Assembly and activation of site-specific recombination complexes

Site-specific recombination is responsible for a broad range of biological phenomena, including DNA inversion, resolution of transposition intermediat es, and the integration and excision of bacteriophage genomes. Integration of mycobacteriophage L5 is catalyzed by a phage-encoded integrase with reco mbination occurring between specific attachment sites on the phage and myco bacterial chromosomes (attP and attB, respectively). Although some site-spe cific recombination systems simply involve binding of the recombinase to th e sites of strand exchange, synapsis, and recombination, phage systems typi cally require the assembly of higher-order structures within which the reco mbinational potential of integrase is activated. The requirement for these structures derives from the necessity to regulate the directionality of rec ombination-either integration or excision-which must be closely coordinated with other aspects of the phage growth cycles. We show herein that there a re multiple pathways available for the assembly of L5 recombination complex es, including the early synapsis of the attP and attB DNAs. This process is in contrast to the model for lambda integration and illustrates the differ ent usage of molecular machineries to accomplish the same biological outcom e.