The RNase P associated with HeLa cell mitochondria contains an essential RNA component identical in sequence to that of the nuclear RNase P

The mitochondrion-associated RNase P activity (mtRNase P) was extensively p urified from HeLa cells and shown to reside in particles with a sedimentati on constant (similar to 17S) very similar to that of the nuclear enzyme (nu RNase P). Furthermore, mtRNase P, like nuRNase P, was found to process a mi tochondrial tRNA(Ser(UCN)) precursor [ptRNA(Ser(UCN))] at the correct site. Treatment with micrococcal nuclease of highly purified mtRNase P confirmed earlier observations indicating the presence of an essential RNA component . Furthermore, electrophoretic analysis of 3'-end-labeled nucleic acids ext racted from the peak of glycerol gradient-fractionated mtRNase P revealed t he presence of a 340-nucleotide RNA component, and the full-length cDNA of this RNA was found to be identical in sequence to the H1 RNA of nuRNase P. The proportions of the cellular H1 RNA recovered in the mitochondrial fract ions from HeLa cells purified by different treatments were quantified by No rthern blots, corrected on the basis of the yield in the same fractions of four mitochondrial nucleic acid markers, and shown to be 2 orders of magnit ude higher than the proportions of contaminating nuclear U2 and U3 RNAs. In particular, these experiments revealed that a small fraction of the cell H 1 RNA (of the order of 0.1 to 0.5%), calculated to correspond to similar to 33 to similar to 175 intact molecules per cell, is intrinsically associate d with mitochondria and can be removed only by treatments which destroy the integrity of the organelles. In the same experiments, the use of a probe s pecific for the RNA component of RNase MRP showed the presence in mitochond ria of 6 to 15 molecules of this RNA per cell. The available evidence indic ates that the levels of mtRNase P detected in HeLa cells should be fully ad equate to satisfy the mitochondrial tRNA synthesis requirements of these ce lls.