Mapping the active site of ribonuclease P RNA using a substrate containing a photoaffinity agent.

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Abstract
Ribonuclease P RNA is the catalytic moiety of the ribonucleoprotein enzyme that removes precursor sequences from 5′-ends of pre-tRNAs. A photoaffinity cross-linking agent was coupled to the substrate phosphate on which RNase P acts and used to map nucleotides in the vicinity of the catalytic site of this ribozyme. Mature tRNA(Phe) containing a 5′-thiophosphate was synthesized by transcription in vitro using phage T7 RNA polymerase in the presence of guanosine 5′-phosphorothioate. The photoagent (azidophenacyl) was coupled uniquely to the 5′-thiophosphate of the tRNA, the site of action by RNase P. The photoagent-containing tRNA binds to RNase P RNA and is cross-linked by UV irradiation to it at high efficiency (10-30%). Cross-linked conjugates are enzymatically inactive, consistent with the occupancy of the active site of the RNase P RNA by the tRNA. Reversal of the cross-link by phenylmercuric acetate restores activity. The sites of cross-linking in RNase P RNA were determined by primer extension. In order to identify generalities and detect idiosyncrasies, analyses were carried out using RNase P RNAs from three phylogenetically diverse organisms: Bacillus subtilis, Chromatium vinosum and Escherichia coli. In the context of a phylogenetic structure model, two regions of cross-linking are observed in all three RNAs. Two of the RNAs cross-link to a lesser extent at a third structural region and one of the RNAs is cross-linked to a small extent to a fourth region. All the sites of cross-linking between the substrate phosphate in tRNA and the RNase P RNAs are in the conserved core of the structure model, consistent with the importance of the cross-linked residues to the action of this RNA enzyme.