Model for a DNA‐mediated synaptic complex suggested by crystal packing of gamma delta resolvase subunits.

The packing arrangement of the 12 subunits of intact gamma delta resolvase in the unit cell of a hexagonal crystal form suggests a model for site‐specific recombination that involves a DNA‐mediated synaptic intermediate. The crystal structure has been determined by molecular replacement and partially refined at 2.8/3.5 A resolution. Although the small DNA‐binding domain is disordered in these crystals, packing considerations show that only a small region of space in the crystal could accommodate a domain of its size. A family of related models for a synaptic complex between two DNA duplexes and 12 monomers that are arranged as situated in the crystal is consistent with the known topology of the complex and the distances between the three resolvase dimer‐binding sites per DNA; further, these models place the two DNA recombination sites in contact with each other between two resolvase dimers, implying that strand exchange is accomplished through direct DNA‐DNA interaction. A major role postulated, then, for the resolvase protein assembly is to stabilize a res DNA structure that is close to the topological transition state of the reaction.