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Published June 2000 | Published
Journal Article Open

An unconventional origin of metal-ion rescue and inhibition in the Tetrahymena group I ribozyme reaction


The presence of catalytic metal ions in RNA active sites has often been inferred from metal-ion rescue of modified substrates and sometimes from inhibitory effects of alternative metal ions. Herein we report that, in the Tetrahymena group I ribozyme reaction, the deleterious effect of a thio substitution at the pro-Sp position of the reactive phosphoryl group is rescued by Mn2+. However, analysis of the reaction of this thio substrate and of substrates with other modifications strongly suggest that this rescue does not stem from a direct Mn2+ interaction with the Sp sulfur. Instead, the apparent rescue arises from a Mn2+ ion interacting with the residue immediately 3' of the cleavage site, A(+1), that stabilizes the tertiary interactions between the oligonucleotide substrate (S) and the active site. This metal site is referred to as site D herein. We also present evidence that a previously observed Ca2+ ion that inhibits the chemical step binds to metal site D. These and other observations suggest that, whereas the interactions of Mn2+ at site D are favorable for the chemical reaction, the Ca2+ at site D exerts its inhibitory effect by disrupting the alignment of the substrates within the active site. These results emphasize the vigilance necessary in the design and interpretation of metal-ion rescue and inhibition experiments. Conversely, in-depth mechanistic analysis of the effects of site-specific substrate modifications can allow the effects of specific metal ion-RNA interactions to be revealed and the properties of individual metal-ion sites to be probed, even within the sea of metal ions bound to RNA.

Additional Information

© 2000 RNA Society. The Authors acknowledge that six months after the full-issue publication date, the Article will be distributed under a Creative Commons CC-BY-NC License (Attribution-NonCommercial 4.0 International License, http://creativecommons.org/licenses/by-nc/4.0/). Manuscript accepted without revision April 5, 2000. We thank members of the Piccirilli group for modified oligonucleotides, L. Beigelman for oligonucleotide substrates, and members of the Herschlag group for comments on the manuscript. This work was supported by National Institutes of Health grant GM49243 to D.H.

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