Distinctive activities of DNA polymerases during human DNA replication

The contributions of human DNA polymerases (pols) α, δ and ε during S-phase progression were studied in order to elaborate how these enzymes co-ordinate their functions during nuclear DNA replication. Pol δ was three to four times more intensely UV cross-linked to nascent DNA in late compared with early S phase, whereas the cross-linking of pols α and ε remained nearly constant throughout the S phase. Consistently, the chromatin-bound fraction of pol δ, unlike pols α and ε, increased in the late S phase. Moreover, pol δ neutralizing antibodies inhibited replicative DNA synthesis most efficiently in late S-phase nuclei, whereas antibodies against pol ε were most potent in early S phase. Ultrastructural localization of the pols by immuno-electron microscopy revealed pol ε to localize predominantly to ring-shaped clusters at electron-dense regions of the nucleus, whereas pol δ was mainly dispersed on fibrous structures. Pol α and proliferating cell nuclear antigen displayed partial colocalization with pol δ and ε, despite the very limited colocalization of the latter two pols. These data are consistent with models where pols δ and ε pursue their functions at least partly independently during DNA replication.