Fig. 4. The coordination geometries of the metal ions and
the spermine molecule. (a) The Sr2+ ion (purple) is coordinated
to eight guanine O6 atoms. (b) Three Ca^2+ ions are coordinated
to U6 (Ca1), U7 (Ca2), and U12 (Ca3) residues, respectively. (c)
Coordination of the spermine molecule (blue) in stereoview.
Hydrogen bonds are shown in white dashed lines. Individual RNA
strands are depicted in different colors.