| Abstract
| - We have studied the interaction between metal ions and the metal ion-binding motif inhammerhead ribozymes, as well as the functions of the metal ion at the motif, with heteronuclear NMRspectroscopy. In this study, we employed model RNA systems which mimic the metal ion-binding motifand the altered motif. In Co(NH3)6(III) titrations, we observed large 1H and 31P chemical shift perturbationsfor the motif and found that outer-sphere complexation of Co(NH3)6(III) is possible for this motif. From thereinvestigation of our previous 15N chemical shift data for Cd(II) binding, in comparison with those oforganometallic compounds, we conclude that Cd(II) can form an inner-sphere complex with the nucleobasein the motif. Therefore, the A9/G10.1 site was found to accept both inner-sphere and outer-spherecomplexations. The Mg(II) titration for a slightly different motif from the A9/G10.1 site (G10.1−C11.1 toA10.1−U11.1) revealed that its affinity to Mg(II) was drastically reduced, although the ribozyme with thisaltered motif is known to retain enzymatic activities. This observation suggests that the metal ion at thesemotifs is not a catalytic center of hammerhead ribozymes.
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