Distinct populations of human PCNA are required for initiation of chromosomal DNA replication and concurrent DNA repair

The integrity of genomic DNA during the cell division cycle in eukaryotic cells is maintained by regulated chromosomal DNA replication and repair of damaged DNA. We have used fractionation and reconstitution experiments to purify essential factors for the initiation of human chromosomal DNA replication in late G1 phase template nuclei from human cells. Here, we report the identification of soluble PCNA as an essential initiation factor in this system. Recombinant histidine-tagged human PCNA can substitute for purified endogenous human PCNA to initiate human chromosomal DNA replication. It is recruited specifically to discrete DNA replication foci formed during initiation in vitro. The template nuclei also contain DNA breaks as result of the synchronisation procedure. A separate population of chromatin-bound PCNA is already present in these template nuclei at discrete DNA damage foci, co-localising with gamma-H2AX, RPA and Rad51. This DNA damage-associated PCNA population is marked by mono-ubiquitination, suggesting that it is involved in DNA repair. Importantly, the population of damage focus-associated PCNA is neither involved in, nor required for, the initiation of chromosomal DNA replication in the same nuclei.