Nucleic Acids Research, 2000, Vol. 28, No. 8 1730-1742
© 2000 Oxford University Press
A further investigation and reappraisal of the thio effect in the cleavage reaction catalyzed by a hammerhead ribozyme
1National Institute for Advanced Interdisciplinary Research, Tsukuba Science City 305-8562, Japan, 2National Institute of Bioscience and Human Technology, Tsukuba Science City 305-8566, Japan and 3Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-8656, Japan
We synthesized three types of 11mer substrate, namely the natural substrate S11O and the thiosubstituted substrates S11SpS and S11RpS, in which the respective pro-Sp and pro-Rp oxygen atoms were replaced by sulfur, and subjected them to detailed kinetic analysis in the cleavage reaction catalyzed by a hammerhead ribozyme. In agreement with previous findings, in the presence of Mg2+ or Ca2+ ions the rate of ribozyme-catalyzed cleavage of S11SpS was as high as that of S11O, whereas the corresponding rate for S11RpS was nearly four orders of magnitude lower than that for either S11O or S11SpS. However, the rate of the ribozyme-catalyzed reaction with each of the three substrates was enhanced by Cd2+ ions. Such results have generally been taken as evidence that supports the direct interaction of the sulfur atom at the Rp position of the cleavage site with the added Cd2+ ion. However, our present analysis demonstrates that (i) the added Cd2+ ion binds at the P9 site; (ii) the bound Cd2+ ion at the P9 site replaces two Mg2+ or two Ca2+ ions, an observation that suggests a different mode of interaction with the added Cd2+ ion; and, most importantly and in contrast to the conclusion reached by other investigators, (iii) the Cd2+ ion does not interact with the sulfur atom at the Rp position of the scissile phosphate either in the ground state or in the transition state.
* To whom correspondence should be addressed at: Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-8656, Japan. Tel: +81 3 5841 8828 or +81 298 61 3015; Fax: +81 298 61 3019; Email: taira@chembio.t.u-tokyo.ac.jp
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