Meiotic recombination is initiated by Spo11-generated DNA double-strand breaks (DSBs) . A fraction of total DSBs is processed into crossovers (CRs) between homologous chromosomes, which promote their accurate segregation at meiosis I (MI) . The coordination of recombination-associated events and MI progression is governed by the "pachytene checkpoint", which in budding yeast requires Rad17, a component of a PCNA clamp-like complex, and Pch2, a putative AAA-ATPase . We show that two genetically separable pathways monitor the presence of distinct meiotic recombination-associated lesions: First, delayed MI progression in the presence of DNA repair intermediates is suppressed when RAD17 or SAE2, encoding a DSB-end processing factor , is deleted. Second, delayed MI progression in the presence of aberrant synaptonemal complex (SC) is suppressed when PCH2 is deleted. Importantly, ZIP1, encoding the central element of the SC , is required for PCH2-dependent checkpoint activation. Analysis of the rad17Deltapch2Delta double mutant revealed a redundant function regulating interhomolog CR formation. These findings suggest a link between the surveillance of distinct recombination-associated lesions, control of CR formation kinetics, and regulation of MI timing. A PCH2-ZIP1-dependent checkpoint in meiosis is likely conserved among synaptic organisms from yeast to human .

Summary Background During meiosis, recombination between homologous chromosomes promotes their proper segregation. In budding yeast, programmed double-strand breaks (DSBs) promote recombination between homologs versus sister chromatids by dimerizing and activating Mek1, a chromosome axis-associated kinase. Mek1 is also a proposed effector kinase in the recombination checkpoint that arrests exit from pachytene in response to aberrant DNA/axis structures. Elucidating a role for Mek1 in the recombination checkpoint has been difficult since in mek1 loss-of-function mutants DSBs are rapidly repaired using a sister chromatid thereby bypassing formation of checkpoint-activating lesions. Here we tested the hypothesis that a MEK1 gain-of-function allele would enhance interhomolog bias and the recombination checkpoint response. Results When Mek1 activation was artificially maintained through GST-mediated dimerization, there was an enhanced skew toward interhomolog recombination and reduction of intersister events including multi-chromatid joint molecules. Increased interhomolog events were specifically repaired as noncrossovers rather than crossovers. Ectopic Mek1 dimerization was also sufficient to impose interhomolog bias in the absence of recombination checkpoint functions, thereby uncoupling these two processes. Finally, the stringency of the recombination checkpoint was enhanced in weak meiotic recombination mutants by blocking prophase exit in a subset of cells where arrest is not absolute. Conclusions We propose that Mek1 plays dual roles during meiotic prophase I by phosphorylating targets directly involved in the recombination checkpoint as well as targets involved in sister chromatid recombination. We discuss how regulation of pachytene exit by Mek1 or similar kinases could influence checkpoint stringency, which may differ among species and between sexes.

