RAD51 and RAD51B Play Diverse Roles in the Repair of DNA Double Strand Breaks in Physcomitrium patens

RAD51 is involved in finding and invading homologous DNA sequences for accurate homologous recombination (HR). Its paralogs have evolved to regulate and promote RAD51 functions. The efficient gene targeting and high HR rates are unique in plants only in the moss ( ). In addition to two functionally equivalent genes ( and ), other paralogues were also identified in . For elucidation of RAD51's involvement during DSB repair, two knockout lines were constructed, one mutated in both genes ( ) and the second with mutated gene ( ). Both lines are equally hypersensitive to bleomycin, in contrast to their very different DSB repair efficiency. Whereas DSB repair in is even faster than in WT, in , it is slow, particularly during the second phase of repair kinetic. We interpret these results as and - being true functional homologs of ancestral involved in the homology search during HR. Absence of RAD51 redirects DSB repair to the fast NHEJ pathway and leads to a reduced 5S and 18S rDNA copy number. The exact role of the RAD51B paralog remains unclear, though it is important in damage recognition and orchestrating HR response.