NMR structure of the N-terminal domain of E-coli DnaB helicase:: implications for structure rearrangements in the helicase hexamer

Background: DnaB is the primary replicative helicase in Escherichia coli. Native DnaB is a hexamer of identical subunits, each consisting of a larger C-terminal domain and a smaller N-terminal domain. Electron-microscopy data show hexamers with C-6 or C-3 symmetry, indicating large domain movements and reversible pairwise association. Results: The three-dimensional structure of the N-terminal domain of E. coli DnaB was determined by nuclear magnetic resonance (NMR) spectroscopy. Structural similarity was found with the primary dimerisation domain of a topoisomerase, the gyrase A subunit from E. coli. A monomer-dimer equilibrium was observed for the isolated N-terminal domain of DnaB. A dimer model with C-2 symmetry was derived from intermolecular nuclear Overhauser effects, which is consistent with all available NMR data. Conclusions: The monomer-dimer equilibrium observed for the N-terminal domain of DnaB is likely to be of functional significance for helicase activity, by participating in the switch between C-6 and C-3 symmetry of the helicase hexamer.