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Meiotic recombination is a mandatory process for sexual reproduction. We identified a protein specifically implicated in meiotic homologous recombination that we named: meiosis specific with OB domain (MEIOB). This protein is conserved among metazoan species and contains single-strand DNA binding sites similar to those of RPA1. Our studies in vitro revealed that both recombinant and endogenous MEIOB can be retained on single-strand DNA. Those in vivo demonstrated the specific expression of Meiob in early meiotic germ cells and the co-localization of MEIOB protein with RPA on chromosome axes. MEIOB localization in Dmc1 −/− spermatocytes indicated that it accumulates on resected DNA. Homologous Meiob deletion in mice caused infertility in both sexes, due to a meiotic arrest at a zygotene/pachytene-like stage. DNA double strand break repair and homologous chromosome synapsis were impaired in Meiob −/− meiocytes. Interestingly MEIOB appeared to be dispensable for the initial loading of recombinases but was required to maintain a proper number of RAD51 and DMC1 foci beyond the zygotene stage. In light of these findings, we propose that RPA and this new single-strand DNA binding protein MEIOB, are essential to ensure the proper stabilization of recombinases which is required for successful homology search and meiotic recombination., Author Summary Homologous recombination allows faithful repair of damaged DNA; in mitotic cells, it necessitates the formation of single strand DNA (ssDNA), which is first protected by RPA and then coated by the RAD51 recombinase to mediate homology search. Specific modifications are made to this mechanism during meiosis, a specialized division that allows halving the ploidy of the genome and the production of haploid gametes. Among others a specialized recombinase DMC1 is added to its somatic paralog RAD51 to perform homology search. We identified a new meiotic protein that we named meiosis specific with OB domains (MEIOB). Our findings indicate that MEIOB binds ssDNA, and we propose that MEIOB is a meiotic paralog of RPA, another OB-domain containing protein. Meiob mutant mice were infertile and unable to complete meiotic recombination, most likely due to destabilization of DMC1 and RAD51 in the absence of MEIOB. Meiosis appears thus to be a ‘game of two pairs’ using both the canonical players in homologous recombination (RPA and RAD51) and a second set of paralogs (MEIOB and DMC1). Identifying such new players should help clarify some genetic causes of infertility and shed new light on the interplay between the molecular actors involved in maintaining genome stability.

Orderly chromosome segregation during the first meiotic division requires meiotic recombination to form crossovers between homologous chromosomes (homologues). Members of the minichromosome maintenance (MCM) helicase family have been implicated in meiotic recombination. In addition, they have roles in initiation of DNA replication, DNA mismatch repair and mitotic DNA double-strand break repair. Here, we addressed the function of MCMDC2, an atypical yet conserved MCM protein, whose function in vertebrates has not been reported. While we did not find an important role for MCMDC2 in mitotically dividing cells, our work revealed that MCMDC2 is essential for fertility in both sexes due to a crucial function in meiotic recombination. Meiotic recombination begins with the introduction of DNA double-strand breaks into the genome. DNA ends at break sites are resected. The resultant 3-prime single-stranded DNA overhangs recruit RAD51 and DMC1 recombinases that promote the invasion of homologous duplex DNAs by the resected DNA ends. Multiple strand invasions on each chromosome promote the alignment of homologous chromosomes, which is a prerequisite for inter-homologue crossover formation during meiosis. We found that although DNA ends at break sites were evidently resected, and they recruited RAD51 and DMC1 recombinases, these recombinases were ineffective in promoting alignment of homologous chromosomes in the absence of MCMDC2. Consequently, RAD51 and DMC1 foci, which are thought to mark early recombination intermediates, were abnormally persistent in Mcmdc2-/- meiocytes. Importantly, the strand invasion stabilizing MSH4 protein, which marks more advanced recombination intermediates, did not efficiently form foci in Mcmdc2-/- meiocytes. Thus, our work suggests that MCMDC2 plays an important role in either the formation, or the stabilization, of DNA strand invasion events that promote homologue alignment and provide the basis for inter-homologue crossover formation during meiotic recombination., Author Summary Each chromosome is present in two distinct but homologous copies in diploid organisms. To generate haploid gametes suitable for fertilization, these homologous chromosomes must segregate during meiosis. To ensure correct chromosome segregation, homologous chromosomes must align and become connected by inter-homologue crossovers during early meiosis in most taxa including mammals. Defects in these processes result in infertility and aneuploidies in gametes. Alignment of homologous chromosomes and crossover formation entail generation of DNA double-strand breaks and repair of DNA breaks by meiotic recombination. As part of the repair process, single-stranded DNA ends resulting from DNA breaks invade homologous DNA sequences and use them as repair templates. DNA strand invasion events lead to the alignment of homologous chromosomes, and serve as precursors for crossovers. We discovered that meiotic recombination critically depends on the helicase-related minichromosome maintenance domain containing 2 protein (MCMDC2). MCMDC2 likely promotes the formation and/or stabilization of DNA strand invasion events that connect homologous chromosomes. Thus, MCMDC2 is required for DNA breaks to effectively promote alignment of homologous chromosomes. This work reveals a crucial role for MCMDC2 in recombination in mammals, and constitutes an important step in understanding how recombination establishes connections between homologous chromosomes during meiosis.