A structural view of Cre-loxP site-specific recombination

Structural models of site-specific recombinases from the lambda integrase f amily of enzymes have in the last four years provided an important new pers pective on the three-dimensional nature of the recombination pathway. Membe rs of this family, which include the bacteriophage pi Cre recombinase, bact eriophage lambda integrase, the yeast Flp recombinase, and the bacterial Xe rCD recombinases, exchange strands between DNA substrates in a stepwise pro cess. One pair of strands is exchanged to form a Holliday junction intermed iate, and the second pair of strands is exchanged during resolution of the junction to products. Crystal structures of reaction intermediates in the C re-loxP site-specific recombination system, together with recent biochemica l studies in the field, support a "strand swapping" model for recombination that does not require branch migration of the Holliday junction intermedia te in order to test homology between recombining sites.