RNase P is one of the key enzymes in the processing of tRNA primary transcripts [Transfer RNA: Synthesis].
RNase P is a ribozyme. Most of the enzyme consists of an RNA molecule called RNA P and the rest is composed of small proteins. In bacteria there is a single protein subunit while in eukaryotes there are up to eight small proteins bound to the RNA component.
RNA P, by itself, can catalyze the cleavage reaction [Monday's Molecule #58]. The role of the protein is simply to facilitate the reaction.1
The structure of the RNA component from two different species has recently been published. The one shown here is RNA P from Thermophilus maritima (reviewed in Baird et al. 2007). This catalytic RNA is found in all species and it's the classic example of an RNA that can catalyze a reaction in the absence of protein. Sidney Altman received the Nobel Prize in 1989 for demonstrating that the activity was confined to the RNA part of the holoenzyme.
The exact structure of the complete holoenzyme (RNA + protein) is not known but the evidence suggest a model such as the one shown on the left (Smith et al. 2007). The RNA is blue, the protein subunit is red, and the bound tRNA precursor is brown. Note that the protein subunit is positioned at the site of the cleavage near the 5′ end of the mature tRNA.
Part of the RNA ribozome is interacting with the TΨC loop of the tRNA molecule. This loop is present in all tRNAs which explains why the RNase P enzyme can cleave all tRNA precusors no matter which particular tRNA going to be produced.
There are two different types of RNase P depending on the species. Although both of them have similar catalytic RNAs they differ in size of the RNA and in the proteins that are bound to it.
1 When the reaction is carried out under in vivo concentrations of ionic strength, temperature etc., the protein component is absolutely required in order to get significant activity.
Baird, N.J., Fang, X.W., Srividya, N., Pan, T. and Sosnick, T.R. (2007) Folding of a universal ribozyme: the ribonuclease P RNA. Quarterly Rev. Biophys. 40:113-161. [doi:10.1017/S0033583507004623] [PubMed]
Smith, J.K., Hsieh, J. and Fierke, C.A. (2007) Importance of RNA-protein interactions in bacterial ribonuclease P structure and catalysis. Biopolymers 87:329-38. [PubMed]