L'analyse fonctionnelle de BRCA2 canonique et non-canonique dans la réparation de l'ADN par recombinaison homologue.

DNA repair pathways are crucial for the maintenance of genome integrity. They prevent a loss of genetic information which threatens cell viability and regulation of cell division and ensure proper DNA replication. While some repair pathways inherently introduce mutations, this is not the case for the most faithful one: homologous recombination (HR). The moss Physcomitrium patens is an excellent model organism to study HR, as it exhibits a higher rate of HR-mediated repair than other plants. HR relies on a number of proteins, including BRCA2 (BReast CAncer 2), which promotes the activity of the two main enzymes of somatic and meiotic HR, the recombinases RAD51 and the meiosis-specific DMC1. Surprisingly, BRCA2 homologues in the moss have long remained unidentified. In this thesis, I present the identification and characterization of a BRCA2 homologue in P. patens (PpBRCA2). PpBRCA2 lacks a canonical DNA binding domain (DBD) found in most BRCA2 homologues. Therefore, the first objective of my thesis was to analyse the involvement of PpBRCA2 in HR, and to determine its ability to interact with both recombinases. I show that PpBRCA2 recruits PpRAD51 to double strand break sites, thereby promoting HR both in somatic and meiotic cells, and binds DNA through a disordered region, despite the absence of DBD. In other BRCA2 homologues, the DBD associates with DSS1, which regulates the activity and localization of BRCA2. In the absence of DBD, we wondered whether DSS1 could interact with BRCA2 or have a role in DNA repair. My second aim was thus to examine the functions of DSS1 homologues in P. patens in DNA repair. Two DSS1 homologues in P. patens were identified and characterized in this work. I could show that DSS1-1 is not necessary for DNA repair. Concerning DSS1-2, I could show that it is required for the early growth of the moss, through a still unknown function, which prevented the analysis of its potential role in DNA repair. Overall, this work highlights the importance of disordered regions in BRCA2 homologues, which carry essential DNA and RAD51 binding domains in multiple species even though their sequence is poorly conserved, and provides new insights into the moss HR mechanism in mitosis and meiosis.