Structure and Catalytic Function in Ribonuclease P

for removing the 5 ' precursor segments from tRNA during its maturation. RNase P is particularly interest-ing because its catalytic element is an RNA, not a protein (Guerrier-Takada et al. 1983; Gardiner et al. 1985). Although the recognition of RNase P as a catalytic RNA was preceded by the discovery of a self-splicing intron in some Tetrahymena 26S rRNA precursors (Kruger et al. 1982), the RNase P RNA differs in an important way: It engages in intermolecu-lar reactions. In contrast, the self-splicing intron ac-tivity in vivo is a series of intramolecular rearrange-ments that collectively result in the excision of the intron and the ligation of the flanking exons (for re-view, see Cech 1985). RNase P therefore offers not only a model for RNA catalytic mechanisms, but also a