Arm Sequences Contribute to the Architecture and Catalytic Function of a λ Integrase-Holliday Junction Complex

λ integrase (Int) mediates recombination between attachment sites on λ phage and E. coli DNAs. With the assistance of accessory proteins that induce DNA loops, Int bridges pairs of distinct arm- and core-type DNA binding sites to form synapsed recombination complexes, which then recombine via a Holliday junction (HJ) intermediate. We show that, in addition to promoting the proper positioning of Int protomers, the arm sequences facilitate the catalytic activities of the Int tetramer, independent of accessory proteins or physical continuity between the arm and core sites. We have determined the architecture of ternary complexes containing a HJ, Int, and P′1,2 arm-type DNA. These structures accommodate simultaneous binding of Int to direct-repeat arm sites and indirect-repeat core sites and afford a new view of the higher-order recombinogenic complexes.