The A----G transition at nucleotide 3243 of the mitochondrial tRNA(Leu)(UUR)) gene has been associated with MELAS, a maternally-inherited mitochondrial disorder. We recently transferred mitochondria harboring this mtDNA mutation into a human cell line devoid of endogenous mtDNA (rho degrees cells), and showed: (1) decreased rate of synthesis and of steady-state levels of mitochondrial translational products, (2) reduced respiratory chain function and (3) increased amounts of a novel unprocessed RNA species (termed by us RNA 19) derived from transcription of the 16S rRNA + tRNA(Leu)(UUR) + ND 1 genes. Because RNA 19 contains rRNA sequences, we propose that this molecule is incorporated into mitochondrial ribosomes, and interferes disproportionately with mitochondrial translation, thereby causing the phenotypic changes associated with MELAS.