Ribozymes : tructure and mechanism in RNA catalysis

A NEW FIELD in enzymology has emerged in the past decade with the discovery that RNA can act as an enzyme. First discovered in the cellular RNA-splicing and processing machinery in the form of self-splicing group 1 introns’ and precursor tRNA-processing RNase P (Ref. 2) RNA catalytic activity in a number of smaller RNAs has subsequently been identified3-5. The small, naturally occurring catalytic RNAs are generally found in the genomes of RNA viruses and in virus-related RNAs, which are believed to replicate by a rollingcircle mechanism. Recently, the crystal structure of one of these small catalytic RNAs, the hammerhead ribozyme, was elucidated by two research groups using different approaches. The first structure was that of the hammerhead ribozyme, in which the catalytic or ‘enzyme’ strand was composed of RNA and the RNA substrate was replaced with a ‘substrateanalogue’ strand composed of DNA. The