The major site for regulation of replication and transcription of animal mitochondrial DNA is the displacement loop. Transcription of each strand, heavy and light, emanates from this region and the origin of heavy-strand, replication is located down-stream of the light-strand promoter. It is known that the light-strand promoter is responsible both for transcription of light-strand genes as well as for priming of DNA synthesis. The efficient transcription system is composed of at least two protein entities with a transcription factor, capable of binding specific regions of mitochondrial DNA, being required in addition to mitochondrial RNA polymerase. Processing of a transcript complementary to the replication origin sequence is involved in displacement-loop metabolism and is effected by a site-specific endoribonuclease; this processing activity is a ribonucleoprotein. A currently identified RNA component of the endonuclease is a nucleus-encoded species that, by definition, must be imported to the organelle matrix.