The excised leader of human cytochrome c oxidase subunit I mRNA which contains the origin of mitochondrial DNA light-strand synthesis accumulates in mitochondria and is polyadenylated.

We identified a polyadenylated RNA species which contains the origin of human mitochondrial DNA light-strand synthesis and the surrounding complementary sequences of the four light-strand-encoded tRNAs. This RNA (RNA 9L) is probably derived from the leader portion of RNA 6 which is excised during the formation of the mature cytochrome c oxidase subunit mRNA (RNA 9). The high degree of secondary structure of this RNA is presumably responsible for its anomalous electrophoretic behavior in denaturing polyacrylamide gels.