Mapping of control elements in the displacement loop region of bovine mitochondrial DNA

The genomes of mammalian mitochondria are duplex DNA circles. The two major transcriptional promoters and the origin of DNA replication for one DNA strand are located in a single region which contains no structural genes and occupies about 6% of the genome. This region is called the displacement loop (D-loop) region since it is often found as a novel triplex structure in which the heavy strand of the genome has been partially replicated. This nascent single-stranded DNA segment remains hybridized to the light strand, displacing the heavy strand of the genome over much of the D-loop region. The promoters and the sites of initiation of D-loop DNA synthesis have been mapped in the human and mouse genomes and may show limited sequence conservation. We have mapped these sites in the bovine mitochondrial genome. Some features are conserved between all three species; however, the promoters and the sites of initiation of D-loop DNA synthesis show no primary sequence homology among all species. This lack of sequence homology is in contrast to the greater than 80% sequence conservation which has been reported in portions of the D-loop region which are located distal to the origin of DNA replication and far from the transcriptional promoters. These results imply that closely related species may have developed different means of controlling mitochondrial gene expression.