Point mutations in the mitochondrial (mt) tRNA ^Leu(UUR) gene are responsible for mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), a subgroup of mitochondrial encephalomyopathic diseases. We previously showed that mt tRNA ^Leu(UUR) with an A3243G or T3271C mutation derived from patients with MELAS are deficient in a normal taurine-containing modification (τm ⁵ U; 5-taurinomethyluridine) at the anticodon wobble position. To examine decoding disorder of the mutant tRNA due to the wobble modification deficiency independent of the pathogenic point mutation itself, we used a molecular surgery technique to construct an mt tRNA ^Leu(UUR) molecule lacking the taurine modification but without the pathogenic mutation. This “operated” mt tRNA ^Leu(UUR) without the taurine modification showed severely reduced UUG translation but no decrease in UUA translation. We thus concluded that the UUG codon–specific translational defect of the mutant mt tRNAs ^Leu(UUR) is the primary cause of MELAS at the molecular level. This result could explain the complex I deficiency observed clinically in MELAS.