The hammerhead is the smallest of the nucleolytic ribozymes, that undergo backbone cleavage by a transesterification reaction in the presence of magnesium ions. The RNA is induced to fold into its active conformation by the binding of metal ions in two stages. These generate domain 2, the scaffold on which the ribozyme is built, and domain 1, the active centre of the ribozyme. Further local structural rearrangement during the activation of the ribozyme is suggested by a number of crystal structures. The 10(5)-fold rate enhancement is probably brought about by a combination of metal-ion participation and stereochemical factors in the environment of the folded RNA structure.