Mitochondrial tRNALeu isoforms in lung carcinoma cybrid cells containing the np 3243 mtDNA mutation

We have investigated the representation of structural isoforms of the two mitochondrial leucyl tRNAs in lung carcinoma cybrid cell lines containing the np 3243 (MELAS) mtDNA mutation, alone or in combination with the np 12300 suppressor mutation. The mutant tRNA(Leu)(UUR) is aminoacylated very poorly or not at all, whereas the suppressor tRNA(Leu)(CUN) is efficiently aminoacylated. Deacylated mitochondrial tRNA(Leu)(CUN) is present, in all human cells tested, in two structural isoforms that are separable on denaturing gels, indicating a difference in primary structure. The ratio of the two isoforms differs between cell types and is strongly biased towards one isoform in lung carcinoma cybrids containing high levels of the np 3243 mutation, compared with control cybrids, We propose that structural modification of tRNA(Leu)(CUN) could be a natural suppression mechanism for the np 3243 and other mitochondrial tRNA(Leu)(UUR) mutations and could underlie some of the phenotypic variability of np 3243 disease.