Dissociation kinetics of RepA dimers: implications for mechanisms of activation of DNA binding by chaperones

Background: The replication initiator of plasmid P1, RepA, binds DNA as monomer. The binding is stimulated by the chaperones DnaJ, DnaK and GrpE of Escherichia coli. Two models of chaperone action have been proposed. (i) Chaperones dissociate RepA dimers, which are inactive in DNA binding, into active monomers. (ii) The dissociation occurs spontaneously but the monomeric products require the chaperones for refolding into the active form. The latter model was based on the observation that RepA diluted 1000-fold below the KD for dimer dissociation, still required the chaperones for DNA binding. Results: We have confirmed that under the condition of DNA binding experiments, the RepA dimers dissociate reversibly into monomers with a KD value of 1.1 ± 0.1 μm. In the vicinity of this concentration, the sedimentation coefficient of RepA was concentration dependent, allowing estimation of s20,w coefficients for the RepA monomer (2.95 S) and dimer (4.01 S). Dynamic light scattering experiments indicated an increase of the monomer fraction within 5 min of RepA dilution. Circular dichroism (CD) measurements were consistent with these results. Conclusion: RepA monomerization is efficient without the mediation of chaperones. They are required to activate RepA most likely because they are needed to re-fold RepA monomers.