SUMMARYMitochondria are required to convert food into usable energy forms and every cell contains thousands of them. Unlike most other cellular compartments, mitochondria have their own genomes (mtDNA) that encode for 13 of the about 90 proteins present in the respiratory chain. All proteins necessary for mtDNA replication, as well as transcription and translation of mtDNA-encoded genes, are encoded in the nucleus. Mutations in nuclear-encoded proteins required for mtDNA maintenance is an important cause of neurodegeneration and muscle diseases. The common result of these defects is either mtDNA depletion or accumulation of multiple deletions of mtDNA in postmitotic tissues.Research in my laboratory will elucidate the molecular mechanisms and regulation of mitochondrial DNA replication in human cells. We will establish how mtDNA is packaged into nucleoprotein complexes, a.k.a. nucleoids and establish how these nucleoids are selected for mtDNA replication. We will elucidate the molecular mechanisms by which specific mutations in the mtDNA replication machinery affect mtDNA maintenance and cause human disease.Mitochondrial dysfunction is not limited to rare, genetic disorders, but also associated with age-associated common diseases as well as with the normal aging process. I will use my biochemical insights in combination with mouse genetics to address the hypothesis that increased mtDNA mutation load may be an important cause of normal aging.My specific aims will be:Aim 1. To define how initiation of mtDNA replication at OriH is regulated.Aim 2. To identify and characterize regulators of mtDNA replication.Aim 3. To characterize the structure and function of the mtDNA nucleoid in DNA replication.Aim 4. To address the mitochondrial theory of ageing