The Catalytic Activity of RNase P Is Due to RNA

KEY CONCEPTS
- Ribonuclease P (RNase P) is a ribonucleoprotein in which the RNA has catalytic activity.
- RNase P is essential for bacteria, archaea, and eukaryotes.
- RNase MRP in eukaryotes is related to RNase P and is involved in rRNA processing and degradation of cyclin B mRNA.
One of the first demonstrations of the catalytic capabilities of RNA was provided by the analysis of RNase P from E. coli. Although originally identified in bacteria, RNase P has been identified as an essential endonuclease involved in tRNA processing in most, if not all, bacterial, archaeal, and eukaryotic organisms.

In its simplest form, bacterial RNase P can be dissociated into two components: a base RNA of 350 to 400 nucleotides and a single protein subunit. The RNA subunit from bacteria, when isolated in vitro, displays catalytic activity. RNase P from archaea and eukaryotes consists of a single RNA structurally related to that found in bacteria, but it has a higher protein content and the RNA has little, if any, catalytic activity when examined in vitro. Typically, archaeal RNase P has four proteins, whereas the yeast version has 9 proteins and the human version has 10 proteins. In all cases, the protein component is required to support RNase P activity in vivo. Mutations in either the gene for the RNA or the gene for the protein can inactivate RNase P in vivo, proving that both components are necessary for natural enzyme activity. Originally it was assumed that the protein provided the catalytic activity, while the RNA filled some subsidiary role—for example, assisting in the binding of substrate, as it has some short sequences complementary to exposed regions of tRNA. However, these roles are reversed, with the RNA actually providing the catalytic activity while the protein provides structural support.
Analyzing the results as though the RNA were an enzyme, each “enzyme” catalyzes the cleavage of multiple substrates. Although the catalytic activity resides in the RNA, the protein component greatly increases the speed of the reaction, as seen in the increase in turnover number .
In addition to RNase P, eukaryotes have another essential RNAbased endonuclease, RNase MRP (mitochondrial RNA processing). This endonuclease is composed of a structurally related catalytic RNA and shares many of the same protein subunits that are found in RNase P. While originally identified for its role in processing mitochondrial RNAs, RNase MRP functions mainly in the nucleus, processing precursor ribosomal RNA. RNase MRP may also play an important role in cell cycle regulation, given that it is involved in degradation of cyclin B mRNA. Identification of RNase MRP is provocative, as it appears that the protein component is largely conserved between RNase P and RNase MRP, with the change in substrate specificity provided by exchanging the catalytic RNA.

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