The contributions of DNA polymerases alpha, delta, and epsilon to SV40 and nuclear DNA syntheses were evaluated. Proteins were UV‐crosslinked to nascent DNA within replicating chromosomes and the photolabelled polymerases were immunopurified. Only DNA polymerases alpha and delta were detectably photolabelled by nascent SV40 DNA, whether synthesized in soluble viral chromatin or within nuclei isolated from SV40‐infected cells. In contrast, all three enzymes were photolabelled by the nascent cellular DNA. Mitogenic stimulation enhanced the photolabelling of the polymerases in the alpha>delta>epsilon order of preference. The data agree with the notion that DNA polymerases alpha and delta catalyse the principal DNA polymerisation reactions at the replication fork of SV40 and, perhaps, also of nuclear chromosomes. DNA polymerase epsilon, implicated by others as a cell‐cycle checkpoint regulator sensing DNA replication lesions, may be dispensable for replication of the small, fast propagating virus that subverts cell cycle controls.

We have previously proposed that DNA polymerase a-primase provides short RNA-DNA precursors below 40 nucleotides (DNA primers), several ofwhich assemble into an Okazaki piece after intervening RNA has been removed and the gaps have been filled by DNA polymerase 8 (or £) (T.