Spatiotemporal recruitment of human DNA polymerase delta to sites of UV damage

Human DNA polymerase delta (Pol delta) is involved in various DNA damage responses in addition to its central role in DNA replication. The Pol delta 4 holoenzyme consists of four subunits, p125, p50, p68 and p12. It has been established that the p12 subunit is rapidly degraded in response to DNA damage by UV leading to the in vivo conversion of Pol delta 4 to Pol delta 3, a trimeric form lacking the p12 subunit. We provide the first analysis of the time-dependent recruitment of the individual Pol delta subunits to sites of DNA damage produced by UV irradiation through 5 mu m polycarbonate filters by immunofluorescence microscopy and laser scanning cytometry (LSC). Quantitative analysis demonstrates that the recruitments of the three large subunits was near complete by 2 h and did not change significantly up to 4 h after UV exposure. However, the recruitment of p12 was incomplete even at 4 h, with about 70% of the Pol delta lacking the p12 subunit. ChIP analysis of Pol delta after global UV irradiation further demonstrates that only p125, p50 and p68 were present. Thus, Pol delta 3 is the predominant form of Pol delta at sites of UV damage as a result of p12 degradation. Using LSC, we have further confirmed that Pol delta was recruited to CPD damage sites in all phases of the cell cycle. Collectivelly, our results show that Pol delta at the DNA damage site is the Pol delta trimer lacking p12 regardless of the cell cycle phase.