Crystallization, dehydration and preliminary X‐ray analysis of excisionase (Xis) proteins cooperatively bound to DNA

This paper describes the crystallization, dehydration and preliminary X‐ray data analysis of a complex containing several bacteriophage lambda excisionase (Xis) [Bushman et al. (1984). Cell , 39 , 699–706] proteins cooperatively bound to a 33‐mer DNA duplex (Xis–DNA X1‐X2 ). Xis is expected to recognize this regulatory element in a novel manner by cooperatively binding and distorting multiple head‐to‐tail orientated DNA‐binding sites. Crystals of this complex belonged to space group P 3 1 21 or P 3 2 21, with unit‐cell parameters a = b = 107.7, c = 73.5 Å, α = β = 90, γ = 120°. Based on the unit‐cell parameters for the asymmetric unit, V M is 3.0 Å 3  Da −1 , which corresponds to a solvent content of ∼59%. The approaches used to crystallize the unusually long DNA fragment in the complex and the dehydration technique applied that dramatically improved the diffraction of the crystals from 10 to 2.6 Å are discussed.