The Accessory Subunit of Xenopus laevis Mitochondrial DNA Polymerase γ Increases Processivity of the Catalytic Subunit of Human DNA Polymerase γ and Is Related to Class II Aminoacyl-tRNA Synthetases

Peptide sequences obtained from the accessory subunit of Xenopus laevis mitochondrial DNA (mtDNA) polymerase γ (pol γ) were used to clone the cDNA encoding this protein. Amino-terminal sequencing of the mitochondrial protein indicated the presence of a 44-amino-acid mitochondrial targeting sequence, leaving a predicted mature protein with 419 amino acids and a molecular mass of 47.3 kDa. This protein is associated with the larger, catalytic subunit in preparations of active mtDNA polymerase. The small subunit exhibits homology to its human, mouse, and Drosophila counterparts. Interestingly, significant homology to glycyl-tRNA synthetases from prokaryotic organisms reveals a likely evolutionary relationship. Since attempts to produce an enzymatically active recombinant catalytic subunit of Xenopus DNA pol γ have not been successful, we tested the effects of adding the small subunit of the Xenopus enzyme to the catalytic subunit of human DNA pol γ purified from baculovirus-infected insect cells. These experiments provide the first functional evidence that the small subunit of DNA pol γ stimulates processive DNA synthesis by the human catalytic subunit under physiological salt conditions.