CIP1 INHIBITS DNA-REPLICATION BUT NOT PCNA-DEPENDENT NUCLEOTIDE EXCISION-REPAIR

Background: DNA that is damaged by ultraviolet (UV) light is repaired predominantly by nucleotide excision-repair, a process requiring the DNA polymerase auxiliary factor PCNA. UV-irradiation also induces the production of Cip 1 protein via activation of p53. Cip 1 is an inhibitor of the cyclin-dependent kinases, which are required for the cell cycle to proceed through the G1/S-phase transition and initiate DNA replication. inhibition by Cip 1 probably causes the block; to initiation of DNA replication that is seen in irr adiated cells. Cip 1 also directly inhibits the function of PCNA during DNA synthesis. As nucleotide excision-repair requires PCNA, the physiological relevance of PCNA inhibition by Cip 1 is currently unclear. Results: We show that nucleotide excision-repair of UV-damaged DNA occurs in extracts of Xenopus eggs, and that this reaction is PCNA-dependent. The repair reaction is not inhibited by Cip 1, even when the level of PCNA is reduced 100-fold so that it becomes limiting for DNA repair. By contrast, Cip 1 strongly suppresses the function of PCNA in replicative DNA synthesis under these conditions. Conclusions: Cip 1 can potentially inhibit DNA replication in Xenopus egg extracts by inhibiting the cyclin-dependent kinase function required for the initiation of replication forks, and also by inhibiting PCNA function. The inhibition of PCNA is selective for its function in DNA replication, however, as Cip 1 does not affect PCNA function in nucleotide excision-repair. The induction of Cip 1 in response to DNA damage, therefore, allows repair to continue in the genome under conditions in which replication is severely inhibited.