Identification of a β-like DNA Polymerase Activity in Bovine Heart Mitochondria

A new DNA polymerase activity, distinct from DNA polymerase γ, has been identified in bovine heart mitochondria. First detected among proteins isolated in a complex with mitochondrial DNA, the DNA polymerase activity has been partially purified 47,000-fold. Enzyme activity separates from DNA polymerase γ on several chromatographic columns and appears to copurify with a 38 ± 2-kDa polypeptide. Unlike DNA polymerase γ, this enzyme is relatively resistant to inhibition by N-ethylmaleimide and dideoxynucleotides, has moderately low monovalent and high divalent cation requirements, and possesses 20-fold-higher apparent Km values for deoxynucleotides. The enzyme polymerizes deoxynucleotides onto a primed template DNA in a relatively nonprocessive fashion and lacks a detectable 3′ to 5′ exonuclease activity. Many of these characteristics resemble a β-like mitochondrial DNA polymerase previously identified in, and considered unique to, trypanosomes. We propose that the bovine and trypanosomal enzymes are related and represent a new class of ubiquitous mitochondrial DNA polymerases.