Figure 6. The λ excision complex structure.
(A) View towards the attR-derived half of the complex, where the path of the P arm can be seen. A sharp IHF (yellow) directed bend at the H2 site, combined with the cooperative binding and bending by three Xis molecules (gold) and the Int subunit bridging between the B' and P2 sites (magenta), redirects the P arm over the top of the Int tetramer. The Int-C NTD is not present in the model because it could not be unambiguously fit into EM density (approximate location is indicated in the corresponding cartoon). B) View towards the attL-derived half of the complex, where IHF bound at the H' site sharply bends the P' arm and enables Int subunits to bridge between the C' and P'1 sites (brown) and between the B and P'2 sites (blue). (C) View from the top of the excision complex, where the P'-arm (from attL) and P-arm (from attR) run nearly parallel and close to one another. An intimate interface is formed between the Xis subunits and the Int-C' and Int-B NTDs. The unoccupied, but accessible binding site for Fis is indicated. In the schematic views of the excision complex shown below the structural models, the DNA strands near the junction center are omitted for clarity. For reference, the brackets labeled attP/attB indicate the relative positions of the excision products. See Video 1 for multiple views of the excisive complex structure and isolation of individual subunits.