The accessory subunit of Xenopus laevis mitochondrial DNA polymerase gammaincreases processivity of the catalytic subunit of human DNA polymerase gamma and is related to class II aminoacyl-tRNA synthetases

Peptide sequences obtained from the accessory subunit of Xenopus laevis mit ochondrial DNA (mtDNA) polymerase gamma (pol gamma) 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 s equence, leaving a predicted mature protein with 419 amino acids and a mole cular mass of 47.3 kDa. This protein is associated with the larger, catalyt ic subunit in preparations of active mtDNA polymerase, The small subunit ex hibits homology to its human, mouse, and Drosophila counterparts. Interesti ngly, significant homology to glycyl-tRNA synthetases from prokaryotic orga nisms reveals a likely evolutionary relationship. Since attempts to produce an enzymatically active recombinant catalytic subunit of Xenopus DNA poly 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 gamma puri fied from baculovirus-infected insect cells. These experiments provide the first functional evidence that the small subunit of DNA pol gamma stimulate s processive DNA synthesis by the human catalytic subunit under physiologic al salt conditions.