Architecture of the γδ Resolvase Synaptosome Oriented Heterodimers Identify Interactions Essential for Synapsis and Recombination

linked and identically oriented res sites (Reed, 1981). gd resolvase catalyzes recombination within a comRecombination involves the binding of resolvase diplex nucleoprotein structure containing at least 12 remers to the two res sites, their synapsis, double strand solvase subunits bound to two 114 bp res sites. The cleavage at the crossover points, and exchange and 2,39 interaction between resolvase dimers is essential rejoining of the DNA strands. The DNA cleavage is acfor synapsis and recombination. Using oriented resolcomplished by direct attack of a resolvase side chain, vase heterodimers, we have identified half sites of res serine 10, on the phosphodiester bonds at the crossover that must be occupied by a 2,39-proficient protomer. point, forming an intermediate with resolvase covalently For synapsis, only four of the eight subunits bound to linked to the 59-phosphoryl ends of the broken DNA sites II and III, those at II-L and III-L, require a 2,39(Reed and Grindley, 1981; Reed and Moser, 1984; Hatfull proficient interface. This 2,39 interaction, apparently and Grindley, 1986). Analysis of the topological changes between protomers bound to adjacent sites, may nuthat accompany recombination revealed that the ascleate assembly of the core synaptic complex. For sembly of the synaptosome is carefully choreographed recombination, 2,39-proficient subunits are also re(Cozzarelli et al., 1984; Wasserman et al., 1985; Stark et quired at sites I-R and III-R, suggesting an important al., 1989). Synapsis traps three negative interdomainal communication between the crossover site and the supercoils; completion of synapsis activates the recomcore of the synapse. binational potential of the site I–bound dimers, and the