Overexpression of the catalytic subunit of DNA polymerase gamma results indepletion of mitochondrial DNA in Drosophila melanogaster

The mechanisms involved in the regulation of mitochondrial DNA (mtDNA) repl ication, a process that is crucial for mitochondrial biogenesis, are not we ll understood. In this study, we evaluate the role of DNA polymerase gamma (pol gamma), the key enzyme in mtDNA replication, in both Drosophila cell c ulture and in developing flies. We report that overexpression of the pol ga mma catalytic subunit (pol gamma-alpha) in cultured Schneider cells does no t alter either the amount of mtDNA or the growth rate of the culture. The p olypeptide is properly targeted to mitochondria, yet the large excess of po l gamma-alpha does not interfere with mtDNA replication under these conditi ons where the endogenous polypeptide is apparently present in amounts that exceed of the demand for its function in the cell. In striking contrast, ov erexpression of pol gamma-alpha at the same level in transgenic flies inter feres with the mtDNA replication process, presumably by altering the mechan ism of DNA synthesis, suggesting differential requirements for, and/or regu lation of, mtDNA replication in Drosophila cell culture versus the developi ng organism. Overexpression of pol gamma-alpha 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 morpholo gical 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 gamma catalytic subunit under some conditions , its level may be critical in the context of the whole organism.