γδ resolvase dimer and tetramer structures. (a) Inactive dimer: WT γδ resolvase dimer bound to its cognate crossover site (PDBid 1gdt) . Subunits are coloured yellow and pale yellow, with the side chains of the active site serines, which are distant from the DNA backbone, shown as red spheres. Helices B, D, and E are labelled for comparison with later figures. (b) Active tetramer: An activated γδ resolvase tetramer synapsing two crossover sites (PDBid 1zr4) . The DNA is cleaved, with the 5′ ends covalently attached to the serines, and 2nt 3′ overhangs unpaired in the centre.