Continuous synthesis of long DNA chains by chick embryo DNA polymerase γ

Abstract

There is evidence for at least two mechanisms of DNA replication in eukaryotic cells. One is nuclear DNA replication in which DNA strands are synthesised in relatively short pieces (4–5S) that are later elongated and joined together^1–6. These short DNA intermediates are also observed in the replication of viral DNA such as polyoma virus^7,8 and simian virus (SV40)^11,12, but not in adenovirus DNA^13,19 and mitochondrial DNA¹⁵, where longer DNA chains are synthesised continuously. Studies of the subcellular localisation and the inhibitors of DNA polymerases^16,17 suggest that DNA polymerase α, β and γ are involved in nuclear DNA replication, DNA repair and mitochondrial DNA replication, respectively. And it is suggested that DNA polymerase α, β and γ are involved in the replication of SV40 DNA¹⁸ and adenovirus DNA¹⁹, respectively. Does the difference between the two types of replication depend on the difference in the reaction property of DNA polymerases α and γ? DNA polymerase a from human KB cells incorporated about 11 nucleotides per binding event²⁰. Using mouse enzymes we found that one DNA polymerase β molecule polymerises dTMP on the multiple oligo(dT) primers with poly(rA) templates in a highly discontinuous fashion, while DNA polymerase γ may synthesise rather long poly(dT) chains in a one enzyme–one product fashion. We report here that using nearly homogeneous chick embryo DNA polymerase γ, the results clearly indicating that one DNA polymerase γ molecule synthesises one long DNA in a highly progressive fashion.