FIGURE 1.

(A) A schematic diagram of a bidentate RNA clamp of magnesium, formed when adjacent phosphate groups enter the first coordination shell of a common magnesium ion. A 10-membered ring (shaded yellow) characterizes these bidentate RNA clamps. (B) Adjacent phosphate groups enter the first coordination sphere of a common phosphate more commonly than nonadjacent phosphates. A bidentate RNA clamp of magnesium corresponds to one residue in the frequency graph below (frequency from the H. marismortui 23S rRNA, PDB entry 1JJ2). Extending the horizontal axis (not shown) reveals local phosphates enter the first shell of a common magnesium ion preferentially over remote phosphates.