Crystal structure and deletion analysis show that the accessory subunit ofmammalian DNA polymerase gamma, Pol gamma B, functions as a homodimer

Polymerase gamma, which replicates and repairs mitochondrial DNA, requires the Pol gammaB subunit for processivity. We determined the crystal structur e of mouse Pol gammaB, a core component of the mitochondrial replication ma chinery. Pol gammaB shows high similarity to glycyl-tRNA synthetase and dim erizes through an unusual intermolecular four-helix bundle. A human Pol gam maB mutant lacking the four-helix bundle failed to dimerize in solution or to stimulate the catalytic subunit Pol gammaA, but retained the ability to bind with Pol gammaA to a primer-template construct, indicating that the fu nctional holoenzyme contains two Pol gammaB molecules. Other mutants retain ed stimulatory activity but lost the ability to bind folded ssDNA. These re sults suggest that the Pol gammaB dimer contains distinct sites for Pol gam maA binding, dimerization, and DNA binding.