The catalytic core of Escherichia coli DNA polymerase III contains three tightly associated subunits (α, ε, and θ). The refinement of the three-dimensional structure of the θ-subunit was completed by the NMR group. The θ-subunit has three α-helices in the N-terminal two thirds of the protein that fold to form a three helix bundle. As part of a program aimed at understanding the molecular mechanism of the core, we have set out to investigate the association of the θ- and ε-subunits. The structure of the θ-subunit bound to ε- has been refined using an innovative technique that combines NOE restraints with distance and orientation restraints calculated from a paramagnetic centre located in the active site of ε. We have mapped the binding surface of ε on θ to a hydrophobic patch on θ using advanced NMR techniques. The structure of the complex between ε and θ has been assembled using the same paramagnetic restraints that were used to refine the structure of ε. The structure is now complete and the work was recently published in the Journal of Bacteriology.