replicating wild-type pSC101 plasmids. However, at concentrations inhibitory to pSC101 DNA replication, the majority of RepA molecules exist as dimers. Our findings provide experimental support for the proposal that the equilibrium between monomer and dimer forms of RepA has a key role in determining its effect on the replication of pSC101.