A MAGNESIUM-ION CORE AT THE HEART OF A RIBOZYME DOMAIN

Large ribozymes require divalent metal ions to fold. We show here that the tertiary structure of the Tetrahymena group I intron P4-P6 domain nucleates around a magnesium ion core. In the domain crystal structur e, five magnesium ions bind in a three-helix junction at the centre of the molecule. Single atom changes in any one of four magnesium sites in this three-helix junction destroy folding of the entire 160-nucleot ide P4-P6 domain. The magnesium ion core may be the RNA counterpart to the protein hydrophobic core, burying parts of the RNA molecule in th e native structure.