Spatiotemporal regulation of meiotic recombination by Liaisonin

Abstract

Sexual reproduction involves diversification of genetic information in successive generations. Meiotic recombination, which substantially contributes to the increase in genetic diversity, is initiated by programmed DNA double-strand breaks (DSBs) catalyzed by the evolutionarily conserved Spo11 protein. Spo11 requires additional partner proteins for its DNA cleavage reaction. DSBs are preferentially introduced at defined chromosomal sites called "recombination hotspots." Recent studies have revealed that meiotically established higher-order chromosome structures, such as chromosome axes and loops, are also crucial in the control of DSB formation. Most of the DSB sites are located within chromatin loop regions, while many of the proteins involved in DSB formation reside on chromosomal axes. Hence, DSB proteins and DSB sites seem to be distantly located. To resolve this paradox, we conducted comprehensive proteomics and ChIP-chip analyses on Spo11 partners in Schizosaccharomyces pombe, in combination with mutant studies. We identified two distinct DSB complexes, the "DSBC (DSB Catalytic core)" and "SFT (Seven-Fifteen-Twenty four; Rec7-Rec15-Rec24)" subcomplexes. The DSBC subcomplex contains Spo11 and functions as the catalytic core for the DNA cleavage reaction. The SFT subcomplex is assumed to execute regulatory functions. To activate the DSBC subcomplex, the SFT subcomplex tethers hotspots to axes via its interaction with Mde2, which can interact with proteins in both DSBC and SFT subcomplexes. Thus, Mde2 is likely to bridge these two subcomplexes, forming a "tethered loop-axis complex." It should be noted that Mde2 expression is strictly regulated by S phase checkpoint monitoring of the completion of DNA replication. From these observations, we proposed that Mde2 is a central coupler for meiotic recombination initiation to establish a tethered loop-axis complex in liaison with the S phase checkpoint.

Keywords: DNA double-strand break formation; Meiotic recombination; S phase checkpoint; Spo11; higher-order chromosome structure.

Figure 1. (A) DSB proteins in S. pombe, S. cerevisiae and mammals. (B) The two DSB subcomplexes and the network for protein-protein interactions among DSB proteins.

Figure 2. We propose a model for the step-wise and cooperative assembly of DSB subcomplexes on DSB hotspots in S. pombe. During premeiotic DNA replication, chromosomal axes are established by the meiotic cohesin, the major axial protein Rec10, and those associated proteins along meiotic chromosomes. Rec15 is recruited to Rec10 on axis sites. At this point, Mde2 expression is inhibited by the active S phase checkpoint through the regulation of Mei4. After DNA replication, the SFT subcomplex and Mde2 are recruited cooperatively to DSB hotspots located in loop regions. This allows for stabilization of the SFT subcomplex at DSB sites as well as the interaction between axes and DSB sites on loops through the multi-protein complex formation. In such circumstances (stabilized SFT subcomplex in close proximity to axis-components), the SFT subcomplex and Mde2 can recruit the active DSBC subcomplex to DSB sites. Such DSBC recruitment is possibly mediated by the interaction between Rec14 and Mde2. On the other hand, in S. cerevisiae, axis-bound Mer2 recruits Spp1, which recognizes a specific histone modification such as H3K4 di or trimethylation around DSB hotspots, and participates in tethering DSB hotspots to axes, allowing Spo11 activation. These models explain how DSB hotspots can interact with chromosome axes and why axis-bound proteins influence DSB formation.