Chelysheva Liudmila, Vezon Daniel, Belcram Katia, Gendrot Ghislaine, Grelon Mathilde
INRA de Versailles, Institut Jean-Pierre Bourgin, Station de Génétique et d'Amélioration des Plantes UR-254, Versailles, France.
PLoS Genet. 2008 Dec;4(12):e1000309. doi: 10.1371/journal.pgen.1000309. Epub 2008 Dec 19.
In human cells and in Saccharomyces cerevisiae, BLAP75/Rmi1 acts together with BLM/Sgs1 and TopoIIIalpha/Top3 to maintain genome stability by limiting crossover (CO) formation in favour of NCO events, probably through the dissolution of double Holliday junction intermediates (dHJ). So far, very limited data is available on the involvement of these complexes in meiotic DNA repair. In this paper, we present the first meiotic study of a member of the BLAP75 family through characterisation of the Arabidopsis thaliana homologue. In A. thaliana blap75 mutants, meiotic recombination is initiated, and recombination progresses until the formation of bivalent-like structures, even in the absence of ZMM proteins. However, chromosome fragmentation can be detected as soon as metaphase I and is drastic at anaphase I, while no second meiotic division is observed. Using genetic and imunolocalisation studies, we showed that these defects reflect a role of A. thaliana BLAP75 in meiotic double-strand break (DSB) repair -- that it acts after the invasion step mediated by RAD51 and associated proteins and that it is necessary to repair meiotic DSBs onto sister chromatids as well as onto the homologous chromosome. In conclusion, our results show for the first time that BLAP75/Rmi1 is a key protein of the meiotic homologous recombination machinery. In A. thaliana, we found that this protein is dispensable for homologous chromosome recognition and synapsis but necessary for the repair of meiotic DSBs. Furthermore, in the absence of BLAP75, bivalent formation can happen even in the absence of ZMM proteins, showing that in blap75 mutants, recombination intermediates exist that are stable enough to form bivalent structures, even when ZMM are absent.
在人类细胞和酿酒酵母中，BLAP75/Rmi1与BLM/Sgs1以及TopoIIIα/Top3共同作用，通过限制交叉（CO）形成以促进非交叉（NCO）事件来维持基因组稳定性，这可能是通过溶解双Holliday连接中间体（dHJ）实现的。到目前为止，关于这些复合物参与减数分裂DNA修复的可用数据非常有限。在本文中，我们通过对拟南芥同源物的表征，首次对BLAP75家族的一个成员进行了减数分裂研究。在拟南芥blap75突变体中，减数分裂重组启动，并且重组会持续进行直到形成类似二价体的结构，即使在没有ZMM蛋白的情况下也是如此。然而，在中期I就能检测到染色体片段化，在后期I则更为剧烈，同时未观察到第二次减数分裂。通过遗传和免疫定位研究，我们表明这些缺陷反映了拟南芥BLAP75在减数分裂双链断裂（DSB）修复中的作用——它在由RAD51及相关蛋白介导的侵入步骤之后发挥作用，并且对于在姐妹染色单体以及同源染色体上修复减数分裂DSB是必需的。总之，我们的结果首次表明BLAP75/Rmi1是减数分裂同源重组机制的关键蛋白。在拟南芥中，我们发现该蛋白对于同源染色体识别和联会并非必需，但对于减数分裂DSB的修复是必需的。此外，在没有BLAP75的情况下，即使没有ZMM蛋白也能发生二价体形成，这表明在blap75突变体中，存在足够稳定的重组中间体以形成二价体结构，即使在没有ZMM的情况下也是如此。
