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14 Ribonuclease P

Sidney Altman, Leif Kirsebom


RNase P was first characterized in Escherichia coli as an enzyme that is required for the processing of the 5′ termini of tRNA in the pathway of biosynthesis of tRNA from precursor tRNAs (ptRNAs; Altman and Smith 1971; for review, see Altman 1989). Subsequently, it was discovered that this enzyme consisted of one protein and one RNA subunit, the latter being the catalytic subunit (Guerrier-Takada et al. 1983). In retrospect, it should not be surprising that an enzyme with the chemical composition of RNase P exists. Ribosomes are also ribonucleoproteins (RNPs), but they are much more complicated than RNase P. It seems unreasonable that ribosomes, with three RNAs and about 50 proteins, would exist without less complex RNPs having come into existence first and having persisted throughout evolution. Indeed, relatively simple RNPs with and without catalytic activity have been discovered with regularity during the past 20 years.

What important questions about RNase P have not yet been answered? Certainly, for the biochemist, problems not yet solved for E. coli RNase P include a full understanding of enzyme–substrate interactions, RNA–protein subunit interactions, and the chemical details of the hydrolysis reaction. In every case, much progress has been made, but much has yet to be learned.

A second issue of interest to biochemists and “evolutionists” is the puzzle presented by the different compositions of RNase P from eubacteria (one catalytic RNA and one protein subunit) and from eukaryotes (one RNA subunit and several protein subunits: no understanding yet of which...

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