ION-INDUCED FOLDING OF THE HAMMERHEAD RIBOZYME - A FLUORESCENCE RESONANCE ENERGY-TRANSFER STUDY

The ion-induced folding transitions of the hammerhead ribozyme have be en analysed by fluorescence resonance energy transfer. The hammerhead ribozyme may be regarded as a special example of a three-way RNA junct ion, the global structure of which has been studied by comparing the d istances (as energy transfer efficiencies) between the ends of pairs o f labelled arms for the three possible end-to-end vectors as a functio n of magnesium ion concentration. The data support two sequential ion- dependent transitions, which can be interpreted in the light of the cr ystal structures of the hammerhead ribozyme. The first transition corr esponds to the formation of a coaxial stacking between helices II and III; the data can be fully explained by a model in which the transitio n is induced by a single magnesium ion which binds with an apparent as sociation constant of 8000-10 000 M-1. The second structural transitio n corresponds to the formation of the catalytic domain of the ribozyme , induced by a single magnesium ion with an apparent association const ant of similar to 1100 M-1. The hammerhead ribozyme provides a well-de fined example of ion-dependent folding in RNA.