HUMAN MITOCHONDRIAL TRANSFER-RNA PROCESSING

tRNA processing is a central event in mammalian mitochondrial gene exp ression. We have identified key enzymatic activities (ribonuclease P, precursor tRNA 3'-endonuclease, and ATP(CTP)-tRNA-specific nucleotidyl transferase) that are involved in HeLa cell mitochondrial tRNA maturat ion. Different mitochondrial tRNA precursors are cleaved precisely at the tRNA 5'- and 3'-ends in a homologous mitochondrial in vitro proces sing system, The cleavage at the 5'-end precedes that at the 3'-end, a nd the tRNAs are substrates for the specific CCA addition in the same in vitro system, Using a comparative enzymatic approach as well as bio chemical and immunological techniques, we furthermore demonstrate that human cells contain two distinct enzymes that remove 5'-extensions fr om tRNA precursors, the previously characterized nuclear and the newly identified mitochondrial ribonuclease P. These two cellular isoenzyme s have different substrate specificities that seem to be well adapted to their structurally disparate mitochondrial and nuclear tRNA substra tes. This kind of approach may also help to understand the structural diversities and commonalities of tRNAs.