RNase P — a 'Scarlet Pimpernel'

RNase P is responsible for the maturation of the 5'-termini of tRNA molecules in all cells studied to date. This ribonucleoprotein has to recognize and identify its cleavage site on a large number of different precursors. This review covers what is currently known about the function of the catalytic subunit of Escherichia coli RNase P, M1 RNA, and the protein subunit, C5, in particular with respect to cleavage-site selection. Recent genetic and biochemical data show that the two C residues in the 3'-terminal CCA sequence of a precursor interact with the enzyme through Watson-Crick base-pairing. This is suggested to result in unfolding of the amino acid acceptor-stem and exposure of the cleavage site. Furthermore, other close contact points between M1 RNA and its substrate have recently been identified. These data, together with the two existing three-dimensional structure models of M1 RNA in complex with its substrate, establish a platform that will enable us to seek an understanding of the underlying mechanism of cleavage by this elusive enzyme.