Sufficient amounts of functional HOP2/MND1 complex promote interhomolog DNA repair but are dispensable for intersister DNA repair during meiosis in Arabidopsis.

During meiosis, homologous recombination (HR) is essential to repair programmed DNA double-strand breaks (DSBs), and a dedicated protein machinery ensures that the homologous chromosome is favored over the nearby sister chromatid as a repair template. The homologous-pairing protein2/meiotic nuclear division protein1 (HOP2/MND1) protein complex has been identified as a crucial factor of meiotic HR in Arabidopsis thaliana, since loss of either MND1 or HOP2 results in failure of DNA repair. We isolated two mutant alleles of HOP2 (hop2-2 and hop2-3) that retained the capacity to repair meiotic DSBs via the sister chromatid but failed to use the homologous chromosome. We show that in these alleles, the recombinases radiation sensitive51 (RAD51) and disrupted meiotic cDNA1 (DMC1) are loaded, but only the intersister DNA repair pathway is activated. The hop2-2 phenotype is correlated with a decrease in HOP2/MND1 complex abundance. In hop2-3, a truncated HOP2 protein is produced that retains its ability to bind to DMC1 and DNA but forms less stable complexes with MND1 and fails to efficiently stimulate DMC1-driven D-loop formation. Genetic analyses demonstrated that in the absence of DMC1, HOP2/MND1 is dispensable for RAD51-mediated intersister DNA repair, while in the presence of DMC1, a minimal amount of functional HOP2/MND1 is essential to drive intersister DNA repair.

The recombinases DMC1 and RAD51 are functionally and spatially separated during meiosis in Arabidopsis.

Meiosis ensures the reduction of the genome before the formation of generative cells and promotes the exchange of genetic information between homologous chromosomes by recombination. Essential for these events are programmed DNA double strand breaks (DSBs) providing single-stranded DNA overhangs after their processing. These overhangs, together with the RADiation sensitive51 (RAD51) and DMC1 Disrupted Meiotic cDNA1 (DMC1) recombinases, mediate the search for homologous sequences. Current models propose that the two ends flanking a meiotic DSB have different fates during DNA repair, but the molecular details remained elusive. Here we present evidence, obtained in the model plant Arabidopsis thaliana, that the two recombinases, RAD51 and DMC1, localize to opposite sides of a meiotic DSB. We further demonstrate that the ATR kinase is involved in regulating DMC1 deposition at meiotic DSB sites, and that its elimination allows DMC1-mediated meiotic DSB repair even in the absence of RAD51. DMC1's ability to promote interhomolog DSB repair is not a property of the protein itself but the consequence of an ASYNAPTIC1 (Hop1)-mediated impediment for intersister repair. Taken together, these results demonstrate that DMC1 functions independently and spatially separated from RAD51 during meiosis and that ATR is an integral part of the regular meiotic program.

A novel plant gene essential for meiosis is related to the human CtIP and the yeast COM1/SAE2 gene.

Obligatory homologous recombination (HR) is required for chiasma formation and chromosome segregation in meiosis I. Meiotic HR is initiated by DNA double-strand breaks (DSBs), generated by Spo11, a homologue of the archaebacterial topoisomerase subunit Top6A. In Saccharomyces cerevisiae, Rad50, Mre11 and Com1/Sae2 are essential to process an intermediate of the cleavage reaction consisting of Spo11 covalently linked to the 5' termini of DNA. While Rad50 and Mre11 also confer genome stability to vegetative cells and are well conserved in evolution, Com1/Sae2 was believed to be fungal-specific. Here, we identify COM1/SAE2 homologues in all eukaryotic kingdoms. Arabidopsis thaliana Com1/Sae2 mutants are sterile, accumulate AtSPO11-1 during meiotic prophase and fail to form AtRAd51 foci despite the presence of unrepaired DSBs. Furthermore, DNA fragmentation in AtCom1 is suppressed by eliminating AtSPO11-1. In addition, AtCOM1 is specifically required for mitomycin C resistance. Interestingly, we identified CtIP, an essential protein interacting with the DNA repair machinery, as the mammalian homologue of Com1/Sae2, with important implications for the molecular role of CtIP.