How many catalytic RNAs? Ions and the Cheshire cat conjecture.

Three well-characterized RNA catalysts not only require Mg2+ for activity, but also bind a metal ion (or ions) within the active site, apparently in a catalytic rather than solely structural role. I suggest, in view of the general catalytic utility of bound ions, that catalytic RNAs be viewed as Cheshire cats, by dimming their complex three-dimensional ribonucleotide structure to leave only the sharp mineral parts in view. That is, catalytic RNAs may be viewed as metalloenzymes, with the burdens of catalysis frequently borne by specifically poised metal ions. Comparison to modern protein metalloenzymes predicts particular RNA metallocatalysts that may be possible presently, and in a hypothetical ancestral RNA world that did not encode peptide catalysts. In support of this view, known catalytic RNAs can be considered Cheshire catalysts; that is, they have apparent cognates among the protein metalloenzymes.