Structural diversity of self-cleaving ribozymes

In vitro selection was used to isolate Mg2+-dependent self-cleaving ribozym es from random sequence. Characterization of representative clones revealed the emergence of at least 12 classes of ribozymes that adopt distinct seco ndary structure motifs, Only one class corresponds to a previously known st ructural motif, that of the naturally occurring hammerhead ribozyme, Each r ibozyme promotes self-cleavage via an internal phosphoester transfer reacti on involving the adjacent 2'-hydroxyl group with a chemical rate enhancemen t of between 10(3)- and 10(6)-fold greater than the corresponding uncatalyz ed rate. These findings indicate that RNA can form a multitude of secondary and tertiary structures that promote cleavage by internal phosphoester tra nsfer. Upon further in vitro selection, a class I ribozyme that adopts an " X motif" structure dominates over all other ribozymes in the population. Th us, self-cleaving RNAs isolated by in vitro selection from random-sequence populations can rival the catalytic efficiency of natural ribozymes.