Catalytic RNA

Until recently, protein enzymeswere thought to be the only biologically active catalysts. It was believed that DNA stored the genetic information and that RNA played the role of an intermediate courier between the genetic messages contained in the DNA and the ribosomes, where proteins are synthesized. Other RNAs, like transfer RNAs (tRNAs) and ribosomal RNAs (rRNAs), were considered as helper molecules to assist the function of proteins. The discovery in the 1980s of catalytic RNAs revolutionized molecular biology (Cech et al., 1981; Guerrier-Takada et al., 1983). Today,RNAmolecules are the onlymolecules known both to store genetic information (as in the RNA viruses and viroids or during transport in the form of mRNA) and to exert biological catalysis. Catalytic RNAs, or ribozymes, are found widely in nature and therefore have a nondispensable biological activity. In the tRNA-processing enzyme ribonuclease P (RNase P), theRNAmoiety contains the catalytic activity. Self-splicing introns often occurwithin ribosomal genes, as in the rRNA of Tetrahymena thermophila. The human hepatitis delta virusRNAand some plant virusRNAs, like the tobacco ringspot virus satellite RNAs, require RNA self-cleavage and ligation for replication. The discovery of catalytic RNAs opened new vistas on the evolution of RNA and the origins of life. It is now tantalizing to imagine anRNAworld in whichRNA could store information for self-reproduction and self-processing without the need of proteins. Very recently, it has been established that protein synthesis on the ribosome is catalysed by the 23S ribosomal RNA compound and, thus, that the ribosome is actually a ribozyme (Nissen et al., 2000). Ribozymes need to acquire three-dimensional architectures to promote specific interactions with cofactors, especially divalent metal ions, and other functional domains for processing RNA substrates. Ribozymes are generally built upof several structural subdomainsmadeof helical segments connected by tertiary contacts. Functional regions are usually located in single-stranded regions, such as internal loops or bulges. Specific tertiary contacts occur between hairpin and internal loops, especially those positioned on the outside of the molecule. The subdomains have various functions and are responsible for substrate recognition, specific sequence alignment and catalytic activity, leading to a modular and hierarchically organized architecture. Some ribozymes, like the hammerhead ribozyme, the hairpin ribozyme and the RNAase P RNA, are under extensive clinical research for their ability to cleave other specifically chosen substrate RNAs. The therapeutic applications range from cleavage of viral RNAs, like the acquired immune deficiency syndrome (AIDS)-causing human immunodeficiency virus (HIV) RNA, to silencing of carcinogenic ormutated cellularRNAs, or the control of gene expression in vivo.