Crystallization and preliminary X-ray crystallographic analysis of the excisionase-DNA complex from bacteriophage λ

Bacteriophage λ uses an elegantly regulated and highly directional site‐specific DNA‐recombination reaction to integrate and excise its genome. A critical regulator of this process is the phage‐encoded excisionase (Xis) protein, which dramatically stimulates excision by orchestrating the assembly of a higher order nucleoprotein structure that excises the prophage. The Xis protein stabilizes this recombination intermediate by substantially altering the trajectory of viral DNA and by cooperatively interacting with the λ integrase (Int) protein. In an attempt to understand how Xis controls the directionality of bacteriophage λ recombination, co‐crystals of the DNA‐binding domain of Xis in complex with its binding site within the P‐arm of the phage have been obtained using the hanging‐drop vapor‐diffusion method. Using sodium acetate as a precipitating reagent, the Xis–DNA complex crystallizes in space group C2, with unit‐cell parameters a = 80.2, b = 72.7, c = 38.8 Å, β = 104.1°. These crystals diffract beyond 1.5 Å resolution and are well suited for structural analysis using X‐ray crystallography.