B Cocrystallisation of FICD variants with MgATP results in electron densities for nucleotide and the inhibitory Glu234. Unbiased polder (OMIT) maps for ATP (± Mg) and Glu234 are shown as blue and purple meshes, respectively. (i) The wild-type dimer FICD structure displays a lack of density corresponding to a Mg2+ ion. The ATP density is contoured at 3.5 σ and the Glu234 at 5.0 σ. (ii) The dimeric dimer relay mutant FICDK256A displays a clear MgATP density up to and including the γ-phosphate phosphorous atom. The ATP density and Glu234 densities are both contoured at 3.0 σ. (iii) Monomeric FICDL258D shows a clear MgATP density. The ATP density is contoured at 3.0 σ and the Glu234 at 5.0 σ. All residues and water molecules interacting with ATP (± Mg) are shown as sticks and coloured by heteroatom. Mg2+ coordination complex pseudo-bonds are show in purple dashed lines. 