DNA Polymerase γ, The Mitochondrial Replicase¹

DNA polymerase (pol) γ is the sole DNA polymerase in animal mitochondria. Biochemical and genetic evidence document a key role for pol γ in mitochondrial DNA replication, and whereas DNA repair and recombination were thought to be limited or absent in animal mitochondria, both have been demonstrated in recent years. Thus, the mitochondrial replicase is also apparently responsible for the relevant DNA synthetic reactions in these processes. Pol γ comprises a catalytic core in a heterodimeric complex with an accessory subunit. The two-subunit holoenzyme is an efficient and processive polymerase, which exhibits high fidelity in nucleotide selection and incorporation while proofreading errors with its intrinsic 3′ → 5′ exonuclease. Incorporation of nucleotide analogs followed by proofreading failure leads to mitochondrial toxicity in antiviral therapy, and misincorporation during DNA replication leads to mitochondrial mutagenesis and dysfunction. This review describes our current understanding of pol γ biochemistry and biology, and it introduces other key proteins that function at the mitochondrial DNA replication fork.