Tetraloop–receptor interaction with metal ions observed by NMR and by X-ray crystallography. Superimposition of the 20 lowest-energy NMR structures (with cobalt hexammine ions and with manganese ions) and the three crystal structures (PDB ID 1GID, 1HR2, and 1U6B) over the tetraloop–receptor interaction. For simplicity, only the RNA from the lowest-energy structure with manganese ions is shown, with the tetraloop region in red and the receptor in blue. Ions from all five structures are displayed, including the superimposition of 20 ions from 20 NMR structures. Metal ion association sites are numbered, corresponding to numbers in Figures 5 and 6. Site 2 contains manganese (green, this study), magnesium (pink, PDB ID 1HR2), and potassium (violet, PDB ID 1U6B). Site 3 contains manganese (green, this study), cobalt hexammine (gold, this study), and magnesium (pink, PDB ID 1GID). Site 5 contains manganese (green, this study), cobalt hexammine (gold, this study), and cobalt hexammine (pink, PDB ID 1GID).