The role of metal ions in RNA catalysis.

Understanding the catalytic mechanisms of RNA enzymes remains an important and intriguing challenge - one that has grown in importance since the recent demonstration that the ribosome is a ribozyme. At first, it seemed that all RNA enzymes compensate for the limited chemical versatility of ribonucleotide functional groups by recruiting obligatory metal ion cofactors to carry out catalytic chemistry. Mechanistic studies of the large self-splicing and pre-tRNA-processing ribozymes continue to support this idea, yielding increasingly detailed views of RNA active sites as scaffolds for positioning catalytic metal ions. Re-evaluation of the methodologies used to distinguish catalytic and structural roles for metal ions, however, has challenged this notion in the case of the small self-cleaving RNAs. Recent studies of the small ribozymes blur the distinction between catalytic and structural roles for metal ions, and suggest that RNA nucleobases have a previously unrecognized capacity for mediating catalytic chemistry.