Using two-dimensional agarose gel electrophoresis, we show that mitochondrial DNA (mtDNA) replication of birds and mammals frequently entails ribonucleotide incorporation throughout the lagging strand (RITOLS). Based on a combination of two-dimensional agarose gel electrophoretic analysis and mapping of 5' ends of DNA, initiation of RITOLS replication occurs in the major non-coding region of vertebrate mtDNA and is effectively unidirectional. In some cases, conversion of nascent RNA strands to DNA starts at defined loci, the most prominent of which maps, in mammalian mtDNA, in the vicinity of the site known as the light-strand origin.

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 taurinecontaining modification ( m 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.

The mitochondrial tRNA Leu