Dissociation of p53-mediated suppression of homologous recombination from G1/S cell cycle checkpoint control

The tumor suppressor p53 is considered as the guardian of the genome which is activated following genotoxic stress. In many cell types, p53 mediates G 1 cell cycle arrest as the predominant cellular response. Inactivation of w ild-type p53 leads to loss of G1/S checkpoint control and to genomic instab ility, including increased spontaneous homologous recombination (HR), To de termine whether regulation of the G1/S checkpoint is required for suppressi on of HR, we assessed recombination events using a plasmid substrate that s tably integrated into the genome of p53-null mouse fibroblasts. Exogenous e xpression of a temperature-sensitive p53 protein (Ala135 to Val), which had lost trans-activation function and could not regulate G1/S transition,when in mutant conformation, reduced HR rates to the same extent as wild-type p 53. Furthermore, a p53 construct with an alternatively-spliced carboxy term inus also retained this ability in the absence of both activities, G1/S con trol and non-sequence specific DNA binding as mediated by the carboxy termi nus. Our data dissociate regulation of HR by p53 from its role as a cell cy cle checkpoint protein. The results support a model which extends p53's rol e as a guardian of the genome to include transactivation-independent regula tory functions in DNA repair, replication and recombination.