Figure 4.

(A) Stereoview of the 3F_obs–2F_calc electron density map (contoured at 1.4 σ level) around the active site of the ‘cleavage-competent’ Cre subunit and the kink region of the loxP site. A metal ion (possibly a magnesium) is indicated by an orange sphere. (B) DNA backbone superposition of the wild-type loxP sequence as observed in the present structure (in blue) and the symmetrized loxS sequence (PDBID 4CRX; the second duplex was generated by applying 2-fold symmetry). While the overall bending is the same, marked differences are found in the spacer region resulting from different locations of kinks. (C) Stereoview of the 3F_obs–2F_calc electron density map (contoured at 1.4 σ level) showing a magnesium-mediated protein–DNA contact.