Poly(ADP-RIBOSE) polymerase-1 (Parp-1) antagonizes topoisomerase I-dependent recombination stimulation by p53

PARP-1 interacts with and poly(ADP-ribosyl)ates p53 and topoisomerase I, which both participate in DNA recombination. Previously, we showed that PARP-1 downregulates homology-directed double-strand break (DSB) repair. We also discovered that, despite the well-established role of p53 as a global suppressor of error-prone recombination, p53 enhances homologous recombination (HR) at the RARa breakpoint cluster region (bcr) comprising topoisomerase I recognition sites. Using an SV40-based assay and isogenic cell lines differing in the p53 and PARP-1 status we demonstrate that PARP-1 counteracts HR enhancement by p53, although DNA replication was largely unaffected. When the same DNA element was integrated in an episomal recombination plasmid, both p53 and PARP-1 exerted anti-recombinogenic rather than stimulatory activities. Strikingly, with DNA substra-tes integrated into cellular chromosomes, enhance-ment of HR by p53 and antagonistic PARP-1 action was seen, very similar to the HR of viral minichromo-somes. siRNA-mediated knockdown revealed the essential role of topoisomerase I in this regulatory mechanism. However, after I-SceI-meganuclease-mediated cleavage of the chromosomally integra-ted substrate, no topoisomerase I-dependent effects by p53 and PARP-1 were observed. Our data further indicate that PARP-1, probably through topoisomerase I interactions rather than poly(ADP-ribosyl)ation, prevents p53 from stimulating spon-taneous HR on chromosomes via topoisomerase I activity