The gamma subfamily of DNA polymerases: cloning of a developmentally regulated cDNA encoding Xenopus laevis mitochondrial DNA polymerase gamma

Abstract

We used the known sequence of the Saccharomyces cerevisiae DNA polymerase γ to clone the genes or cDNAs encoding this enzyme in two other yeasts, Pychia pastoris and Schizosaccharomyces pombe, and one higher eukaryote, Xenopus laevis. To confirm the identity of the final X.laevis clone, two antisera raised against peptide sequences were shown to react with DNA polymerase γ purified from X.laevis oocyte mitochondria. A developmentally regulated 4.6 kb mRNA is recognized on Northern blots of oocyte RNA using the X.laevis cDNA. Comparison of the four DNA polymerase γ gene sequences revealed several highly conserved sequence blocks, comprising an N-terminal 3′→5′ exonuclease domain and a C-terminal polymerase active center interspersed with γ-specific gene sequences. The consensus sequences for the DNA polymerase γ exonuclease and polymerase domains show extensive sequence similarity to DNA polymerase I from Escherichia coli. Sequence conservation is greatest for residues located near the active centers of the exo and pol domains of the E.coli DNA polymerase I structure. The domain separating the exonuclease and polymerase active sites is larger in DNA polymerase γ than in other members of family A (DNA polymerase I-like) polymerases. The S.cerevisiae DNA polymerase γ is atypical in that it includes a 240 residue C-terminal extension that is not found in the other members of the DNA polymerase γ family, or in other family A DNA polymerases.