Mapping of nascent light and heavy strand transcripts on the physical map of HeLa cell mitochondrial DNA.

The sequences complementary to the nascent RNA molecules isolated from transcription complexes of HeLa cell mtDNA have been mapped on the H and L strands of mtDNA by the S1 protection technique. The distribution of these sequences among different Hpa II restriction fragments was found to reflect the position of these fragments in the Hpa II map of mtDNA. Thus, the S1-resistant hybrids formed with the L strand corresponded almost exclusively to the right half of the genome past the origin of replication in the direction of L strand transcription, and were especially concentrated in the region immediately adjacent to the origin. By contrast, the hybrid duplexes involving the H strand appeared to be localized in the left half of the genome, and in particular in the quadrant of the map adjacent to the origin in the direction of H strand transcription. These results strongly suggest that the region of mtDNA around the origin of replication contains an initiation site for L strand transcription and an initiation site for H strand transcription.