Thirty-three phage clones carrying DNAs homologous to human mitochondrial DNA (mtDNA) were isolated from two independently constructed human gene libraries, and the region and extent of homology of mtDNA-like sequences carried by these clones were examined in hybridization experiments. Each phage clone contained DNA sequences homologous to various parts of the mtDNA and the extent of homology differed from clone to clone. From the efficiency of the library screening, it was estimated that human nuclear DNA contains at least several hundred copies of mtDNA-like fragments. Four clones carrying nuclear DNA sequences homologous to the mitochondrial Unidentified Reading Frame (URF) 4 and URF5 regions were chosen for further studies, and their structures were analyzed by DNA sequencing. Comparison of these mtDNA-like sequences with that of mtDNAs of several mammalian species revealed conservation of a part of the structures present in direct ancestral mtDNAs. The mtDNA fragments seem to have been continuously integrated into mammalian nuclear DNA during evolution.
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