Structural features of the RihA-DAPIR complex. A) Superposition of the four crystallographically-independent molecules of RihA. The Cα trace of the four RihA monomers are shown in grey, and the αL (red) and α9 (green) segments are labelled for clarity. The four independent RihA chains are virtually undistinguishable. The αL loop is completely visible in chain A, while the full α9 helix could be traced only in chain D, as per nomenclature of the deposited coordinates. B) The DAPIR molecule bound to the active site of chain A superimposed with a (2mFo-DFc, ϕc) shake-omit electron density map contoured at 1.2σ. C) Octacoordination of the active site Ca2+ ion. Five oxygen atoms from RihA amino acid residues, the O2' and O3' hydroxyls of DAPIR, and an highly-ordered water molecule complete the coordination sphere of the metal.