Ribozymes as therapeutic agents and genetic tools
INTRODUCTION Ribozymes are RNA molecules capable of acting as enzymes even in the complete absence of proteins. They have the catalytic activity of breaking and/or forming covalent bonds with extraordinary specificity, accelerating the rate of these catalytic reactions. The ability of RNA to serve as a catalyst was first shown for the self-splicing Group I intron of Tetrahymena and the RNA moiety of RNAse P (Cech, 1990; GuerrierTakada et al., 1983; Kruger et al., 1982). Group I introns, which range in size from 200 to 1500 nucleotides, are classified by phylogenetically conserved secondary structures, as well as by certain functional characteristics. These introns possess many of the attributes of protein enzymes, including sub-strate and cofactor binding domains. The well studied Tetrahymena intron has been shown to harbor important tertiary interactions with the substrate molecule and to undergo conformational shifts. It can also be engineered to bind, cleave and release multiple substrates. RNAse P is a ubiquitous enzyme that cleaves the 5′ precursor segment from pre-transfer RNA molecules. The holoenzyme is comprised of at least one protein and one RNA subunit (ranging in size from 140 to 490 nucleotides, depending on the organism) (Kurz and Fierke, 2000). The RNA of the bacterial enzyme alone, under the appropriate salt and ionic conditions, can site specifically cleave the 5′ leader segment from precursor t-RNAs (Guerrier-Takada et al., 1983).