RNA folding: beyond Watson-Crick pairs

Several crystal structures of RNA fragments, alone or in complex with a spe cific protein, have been recently solved. In addition, the structures of an artificial ribozyme, the leadzyme, and the cleavage product of a human pat hogen ribozyme, have extended the structural diversity of ribozyme architec tures. The attained set of folding rules and motifs expand the repertoire s een previously in tRNA structures.