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RNase P: Variations and Uses*

This essay will bring to date a picture of the properties of RNase P from several organisms and a summary of how this enzyme can be used to decrease specific gene expression. Current details of how the enzyme works and other features governing its reaction are reviewed elsewhere (1–3). RNase P is responsible for generating the mature 5 -end of tRNAs by a single endonucleolytic cleavage of their precursors. It is an essential, ubiquitous enzyme present in all cells and cellular compartments that synthesize tRNA: bacterial cells, eukaryotic nuclei, mitochondria, and chloroplasts. The essential function in vivo of RNase P has been demonstrated in those systems amenable to genetic analysis such as bacteria (4) and yeast nuclei (5) and mitochondria (6, 7). All known RNase P enzymes are ribonucleoproteins and contain an RNA subunit essential for catalysis with the possible exception of RNase P in some plant chloroplasts and trypanosome mitochondria (8, 9). The chemical mechanism of RNase P involves essential divalent metal ions (2) and is thought to be an in-line SN2 displacement reaction (1). The endonucleolytic cleavage generates 5 -phosphate and 3 -hydroxyl end groups in the products. For our purposes, the way in which the enzyme recognizes substrates is an important feature of its ability to lower the amount of any particular RNA and expression inside cells. Natural substrates can be reduced to two oligonucleotides, which when hydrogen bonded together (Fig. 1) resemble sufficiently the essential features of a substrate so that one of the oligonucleotides, the target RNA (i.e. any RNA inside the cell), is cleaved efficiently by the enzyme and inactivated. This important aspect of RNase P revolves entirely around its substrate recognition mechanism and does not depend on the fact that there is an RNA subunit in the enzyme.