Characterization of human mitochondrial RNase P: novel aspects in tRNA processing

Eukaryotic ribonuclease P: increased complexity to cope with the nuclear pre-tRNA pathway

Ribonuclease P is an ancient enzyme that cleaves pre-tRNAs to generate mature 5' ends. It contains an essential RNA subunit in Bacteria, Archaea, and Eukarya, but the degree to which the RNA subunit relies on proteins to supplement catalysis is highly variable. The eukaryotic nuclear holoenzyme has recently been found to contain almost twenty times the protein content of the bacterial enzymes, in addition to having split into at least two related enzymes with distinct substrate specificity. In this review, recent progress in understanding the molecular architecture and functions of nuclear forms of RNase P will be considered.

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 purified from HeLa cells and shown to reside in particles with a sedimentation constant ( approximately 17S) very similar to that of the nuclear enzyme (nuRNase P). Furthermore, mtRNase P, like nuRNase P, was found to process a mitochondrial 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 extracted from the peak of glycerol gradient-fractionated mtRNase P revealed the 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 fractions from HeLa cells purified by different treatments were quantified by Northern 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 magnitude higher than the proportions of contaminating nuclear U2 and U3 RNAs. In particular, these experiments revealed that a small fraction of the cell H1 RNA (of the order of 0.1 to 0.5%), calculated to correspond to approximately 33 to approximately 175 intact molecules per cell, is intrinsically associated 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 specific for the RNA component of RNase MRP showed the presence in mitochondria of 6 to 15 molecules of this RNA per cell. The available evidence indicates that the levels of mtRNase P detected in HeLa cells should be fully adequate to satisfy the mitochondrial tRNA synthesis requirements of these cells.

Mitochondrial tRNA import: are there distinct mechanisms?

Sequence information from an increasing number of complete mitochondrial genomes indicates that a large number of evolutionary distinct organisms import nucleus-encoded tRNAs. In the past five years, much research has been initiated on the features of imported tRNAs, the mechanism and the energetics of the process as well as on the components of the import machinery. In summary, these studies show that the import systems of different species exhibit some unique features, suggesting that more than one mechanism might exist to import tRNAs.

Effect of peptidyltransferase inhibitors on ribonuclease P activity from Dictyostelium discoideum. Effect of antibiotics on RNase P

The effect of several peptidyltransferase inhibitors on ribonuclease P activity from Dictyostelium discoideum was investigated. Among the inhibitors tested puromycin, amicetin and blasticidin S revealed a dose-dependent inhibition of tRNA maturation. Blasticidin S and amicetin do not compete with puromycin for the same site on the enzyme, suggesting the existence of distinct antibiotic binding sites on D. discoideum RNase P. Inhibition experiments further indicate that binding sites for blasticidin S and amicetin overlap.

Impairment of tRNA processing by point mutations in mitochondrial tRNA(Leu)(UUR) associated with mitochondrial diseases

Several point mutations in mitochondrial tRNA genes have been linked to distinct clinical subgroups of mitochondrial diseases. A particularly large number of different mutations is found in the tRNA(Leu)(UUR) gene. We show that base substitutions at nucleotide position 3256, 3260, and 3271 of the mitochondrial genome, located in the D and anticodon stem of this tRNA, and mutation 3243 changing a base involved in a tertiary interaction, significantly impair the processing of the tRNA precursor in vitro. In correlation with other studies, our results suggest that inefficient processing of certain mutant variants of mitochondrial tRNA(Leu)(UUR) is a primary molecular impairment leading to mitochondrial dysfunction and consequently to disease.