Overexpression of the catalytic subunit of DNA polymerase γ results in depletion of mitochondrial DNA in Drosophila melanogaster

The mechanisms involved in the regulation of mitochondrial DNA (mtDNA) replication, a process that is crucial for mitochondrial biogenesis, are not well understood. In this study, we evaluate the role of DNA polymerase γ (pol γ), the key enzyme in mtDNA replication, in both Drosophila cell culture and in developing flies. We report that overexpression of the pol γ catalytic subunit (pol γ-α) in cultured Schneider cells does not alter either the amount of mtDNA or the growth rate of the culture. The polypeptide is properly targeted to mitochondria, yet the large excess of pol γ-α does not interfere with mtDNA replication under these conditions where the endogenous polypeptide is apparently present in amounts that exceed of the demand for its function in the cell. In striking contrast, overexpression of pol γ-α at the same level in transgenic flies interferes with the mtDNA replication process, presumably by altering the mechanism of DNA synthesis, suggesting differential requirements for, and/or regulation of, mtDNA replication in Drosophila cell culture versus the developing organism. Overexpression of pol γ-α in transgenic flies produces a significant depletion of mtDNA that causes a broad variety of phenotypic effects. These alterations range from pupal lethality to moderate morphological abnormalities in adults, depending on the level and temporal pattern of overexpression. Our results demonstrate that although cells may tolerate a variable amount of the pol γ catalytic subunit under some conditions, its level may be critical in the context of the whole organism.