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

We have investigated the representation of structural isoforms of the twomitochondrial leucyl tRNAs in lung carcinoma cybrid cell lines containingthe np 3243 (MELAS) mtDNA mutation, alone or in combination with the np12300 suppressor mutation. The mutant tRNALeu(UUR) is aminoacylated verypoorly or not at all, whereas the suppressor tRNALeu(CUN) is efficientlyaminoacylated. Deacylated mitochondrial tRNALeu(CUN) is present, in allhuman cells tested, in two structural isoforms that are separable ondenaturing gels, indicating a difference in primary structure. The ratio ofthe two isoforms differs between cell types and is strongly biased towardsone isoform in lung carcinoma cybrids containing high levels of the np 3243mutation, compared with control cybrids. We propose that structuralmodification of tRNALeu(CUN) could be a natural suppression mechanism forthe np 3243 and other mitochondrial tRNALeu(UUR) mutations and couldunderlie some of the phenotypic variability of np 3243 disease.