Meiotic chromosomes assemble characteristic “axial element” structures that are essential for fertility and provide the chromosomal context for meiotic recombination, synapsis and checkpoint signaling. Whether these meiotic processes are equally dependent on axial element integrity has remained unclear. Here, we investigated this question in S. cerevisiae using the putative condensin allele ycs4S. We show that the severe axial element assembly defects of this allele are explained by a linked mutation in the promoter of the major axial element gene RED1 that reduces Red1 protein levels to 20–25% of wild type. Intriguingly, the Red1 levels of ycs4S mutants support meiotic processes linked to axis integrity, including DNA double-strand break formation and deposition of the synapsis protein Zip1, at levels that permit 70% gamete survival. By contrast, the ability to elicit a meiotic checkpoint arrest is completely eliminated. This selective loss of checkpoint function is supported by a RED1 dosage series and is associated with the loss of most of the cytologically detectable Red1 from the axial element. Our results indicate separable roles for Red1 in building the structural axis of meiotic chromosomes and mounting a sustained recombination checkpoint response.

Histone modification is a critical determinant of the frequency and location of meiotic double-strand breaks (DSBs), and thus recombination. Set1-dependent histone H3K4 methylation and Dot1-dependent H3K79 methylation play important roles in this process in budding yeast. Given that the RNA polymerase II associated factor 1 complex, Paf1C, promotes both types of methylation, we addressed the role of the Paf1C component, Rtf1, in the regulation of meiotic DSB formation. Similar to a set1 mutation, disruption of RTF1 decreased the occurrence of DSBs in the genome. However, the rtf1 set1 double mutant exhibited a larger reduction in the levels of DSBs than either of the single mutants, indicating independent contributions of Rtf1 and Set1 to DSB formation. Importantly, the distribution of DSBs along chromosomes in the rtf1 mutant changed in a manner that was different from the distributions observed in both set1 and set1 dot1 mutants, including enhanced DSB formation at some DSB-cold regions that are occupied by nucleosomes in wild-type cells. These observations suggest that Rtf1, and by extension the Paf1C, modulate the genomic DSB landscape independently of H3K4 methylation. KEYWORDS meiosis; recombination; H3K4 methylation; PAF; Rtf1; double-strand breaks D NA double-strand breaks (DSBs) are a kind of DNA damage that is deleterious to cells unless it is repaired. Unrepaired DSBs lead to genome instability by forming broken chromosomes. DSBs are repaired by either homologous recombination or nonhomologous end joining (Krogh and Symington 2004;Symington and Gautier 2011). During vegetative growth, cells have to repair DSBs, which are accidentally created by exogenous impacts, such as ionizing radiation, as well as by internal sources, e.g., due to oxidation. Meiotic cells are equipped with a program to introduce DSBs along the genome for the induction of homologous recombination. Recombination generates a crossover, a reciprocal exchange of parental DNAs, which is essential for the correct segregation of homologous chromosomes during meiosis I (Petronczki et al. 2003;Lichten and de Massy 2011). Diversity in the gamete genome is also produced by meiotic recombination.Programmed formation of DSBs on meiotic chromosomes is catalyzed by the topoisomerase II-like protein Spo11 and its binding partner Ski8 (Keeney et al. 1997;Lam and Keeney 2015). The activity of Spo11 is also regulated by essential accessory proteins or protein complexes: the Mre11-Rad50-Xrs2 (MRX) complex, the Rec114-Mer2-Mei4 (RMM) complex, Rec102, and Rec104 (de Massy 2013). In addition, DSB formation during meiosis is controlled by the chromosomal structure. Three meiosis-specific chromosomal proteins, Hop1, Red1, and Mek1/Mre4, are necessary for efficient DSB formation (Xu et al. 1997). Furthermore, a meiosis-specific cohesin complex containing Rec8 kleisin regulates the distribution of DSBs in the genome (Klein et al. 1999;Kugou et al. 2009;Sun et al. 2015). Interestingly, the majority of proteins required for DSB formation, including co...

The H2A.Z histone variant is deposited into the chromatin by the SWR1 complex, affecting multiple aspects of meiosis. We describe here a SWR1-independent localization of H2A.Z at meiotic telomeres and the centrosome. We demonstrate that H2A.Z colocalizes and interacts with Mps3, the SUN component of the linker of nucleoskeleton, and cytoskeleton (LINC) complex that spans the nuclear envelope and links meiotic telomeres to the cytoskeleton, promoting meiotic chromosome movement. H2A.Z also interacts with the meiosis-specific Ndj1 protein that anchors telomeres to the nuclear periphery via Mps3. Telomeric localization of H2A.Z depends on Ndj1 and the N-terminal domain of Mps3. Although telomeric attachment to the nuclear envelope is maintained in the absence of H2A.Z, the distribution of Mps3 is altered. The velocity of chromosome movement during the meiotic prophase is reduced in the htz1 mutant lacking H2A.Z, but it is unaffected in swr1 cells. We reveal that H2A.Z is an additional LINC-associated factor that contributes to promote telomere-driven chromosome motion critical for error-free gametogenesis.

ABSTRACTThe H2A.Z histone variant is deposited into chromatin by the SWR1 complex affecting multiple aspects of meiosis. Here we describe a SWR1-independent localization of H2A.Z at meiotic telomeres and the centrosome. We demonstrate that H2A.Z colocalizes and interacts with Mps3, the SUN component of the LINC complex that spans the nuclear envelope and links meiotic telomeres to the cytoskeleton promoting meiotic chromosome movement. H2A.Z also interacts with the meiosis-specific Ndj1 protein that anchors telomeres to the nuclear periphery via Mps3. Telomeric localization of H2A.Z depends on Ndj1 and the N-terminal domain of Mps3. Although telomeric attachment to the nuclear envelope is maintained in the absence of H2A.Z, the distribution of Mps3 is altered. The velocity of chromosome movement during meiotic prophase I is reduced in the htz1Δ mutant lacking H2A.Z, but it is unaffected in swr1Δ cells. We reveal that H2A.Z is an additional LINC-associated factor that contributes to promote telomere-driven chromosome motion critical for error-free gametogenesis.

Cytoskeletal forces acting upon telomeres promote active chromosome motion needed to pair homologous chromosomes during meiosis. The necessary components that allow this force to be applied to telomeres is still unclear, as are the roles of this motion and whether motion is needed primarily for increasing collisions of homologous regions, testing homolog pairing fidelity, or some other role. Here, we show a novel role for telomerase, previously known to be responsible for telomeric end replication, in anchoring telomeres to the nuclear envelope (NE) to provide proper transmission of cytoskeletal forces during meiosis. Reduction in telomerase function in Saccharomyces cerevisiae results in a dramatic decrease in the frequency of high velocity "pulls" resulting in earlier homolog synapsis and increased recombination. These observations are consistent with a model in which telomeric cytoskeletal engagement ensures homolog pairing fidelity by pulling apart improperly associated regions whereas general chromosomal motion aids in increasing homologous contacts.