Dynamic telomere repositioning is a prominent feature of meiosis. Deletion of a telomere-associated protein, Ndj1, results in the failure of both attachment and clustering of telomeres at the nuclear envelope and delays several landmarks of meiosis I, such as pairing, synaptonemal complex formation, and timing of the meiosis I division. We explored the role of Ndj1 in meiotic recombination, which occurs through the formation and repair of programmed double-strand breaks. The ndj1⌬ mutation allows for the formation of the first detectable strand invasion intermediate (i.e., single-end invasion) with wild-type kinetics; however, it confers a delay in the formation of the double-Holliday junction intermediate and both crossover and noncrossover products. These results challenge the widely held notion that clustering of telomeres in meiosis promotes the ability of homologous chromosomes to find one another in budding Saccharomyces cerevisiae. We propose that an Ndj1-dependent function is critical for stabilizing analogous strand invasion intermediates that exist in two separate branches of the bifurcated pathway, leading to either noncrossover or crossover formation. These findings provide a link between telomere dynamics and a distinct mechanistic step of meiotic recombination that follows the homology search.Meiosis is a conserved cell division pathway that generates haploid gametes from diploid parents in sexual eukaryotes. In budding Saccharomyces cerevisiae and most eukaryotes, meiotic recombination produces crossovers (CRs) between homologous chromosomes. Together with sister chromatid cohesion, CRs hold homologs together prior to meiosis I disjunction to ensure proper chromosome segregation. Failure in this conserved process is a major cause of miscarriage and birth defects in humans (reviewed in references 21, 43, 45, and 64).Meiotic recombination is initiated by programmed doublestrand break (DSB) formation catalyzed by a topoisomeraselike protein, Spo11 (3, 30). The repair of DSBs can generate either CR or noncrossover (NCR) products. The subset of DSBs processed to CRs proceed primarily through a pathway (referred to here as the CR-I pathway) involving physically detectable intermediates: single-end invasion (SEI), followed by a double-Holliday junction (dHJ) with ligated DNA strands (26). It is thought that dHJs are resolved primarily as CRs (1,26,47). The CRs formed via the CR-I pathway are thought to be under crossover control and exhibit crossover interference. Their formation is dependent, to a large extent, on the ZMM group of proteins, comprising Zip1, Zip2, Zip3, Msh4-Msh5, and Mer3 (6, 53).NCRs and a smaller subset of CRs (called class CR-II) are likely processed by a synthesis-dependent strand annealing pathway that is dependent in part on Mus81 and Mms4 in budding yeast (reviewed in references 13, 23, and 25). Both the CR-I and NCR-plus-CR-II pathways require Spo11-dependent DSB formation, resection of 5Ј ends of the DSB, and presumably an unstable strand invasion intermediate promoted by the R...

A unique aspect of meiosis is the segregation of homologous chromosomes at the meiosis I division. The pairing of homologous chromosomes is a critical aspect of meiotic prophase I that aids proper disjunction at anaphase I. We have used a site-specific recombination assay in Saccharomyces cerevisiae to examine allelic interaction levels during meiosis in a series of mutants defective in recombination, chromatin structure, or intracellular movement. Red1, a component of the chromosome axis, and Mnd1, a chromosome-binding protein that facilitates interhomolog interaction, are critical for achieving high levels of allelic interaction. Homologous recombination factors (Sae2, Rdh54, Rad54, Rad55, Rad51, Sgs1) aid in varying degrees in promoting allelic interactions, while the Srs2 helicase appears to play no appreciable role. Ris1 (a SWI2/ SNF2 related protein) and Dot1 (a histone methyltransferase) appear to play minor roles. Surprisingly, factors involved in microtubule-mediated intracellular movement (Tub3, Dhc1, and Mlp2) appear to play no appreciable role in homolog juxtaposition, unlike their counterparts in fission yeast. Taken together, these results support the notion that meiotic recombination plays a major role in the high levels of homolog interaction observed during budding yeast meiosis.M EIOSIS is the process by which a parent diploid cell undergoes one round of DNA replication followed by two rounds of chromosome segregation to yield haploid gametes. A unique aspect of meiosis is the segregation of homologous chromosomes at the first meiotic division. Nondisjunction, or improper segregation of homologs, at this stage can lead to gamete aneuploidy, which is a major cause of birth defects in humans (Hassold and Hunt 2001). Homologs are able to correctly orient toward opposite poles of the meiosis I spindle, because a collaboration of DNA crossovers (CR) with sister-chromatid cohesion forms temporary connections between the homologs (Page and Hawley 2003;Petronczki et al. 2003).Homologous chromosomes form progressively stronger associations as cells proceed through meiotic prophase I (Zickler and Kleckner 1999;Storlazzi et al. 2003). In the budding yeast, plants, and mammals, transacting factors required for meiotic recombination are crucial for the pairing and crossing over between homologous chromosomes. In contrast, synaptonemal complex (SC) formation, meiotic nuclear reorganization, and an achiasmate segregation system appear to play supplementary roles in meiotic homolog pairing in the budding yeast (Loidl et al.

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