Chemical shift mapping of γδ resolvase dimer and activated tetramer: Mechanistic implications for DNA strand exchange

Several static structural models exist for γδ resolvase, a self-coded DNA recombinase of the γδ transposon. While these reports are invaluable to formulation of a mechanistic hypothesis for DNA strand exchange, several questions remain. Foremost among them concerns the protomer structural dynamics within the protein/DNA synaptosome. Solution NMR chemical shift assignments have been made for truncated variants of the natural wild-type dimer, which is inactive without the full synaptosome structure, and a mutationally activated tetramer. Of the 134 residues, backbone 1H, 15N, and 13Cα assignments are made for 121–124 residues in the dimer, but only 76–80 residues of the tetramer. These assignment differences are interpreted by comparison to X-ray diffraction models of the recombinase dimer and tetramer. Inspection of intramolecular and intermolecular structural variation between these models suggests a correspondence between sequence regions at subunit interfaces unique to tetramer, and the regions that can be sequentially assigned in the dimer but not the tetramer. The loss of sequential context for assignment is suggestive of stochastic fluctuation between structural states involving protomer–protomer interactions exclusive to the activated tetrameric state, and may be indicative of dynamics which pertain to the recombinase mechanism.