Wild-type p53 stimulates homologous recombination upon sequence-specific binding to the ribosomal gene cluster repeat.

p53 plays a central role in the maintenance of the genome integrity, both as a gatekeeper and a caretaker. Sequence-specific recognition of DNA is underlying the ability of p53 to transcriptionally transactivate target genes during checkpoint control and to regulate DNA replication at the TGCCT repeat from the ribosomal gene cluster (RGC). In contrast, suppression of recombination by p53 has been observed with nonconsensus DNA sequences. In this study, we discovered that wild-type p53 stimulates homologous recombination adjacent to the RGC repeat, whereas downregulation is seen with a mutated version thereof and with a microsatellite repeat sequence. Analysis of the causes possibly underlying the enhancement of homologous recombination revealed that p53 binding to the RGC element delays DNA synthesis. This was demonstrated after integration of the corresponding DNA fragments into our Simian virus 40-based model system, which was used to study recombination on replicating minichromosomes. Differently, with plasmid-based substrates, p53 did not stimulate recombination at the RGC sequence. Thus, in combination with our previous findings, p53 may promote homologous recombination by two separate mechanisms involving either molecular interactions with topoisomerase I or/and by specific binding to certain genomic regions, thereby causing replication fork stalling and recombination.