Regulation of homologous recombination (HR) represents the best-characterized DNA repair function of p53. The role of p53 phosphorylation in DNA repair is largely unknown. Here, we show that wild-type p53 repressed repair of DNA double-strand breaks (DSBs) by HR in a manner partially requiring the ATM/ATR phosphorylation site, serine 15. Cdk-mediated phosphorylation of serine 315 was dispensable for this anti-recombinogenic effect. However, without targeted cleavage of the HR substrate, serine 315 phosphorylation was necessary for the activation of topoisomerase I-dependent HR by p53. Moreover, overexpression of cyclin A1, which mimics the situation in tumors, inappropriately stimulated DSB-induced HR in the presence of oncogenic p53 mutants (not Wtp53). This effect required cyclin A1/cdk-mediated phosphorylation for stable complex formation with topoisomerase I. We conclude that p53 mutants have lost the balance between activation and repression of HR, which results in a net increase of potentially mutagenic DNA rearrangements. Our data provide new insight into the mechanism underlying gain-of-function of mutant p53 in genomic instability.

The mechanism of blocked replication of human adenoviruses in monkey cells was examined. Previous experiments have placed the replicative block at the level of transcription or translation of adenovirus mRNA. Coinfection of the monkey cells with simian virus 40 enhances adenovirus replication in these cells. We compared the adenovirus mRNA transcribed during infection of permissive human cells and enhanced and unenhanced monkey cells. Adenovirus mRNA from enhanced monkey cells appeared to be identical to adenovirus mRNA from human cells. This indicated that simian virus 40 coinfection did not overcome the blocked replication by substituting for a missing adenovirus transcript. Comparison of adenovirus mRNA from enhanced and unenhanced monkey cell infection revealed two types of transcriptional discrepancies. There was a decrease in both the complexity and the relative abundance of several regions of the enhanced adenovirus mRNA. However, neither of these transcriptional defects was sufficient to totally explain the difference in yield of infectious virus and viral protein seen in these two types of infection.