title: Mitochondrial topoisomerase I sites in the regulatory D-loop region of mitochondrial DNA.
abstract: Mitochondrial DNA (mtDNA) is required for mitochondrial activities because it encodes key proteins for oxidative phosphorylation and the production of cellular ATP. We previously reported the existence of a specific mitochondrial topoisomerase gene, Top1mt, in all vertebrates. The corresponding polypeptide contains an N-terminal mitochondrial targeting sequence and is otherwise highly homologous to the nuclear topoisomerase I (Top1). In this study, we provide biochemical evidence of the presence of an endogenous Top1mt polypeptide in human mitochondria. Using novel antibodies against Top1mt, we detected the corresponding 70 kDa polypeptide in mitochondria but not in nuclear fractions. This polypeptide could be trapped to form covalent complexes with mtDNA when mitochondria from human cells were treated with camptothecin. Mapping of Top1mt sites in the regulatory D-loop region of mtDNA in mitochondria revealed the presence of an asymmetric cluster of Top1mt sites confined to a 150 bp segment downstream from, and adjacent to, the site at which replication is prematurely terminated, generating an approximately 650-base (7S DNA) product that forms the mitochondrial D-loop. Moreover, we show that inhibition of Top1mt by camptothecin reduces the level of formation of the 7S DNA. These results suggest novel roles for Top1mt in regulating mtDNA replication.
authors.affiliation: Laboratory of Molecular Pharmacology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892-4255, USA.
authors.forename: Hongliang
authors.initials: H
authors.lastname: Zhang
authors.forename: Yves
authors.initials: Y
authors.lastname: Pommier
mesh_terms: D001483:Base Sequence; D004247:DNA / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D004264:DNA Topoisomerases, Type I / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D004272:DNA, Mitochondrial / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008928:Mitochondria / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D008970:Molecular Weight; D017434:Protein Structure, Tertiary / Q000235:genetics; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D012045:Regulatory Sequences, Nucleic Acid
chemical_list: D004272:DNA, Mitochondrial; D011994:Recombinant Proteins; D004247:DNA; D004264:DNA Topoisomerases, Type I