Fig. 4. The junction crossovers contain unique ion binding positions.

Stereoscopic view, using J21 in the 4 × 5 system, 2F_o–F_c electron density accounting for the bases at the crossover regions are contoured at σ = 2.0, and the individual ion positions 1 and 2 (indicated Pos1 and Pos2) are independently contoured in the corresponding electron density at σ = 4.0. The presence of arsenic at these sites was substantiated by transferring the crystals into TAE-Mg²⁺ (40 mM Tris, 20 mM acetate, and 1 mM EDTA pH = 8.6), and subsequently freezing the crystals. The crystals were scanned at the arsenic K edge (λ = 1.04 Å) where the corresponding arsenate peak was present. No other components within the crystallization buffers could account for the resulting peaks in the F_o–F_c difference maps for the ions at their corresponding sites. Atoms are indicated using the following: carbon (gray), nitrogen (blue), oxygen (red), phosphate (orange), and arsenic ions (green spheres).