Protein sequences conserved in prokaryotic aminoacyl-tRNA synthetases are important for the activity of the processivity factor of human mitochondrial DNA polymerase

Previous studies have shown that the small subunit of Xenopus DNA polymerase γ (pol γB) acts as a processivity factor to stimulate the 140 kDa catalytic subunit of human DNA polymerase γ. A putative human pol γB initially identified by analysis of DNA sequence had not been shown to be functional, and appeared to be an incomplete clone. In this paper, we report the cloning of full-length human and mouse pol γB. Both human and mouse pol γB proteins were expressed in their mature forms, without their apparent mitochondrial localization signals, and shown to stimulate processivity of the recombinant catalytic subunit of human pol γA. Deletion analysis of human pol γB indicated that blocks of sequence conserved with prokaryotic class II aminoacyl-tRNA synthetases are necessary for activity and interaction with human pol γA. Purification of DNA pol γ from HeLa cells indicated that both proteins are associated in vivo.