Characteristics of Ribonuclease P from Various Organisms

Ribonuclease P (RNase P) is an endoribonuclease required for the maturation of the 5 ' termini of tRNAs from their precursors (for review, see Altman et al. 1986). This enzyme, which engages in a single cleavage event to generate a 5 ' phosphate in its tRNA products in vivo and in vitro, has been best characterized in extracts of Escherichia coli. It has a catalytic RNA subunit (M1 RNA) and a protein subunit (C5 protein; Guerrier-Takada etal. 1983). Under certain conditions in vitro, the RNA subunit alone can carry out the cleavage of tRNA precursors. This reaction, catalyzed by M1 RNA, has all the features of a true enzymatic reaction, and its mechanism differs from those of the reactions governed by other RNA enzymes (Table 1). Although RNase P generates the same end groups in its products as do group I self-splicing introns during the cleavage step performed by these introns, the RNase P reaction is not a transesterification (for review, see Cech and Bass 1986; Been et al.; Forster et al.; Epstein and Gall; all this volume), nor has a covalent linkage been detected, transient or otherwise, between the en-zyme and substrate. The RNase P reaction is also clear-ly different from the autocleavage r actions performed by virusoid RNA or transcripts of newt satellite DNA. In these latter cases, the end groups in the cleavage products are 5'-OH and 3'-P. If we assume that these catalytic RNAs were present in primitive biochemical systems, then several different catalytic mechanisms governed by RNAs participated in the earliest stages of evolution.