Role of heteroduplex joints in the functional interactions between human Rad51 and wild-type p53

Our previous work (Dudenhoffer et al., 1999) unveiled a link between the ca pacity of p53 to regulate homologous recombination processes and to specifi cally bind to heteroduplex junction DNAs. Here, we show that p53 participat es in ternary complex formation after preassembly of nucleoproteins, consis ting of the human recombinase hRad51 and junction DNA, The cancer-related m utant p53(273H), which is defective in inhibiting recombination processes, displays a reduced capacity to associate with hRad51-DNA complexes, even un der conditions which support DNA-binding, This suggests that hRad51-p53 con tacts play a role in targeting p53 to heteroduplex joints and indicates an involvement in recombination immediately following hRad51-mediated strand t ransfer. To study the initial phase of strand exchange, when heteroduplex j oints arise, we applied oligonucleotide based strand transfer assays. We ob served that hRad51 stimulates exonucleolytic DNA degradation by p53, when i t generates strand transfer intermediates. In agreement with this observati on, artificial 3-stranded junction DNAs, designed to mimic nascent recombin ation intermediates, were found to represent preferred exonuclease substrat es, especially when comprising a mismatch within the heteroduplex part. Fro m our data, we propose a model according to which, p53-dependent correction of DNA exchange events is triggered by high-affinity binding to joint mole cules and by stabilizing contacts with hRad51 oligomers.