Kinetic mechanism of rat polymerase β-dsDNA interactions. Fluorescence stopped-flow analysis of the cooperative ligand binding to a two-site one-dimensional lattice

Kinetics of cooperative binding of rat polymerase β to a double-stranded DNA has been studied using the fluorescence stopped-flow techniques. The data have been analyzed by an approach developed to examine complete kinetics of cooperative large ligand binding to a one-dimensional lattice. The method is based on using the smallest possible system that preserves key ingredients of cooperative binding; i.e., at saturation, the lattice can accept only two ligand molecules. It allows the identification of collective amplitudes as well as amplitudes describing particular normal modes of the reaction. The mechanism of the intrinsic binding of pol β to the dsDNA is different from the analogous mechanism for the ssDNA. The difference originates from different enzyme orientations in the corresponding complexes. Intrinsic binding to the dsDNA includes only two sequential steps: a very fast bimolecular association followed by an energetically favorable conformational transition of the complex. The transition following the bimolecular step does not facilitate the engagement of the enzyme in cooperative interactions. Its role seems to be reinforcing the affinity of the bimolecular step. Salt and magnesium cations affect both the bimolecular step and the conformational transition. As a result, the bimolecular step is less sensitive to the increased salt concentration, allowing the enzyme to preserve its initial dsDNA affinity. The changing character of cooperative interactions between bound enzyme molecules as a function of NaCl concentration and MgCl₂ concentration does not affect the binding mechanism. The engagement in cooperative interactions is ∼3-4 orders of magnitude slower than the conformational transition of the DNA-bound polymerase. The importance of the obtained results for the pol β activities is discussed.