Effects of XRCC2 and RAD51B mutations on somatic and meiotic recombination in Arabidopsis thaliana.

Homologous recombination is key to the maintenance of genome integrity and the creation of genetic diversity. At the mechanistic level, recombination involves the invasion of a homologous DNA template by broken DNA ends, repair of the break and exchange of genetic information between the two DNA molecules. Invasion of the template in eukaryotic cells is catalysed by the RAD51 and DMC1 recombinases, assisted by a number of accessory proteins, including the RAD51 paralogues. Eukaryotic genomes encode a variable number of RAD51 paralogues, ranging from two in yeast to five in animals and plants. The RAD51 paralogues form at least two distinct protein complexes, believed to play roles in the assembly and stabilization of the RAD51-DNA nucleofilament. Somatic recombination assays and immunocytology confirm that the three 'non-meiotic' paralogues of Arabidopsis, RAD51B, RAD51D and XRCC2, are involved in somatic homologous recombination, and that they are not required for the formation of radioinduced RAD51 foci. Given the presence of all five proteins in meiotic cells, the apparent absence of a meiotic role for RAD51B, RAD51D and XRCC2 is surprising, and perhaps simply the result of a more subtle meiotic phenotype in the mutants. Analysis of meiotic recombination confirms this, showing that the absence of XRCC2, and to a lesser extent RAD51B, but not RAD51D, increases rates of meiotic crossing over. The roles of RAD51B and XRCC2 in recombination are thus not limited to mitotic cells.