Inhibition of nucleotide excision repair by the cyclin-dependent kinase inhibitor p21

p21, a p53-induced gene product that blocks cell cycle progression at the G₁ phase, interacts with both cyclin-dependent kinases and proliferating cell nuclear antigen (PCNA). PCNA functions as a processivity factor for DNA polymerases δ and ε and is required for both DNA replication and nucleotide excision repair. Previous studies have shown that p21 inhibits simian virus 40 (SV40) DNA replication in HeLa cell extracts by interacting with PCNA. In this report we show that p21 blocks nucleotide excision repair of DNA that has been damaged by either ultraviolet radiation or alkylating agents, and that this inhibition can be reversed following addition of PCNA. We have determined that p21 is more effective in blocking DNA resynthesis than in inhibiting the excision step. We further show that a peptide derived from the carboxyl terminus of p21, which specifically interacts with PCNA, inhibits polymerase δ-catalyzed elongation of DNA chains almost stoichiometrically relative to the concentration of PCNA. When added at higher levels, this peptide also blocks both SV40 DNA replication and nucleotide excision repair in HeLa cell extracts. These results indicate that p21 interferes with the function of PCNA in both in vitro DNA replication and nucleotide excision repair.