No abstract

We report seven unrelated families with mitochondrial tRNA(Ser(UCN)) gene mutations at three different loci. A novel G7497A mutation is found in two families, both of which present with progressive myopathy, ragged-red fibers, lactic acidosis, and deficiency of respiratory chain complexes I and IV. This mutation presumably affects the tertiary tRNA(Ser(UCN)) dihydrouridine interaction. Mutations 7472 insC and T7512C, found in three and two families, respectively, are associated with myoclonus epilepsy, deafness, ataxia, cognitive impairment, and complex IV deficiency. No ragged-red fibers or ultrastructural abnormalities are seen. It is interesting that 6 of our 7 index patients are apparently homoplasmic, indicating a minor pathogenetic power of the tRNA(Ser(UCN)) mutations.

In marsupial mitochondria, the nucleotide residue at the second position of the anticodon of the tRNA for aspartic acid is changed post-transcriptionally such that the translational machinery recognizes it as a uracil rather than the cytosine residue encoded in the gene. By postlabeling nucleotide analysis, we show here that the cytosine residue is converted to a conventional uracil residue in an RNA editing event that affects approximately half of the tRNA molecules under steady state conditions. Furthermore, we have identified three different tRNAASP species which all carry three pseudouridines and two methylations but have the anticodons GCC, GUC and QUC respectively, the latter representing a rare example of queuine incorporation into a mitochondrial tRNA. This allows us to describe a likely sequential order of modification of the tRNAASP, where methylations and conversions of uridines to pseudouridines precede the editing event, while the exchange of guanine by queuine takes place after the C to U editing event.

The mitochondrial tRNA gene for lysine was analyzed in 11 different marsupial mammals. Whereas its location is conserved when compared with other vertebrate mitochondrial genomes, its primary sequence and inferred secondary structure are highly unusual and variable. For example, eight species lack the expected anticodon. Because the corresponding transcripts are not altered by any RNA-editing mechanism, the lysyl-tRNA gene seems to represent a mitochondrial pseudogene. Purification of marsupial mitochondria and in vitro aminoacylation of isolated tRNAs with lysine, followed by analysis of aminoacylated tRNAs, show that a nuclear-encoded tRNALys is associated with marsupial mitochondria. We conclude that a functional tRNALys encoded in the nuclear genome is imported into mitochondria in marsupials. Thus, tRNA import is not restricted to plant, yeast, and protozoan mitochondria but also occurs also in mammals.

RNA editing subsumes a number of functionally different mechanisms which have in common that they change the nucleotide sequence of RNA transcripts such that they become different from what would conventionally be predicted from their gene sequences. RNA editing has now been found in the organelles of numerous organisms as well as in a few nuclear transcripts. Most recently, it was shown to affect tRNAs in the mitochondria of several animals. The occurrence and evolutionary persistence of RNA editing is perplexing since backmutations in the genes might be assumed rapidly to eliminate the need for 'correction' of the gene sequences at the posttranscriptional level. Here, we review the recent RNA editing systems discovered in animal mitochondria and propose that they have arisen as a mechanism counteracting the accumulation of mutations that occurs in asexual genetic system.