Ribonuclease P (RNase P) recognizes precursor transfer RNA (pre-tRNA) and processes it to generate mature tRNAs that are used for assembling proteins. Unlike almost all other enzymes, RNase P is a ribozyme, an enzyme with an active site that is composed of RNA, and it is present in every living organism. RNase P is among the most ancient of enzymes, a living molecular fossil from an “RNA world” in which life is thought to have originated. On page 657 of this issue, Lan et al. (1) present structures of the yeast RNase P enzyme by itself and bound to its pre-tRNA substrate. Additionally, the structure of the human form, by itself and bound to its tRNA product, is reported by Wu et al. (2). These reveal the detailed mechanism by which RNase P hydrolyzes pre-tRNA to produce the required 5′-phosphorylated tRNA of exactly the correct length. These structures unambiguously reveal how an assortment of proteins conspire to form a measuring device that ensures that the pre-tRNA substrate is correctly processed by the catalytic RNA subunit of this universal and essential enzyme.
