Evolutionary conservation of meiotic DSB proteins: more than just Spo11

Abstract

Meiotic recombination is initiated by programmed DNA double-strand breaks (DSBs) generated by the Spo11 protein. In budding yeast, five other meiotic-specific proteins are also required for DSB formation, but, with rare exception, orthologs had not been identified in other species. In this issue of Genes & Development, Kumar and colleagues (pp. 1266-1280) used a phylogenomic approach to identify two of these proteins across multiple clades, and confirmed that one of these, MEI4, is a functional ortholog in mouse.

Figure 1.

Interhomolog recombination is essential for proper chromosome segregation during meiosis I. Mouse oocytes were cultured in vitro and then stained for DNA (red) and β-tubulin (green). In oocytes from wild-type mice, chromosomes congress at the metaphase plate (A), and homologs segregate at anaphase, accompanied by extrusion of the first polar body (arrow) (B). In Spo11-null oocytes, homologs are unconnected to each other such that bipolar attachment and chromosome congression at the metaphase plate are impossible. (C) As a result, chromosomes are chaotically distributed along the spindle. (D) Eventually, the spindle can break down, without oocytes proceeding through meiosis I. Images are courtesy of Monica Di Giacomo. Schematics of chromosome dynamics. In wild-type oocytes, the two homologs (red and blue) of this metacentric chromosome have undergone a CO on each chromosome arm. (A′) Sister chromatids are attached to each other by cohesion (gray circles), which holds the recombinant homologs at the metaphase plate. (B′) When cohesion is released at anaphase, homologs segregate to opposite poles. In Spo11-null mice, homologs are not connected because there are no COs. (C′,D′) Thus, they attach independently of one another and frequently attach to the same pole. Bundles of lines represent spindle microtubules.

Figure 2.

Spo11-mediated DSBs and homologous recombination pathways. (A) The Spo11 homodimer (green and gray shading) is transparent in order to see the duplex DNA underneath (red). Each Spo11 monomer attacks the phosphodiester backbone of the DNA at adjacent base pairs (yellow lines), using a catalytic tyrosine residue with assistance from a metal binding pocket (green arrows). The outline of the structure is based on the structure of the Methanococcus jannaschi TopoVIA (Nichols et al. 1999). (B) After DSB formation, Spo11 is covalently linked (green arcs) to the 5′ ends of DNA, leaving 3′ hydroxyls and nicks offset by 2 base pairs (bp) (yellow lines). (C) MRX/Sae2 endonucleolytically cleaves DNA asymmetrically (brackets) (Neale et al. 2005). (D) The released Spo11 dimer is covalently attached to two oligonucleotides of different lengths, sometimes called “spolligos.” (E) The resulting DSB in the DNA has short 3′ single-stranded tails of two different lengths. Note that the DSB is associated with a 2-bp gap because Spo11 cleaved at 5′ recessed phosphates. (F–H) An expanded view of the DNA shown in A–E. (F) Further 5′-to-3′ resection of the DNA ends, followed by strand invasion of the intact homologous duplex (blue) to form a D-loop. The 3′ end serves as a primer to initiate repair synthesis using the homologous DNA as template (dotted lines). At this point, the D-loop intermediate can proceed by either of two pathways, although it may be more stable in the DSBR pathway. (G) The DSBR pathway. The other 3′ end of the DSB is captured to form a dHJ. Resolution of the dHJ primarily forms COs. (H) The SDSA pathway. The invading 3′ strand is displaced and anneals to the other 3′ end of the DSB to primarily form NCOs.