Perspective News from Arabidopsis on the Meiotic Roles of Blap75/ Rmi1 and Top3a Charles I. White* Ge´ne´tique, Reproduction, et De´veloppement, CNRS UMR 6247, Clermont Universite´, INSERM U931, Aubie`re Cedex, France Two articles published in this issue of PLoS Genetics present novel data concern- ing the members of a key regulator of genetic crossing-over. Working with the plant Arabidopsis thaliana, the authors of the two reports provide exciting new data and further understanding of the meiotic anti– crossing-over function of the Topoisomer- ase 3alpha (Top3a) and Blap75/Rmi1 proteins, and thus presumably that of the protein complex that contains these pro- teins and the RecQ-like helicase BLM. The highly conserved RecQ-like heli- case BLM, which is mutated in patients with Bloom syndrome, acts in a protein complex that can disassociate homologous recombination intermediates in vitro and in vivo (reviewed in [1–3]). The impor- tance of this anti-recombination role is clearly shown by the elevated levels of genetic instability, mitotic recombination, and sister-chromatid exchanges in the somatic tissues of the cancer-prone Bloom syndrome patients. This complex, known as BTB in mammals and RTR in yeast, involves BLM and at least two other proteins: Top3a and Blap75/Rmi1. BLAP75/RMI1 is a highly conserved protein in eukaryotes originally identified through its interactions with the BLM [4,5] and independently as Rmi1/Nce4 in yeast through its genetic interactions with BLM homolog Sgs1 [1,6]. A fourth protein component of this complex, Rmi2, has recently been identified [7,8], and it is likely that others will follow (discussed by [9]). It is proposed that the principal anti-recombinational role of this complex involves BLM helicase-driven migration of double Holliday junctions (dHJs) to form a hemi-catenane interme- diate. The resolution of this structure by the action of a topoisomerase (Top3a) does not lead to the exchange of flanking DNA sequences, and thus BLM acts to avoid crossing-over [3,10–12]. BLM also has affinity for DNA structures other than dHJ and clearly also plays other anti- recombination roles [13–15]. To add to these, a very recent report shows a pro- recombination role for Sgs1/BLM in resection of 59-ended strands at DNA double-strand breaks [16]. What about A. thaliana, the subject of the two reports discussed here? Arabidopsis has a total of seven identified RecQ-like proteins, with RecQ4a being the strongest candidate for the Arabidopsis BLM/Sgs1 ortholog [17–19]. The accompanying papers report the identification of the Arabidopsis othologs of BLAP75/Rmi1 [20,21] and Topo3a [21], as well as the characterization of the mitotic and meiotic phenotypes of the corresponding mutant plants. top3a mutant plants have severe developmental defects, are methyl meth- anesulfonate (MMS)–sensitive, and show elevated levels of mitotic recombination and mitotic chromosome abnormalities. Similar mitotic phenotypes are observed in recQ4a and blap75/rmi1 mutant plants, suggesting a functional interaction be- tween RecQ4a and Top3a. This is further supported by the partial suppression of top3a developmental defects in double recQ4a/top3a mutants. In most (studied) eukaryotes, homolo- gous recombination that occurs during the first meiotic prophase ensures the proper segregation of homologous chromosomes (homologs) at the first meiotic division. These events are initiated by programmed double-strand breaks that generate broken DNA ends that invade homologous se- quences on the homolog, a subset of which are processed to form dHJs. These must be resolved to permit the separation of homologs at the first meiotic anaphase, and the mode of this resolution determines whether or not the recombination is accompanied by physical exchange of chromosome arms of the homologs (cross- ing-over). The potential of crossing-over to cause genome reorganization (insertions, deletions, inversions, translocations) has led to the evolution of multiple controls of recombination. It has long been recognized that the numbers and distribution of meiotic cross- overs are strictly regulated. In the last decade, the existence of cross-over and non–cross-over recombination pathways has been established, and many details of molecular mechanisms elucidated [22– 28]. In this context lies the importance of the characterization of the essential mei- otic anti–crossing-over role of the BTB/ RTR complex in Arabidopsis by the Grelon and Puchta groups, reported in this issue of PLoS Genetics [20,21]. These reports show that Arabidopsis blap75/rmi1 and top3a mutants are capable of full chromosome synapsis, resulting in normal pachytene figures. The structure of the synaptonemal complex at pachytene was verified by staining of blap75 mutant meioses with antisera against Asy1 and Zyp1, two synaptonemal complex pro- teins, and proper chromosome pairing was shown by fluorescence in situ hybridiza- tion (FISH) [20]. Staining with antiserum against Dmc1, a marker for early meiotic recombination intermediates, also shows normal numbers and timing of foci. Although these immunological and FISH analyses haven’t been carried out for top3a mutants, the DAPI-stained pachytene figures of top3a present the same (normal) aspect as those of the blap75 mutants. Epistasis analyses confirm that Blap75/ Rmi1 acts downstream of Spo11 (DNA cleavage/recombination initiation), Rad51, and Mnd1 (homolog invasion). It thus appears that early steps of meiosis, up to homolog pairing and synaptonemal com- plex formation, occur normally in the absence of Blap75/Rmi1 and Top3a in Arabidopsis. However, aberrant diakinesis and interlocked metaphase I figures follow, and chromosomes fragment at snaphase I. The interlocked bivalents observed at Citation: White CI (2008) News from Arabidopsis on the Meiotic Roles of Blap75/Rmi1 and Top3a. PLoS Genet 4(12): e1000306. doi:10.1371/journal.pgen.1000306 Published December 19, 2008 Copyright: 2008 Charles I. White. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. * E-mail: chwhite@univ-bpclermont.fr PLoS Genetics | www.plosgenetics.org 1 December 2008 | Volume 4 | Issue 12 | e1000306