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 P3₁21 or P3₂21, 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 ų 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.