Two separate projects are being pursued. One of these involves studies on the enzymatic replication of mitochondrial DNA. The other involves a recent finding of mitochondrial polymorphism in rats. In connection with the former project, we have isolated and purified a DNA topoisomerase from rat liver mitochondria and plan to study its properties further. In particular, we are interested in its action in concert with mitochondrial DNA polymerase in the replication of mitochondrial DNA. We have also obtained evidence for the presence of a DNA gyrase in rat liver mitochondria and are attempting to isolate it and also use it in a "coupled" enzyme system to study the replication of mitochondrial DNA. In connection with the mitochondrial DNA polymorphism project, we plan to investigate whether the variability of mitochondrial DNA in the rat population is expressed phenotypically, in polymorphism of the protein gene products of mitochondrial DNA. We also plan to extend our studies on mitochondrial DNA polymorphism to the worldwide rat population, looking at intra- as well as interspecific variability of mitochondrial DNA.