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Published June 25, 2000 | Published
Journal Article Open

The Mitochondrial Myopathy, Encephalopathy, Lactic Acidosis, and Stroke-like Episode Syndrome-associated Human Mitochondrial tRNALeu(UUR) Mutation Causes Aminoacylation Deficiency and Concomitant Reduced Association of mRNA with Ribosomes


The pathogenetic mechanism of the mitochondrial tRNALeu(UUR) A3243G transition associated with the mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome has been investigated in transmitochondrial cell lines constructed by transfer of mutant mitochondrial DNA (mtDNA)-carrying mitochondria from three genetically unrelated MELAS patients or of isogenic wild-type mtDNA-carrying organelles into human mtDNA-less cells. An in vivo footprinting analysis of the mtDNA segment within the tRNALeu(UUR) gene that binds the transcription termination factor failed to reveal any difference in occupancy of sites or qualitative interaction with the protein between mutant and wild-type mtDNAs. Cell lines nearly homoplasmic for the mutation exhibited a strong (70-75%) reduction in the level of aminoacylated tRNALeu(UUR) and a decrease in mitochondrial protein synthesis rate. The latter, however, did not show any significant correlation between synthesis defect of the individual polypeptides and number or proportion of UUR codons in their mRNAs, suggesting that another step, other than elongation, may be affected. Sedimentation analysis in sucrose gradient showed a reduction in size of the mitochondrial polysomes, while the distribution of the two rRNA components and of the mRNAs revealed decreased association of mRNA with ribosomes and, in the most affected cell line, pronounced degradation of the mRNA associated with slowly sedimenting structures. Therefore, several lines of evidence indicate that the protein synthesis defect in A3243G MELAS mutation-carrying cells is mainly due to a reduced association of mRNA with ribosomes, possibly as a consequence of the tRNALeu(UUR) aminoacylation defect.

Additional Information

Copyright © 2000 by the American Society for Biochemistry and Molecular Biology. Received for publication, October 28, 1999, and in revised form, March 29, 2000. Originally published In Press as doi:10.1074/jbc.M908734199 on April 14, 2000. We thank Susan Lai, Shili Duan, Benneta Keeley, and Arger Drew for technical assistance. This work was supported by National Institutes of Health Grant GM-11726 (to G. A.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

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