Mismatch-specific 3'----5' exonuclease associated with the mitochondrial DNA polymerase from Drosophila embryos.

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Abstract
The mitochondrial DNA polymerase from Drosophila embryos lacks dNTP turnover activity. However, a potent 3'' .fwdarw. 5'' exonuclease activity can be detected by a specific assay in which the exonuclease excises mispaired nucleotides at the 3'' termini of primed synthetic and natural DNA templates. The excision of a mispaired nucleotide occurs at a significantly greater rate than excision of a correctly paired nucleotide and, under conditions of DNA synthesis, hydrolysis of a mispaired terminal nucleotide occurs prior to primer extension. The 3'' .fwdarw. 5'' exonuclease copurifies quantitatively with DNA polymerase .gamma. and cosediments with the nearly homogeneous enzyme under native conditions. These results suggest that the 3'' .fwdarw. 5'' exonuclease provides a proofreading function to enhance the fidelity of DNA synthesis during Drosophila mitochondrial DNA replication.