ReviewA new light on the meiotic DSB catalytic complex
Introduction
Meiotic recombination is required for the production of balanced gametes and, therefore, is essential for fertility in most eukaryotes. In the early 80s, genetic studies on the outcome of meiotic and mitotic products in the yeast Saccharomyces cerevisiae and in the fungus Ascobolus immersus led to the hypothesis that meiotic recombination had to be initiated by the formation of DNA double-strand breaks (DSBs) at preferential chromosomal sites, called recombination ‘hotspots’ [1]. A few years later, DNA DSBs were detected in the promoter region of the ARG4 gene in S. cerevisiae [2]. These pioneering studies gave the basis for the Szostak model of DSB repair by homologous recombination (Fig. 1).
The following steps of the model were validated at the molecular level with some modifications: DSB 5′ ends are resected to form 3′ single-stranded DNA tails that will be wrapped into a nucleofilament with the strand exchange proteins Rad51 and Dmc1; DSB repair is primed by strand invasion that is biased, but not exclusively, toward the homologous chromosome. The so-called D-loop intermediate goes through various steps of maturation, leading either to reciprocal exchanges of homologous chromosomal fragments (crossovers) or to non-reciprocal exchanges of small homologous chromosomal patches (non-crossovers, or gene conversion events not associated with crossovers) [3]. In most eukaryotes, a large number of DSBs (from tens to hundreds) is generated at the onset of meiotic prophase. They represent a unique threat for the cell. Not surprisingly, multiple layers of control regulate DSB localization, rate of formation, dynamics and fate (reviewed in [4], [5]).
However, only in the late 90s, the Spo11 gene, which is essential for yeast sporulation [6], was identified as the factor potentially responsible for the formation of meiotic DNA DSBs in the yeast S. cerevisiae [7], [8]. Specifically, it was found that the Spo11 protein shows similarities with the catalytic subunit of the DNA topoisomerase VI (TopoVIA) [7]. Furthermore, Spo11 was detected covalently associated with the 5′ end of meiotic DSBs [8]. Spo11 homologues were then identified in many eukaryotes, suggesting a conserved mechanism for initiation of meiotic recombination [9]. However, despite intensive investigations in the last 15 years, Spo11 biochemical activity in DSB formation could not be demonstrated. Very recently, the discovery of a homologue of the TopoVIB subunit that acts in a complex with SPO11 during plant and mouse meiosis revamped our understanding of the mechanisms involved in meiotic DSB formation. In the present chapter, by describing the specific features of the TopoVIA and B subunits, of SPO11 and of the newly identified meiotic TopoVIB-like subunit, we will present a new vision on how a TopoVI-like complex catalyses DSB formation in meiosis.
Section snippets
Type II topoisomerases: a family of enzymes that catalyse strand passage through the formation of a transient DNA double strand break
Topoisomerases are essential enzymes for cell viability. They regulate DNA topology during processes as diverse as DNA replication, transcription or recombination. They carry out their function by combining DNA strand scission, motion and re-joining activities. Topoisomerases are divided in two families (type I and type II) that differ by their ability to cleave one or two strands of DNA, respectively.
Type II topoisomerases catalyse the passage of one DNA duplex through a transient DNA DSB in
An evolutionarily conserved role in meiotic DSB formation
Spo11 was identified as a sporulation-defective S. cerevisiae mutant [6] that was shown later to be defective in meiotic DSB formation [30]. Spo11 direct implication in meiotic DSB formation came from two independent observations. First, based on the results of a protein sequence similarity search for the S. shibatae TopoVIA subunit, Bergerat et al. identified yeast Spo11 as a putative orthologue of the catalytic subunit of TopoVI. They additionally found that only one tyrosine residue
Identification in plants and vertebrates of a meiotic TopoVIB-like protein
Recently, based on a screen of meiotic recombination-deficient mutants in A. thaliana, Vrielynck et al. identified a protein required for meiotic DSB formation that shows striking structural similarities with archaeal TopoVIB [77]. PSI-BLAST searches and HHpred analyses isolated structural homologues in animals. Its essential role in DSB formation has been demonstrated for the mouse homologue [78].
In summary, these studies identified a eukaryotic meiotic-specific TopoVIB-related protein family
A meiotic TopoVI-like complex is required for meiotic DSB formation
The recent identification in plants and animals of a TopoVIB-like protein that directly interacts with SPO11 and that is essential for its DSB catalytic activity [77], [78] sheds new light on the enzymatic complex that achieves this essential function for sexual reproduction. These studies revealed that in plants and animals, the mode of action of the enzymatic complex that catalyses DNA DSBs in meiosis is conserved and is carried out by a TopoVI-like tetrameric enzyme, likely composed of two A
Conclusions
Since the identification of Spo11, an orthologue of the TopoVIA catalytic subunit of TopoVI, the requirement of a TopoVIB-like subunit to produce meiotic DNA double strand breaks has been the source of intense debates and research. The recent identification in plants and mice by Vrielynck et al. and by Robert et al. of such a B subunit that is essential to initiate meiotic DSBs finally closes this question. Moreover, the identification of putative MTOPOVIB/TOPOVIBL orthologues in ascomycetes
Acknowledgements
The IJPB institute benefits from the support of the Labex Saclay Plant Sciences-SPS (ANR-10-LABX-0040-SPS). We thank Henri-Marc Bourbon and Claudine Mayer for the collaboration during the whole project of the identification and analysis of the TOPOVIB-Like protein family and numerous discussions about questions addressed within this review. BdM and TR are funded by grants from the Centre National pour la Recherche Scientifique (CNRS) and the European Research Council Executive Agency under the
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