Analysis of Close Stable Homolog Juxtaposition During Meiosis in Mutants ofSaccharomyces cerevisiae

Abstract

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.

Prelude to a Division

“…Because Cre-mediated recombination does not involve strand-exchange intermediates that might themselves promote pairing, this assay is thought to report on pairing without influencing it (S. Burgess, personal communication). As detected by this assay, CSHJ requires DSBs, DMC1 (Peoples et al 2002), RAD51 (Lui et al 2006), and axial element proteins (Peoples et al 2002). Consistent with cytological analysis of axial associations, CSHJ can be detected in the absence of synapsis and crossovers (Peoples et al 2002), indicating that the stabilization of homolog pairing depends on the initiation of crossovers, but not on their completion.…”

“…Sgs1 is a homolog of the human Bloom Syndrome helicase (BLM), a member of the RecQ family (Ellis et al 1995, Watt et al 1996. Mutations in SGS1 produce the unusual phenotype of elevated numbers of axial associations in the absence of synapsis (Rockmill et al 2003) and increased ectopic recombination in the Cre recombinase-based CSHJ assay (Lui et al 2006). Mutation of SGS1 also results in a mitotic hyper-recombination phenotype (Gangloff et al 1994, Ira et al 2003) similar to defects observed in the cancer-prone cells from Bloom Syndrome patients (Chaganti et al 1974).…”

“…Dernburg, unpublished). Surprisingly, dynein is dispensable for meiotic prophase in S. cerevisiae (Lui et al 2006), and the meiotic bouquet is insensitive to microtubule drugs but is disrupted by latrunculin, which results in actin depolymerization (Trelles-Sticken et al 2005). Latrunculin also reversibly disrupts telomere-led chromosome movement during meiotic prophase (Scherthan et al 2007).…”

Sites of Recombination Are Local Determinants of Meiotic Homolog Pairing inSaccharomyces cerevisiae

“…Measurement of the CSHJ phenotype in a series of mutants has shown that entry into meiosis and meiotic recombination factors are required for achieving wild-type CSHJ (Peoples et al 2002;Lui et al 2006). Mutants with defects in recombination initiation or early stages of DSB repair have severe defects in CSHJ, while those with defects in later stages of repair have moderate defects or no defect.…”

Cooperative Interactions Between Pairs of Homologous Chromatids During Meiosis in Saccharomyces cerevisiae

“…The Cre/loxP site-specific recombination system provides a quantitative genetic probe of meiotic homolog pairing in living yeast cells (Peoples et al 2002;Peoples-Holst and Burgess 2005;Lui et al 2006). In an accompanying article, we show that crossover-associated meiotic recombination increases the probability of nearby Cre-mediated loxP recombination events (''collisions'') between homologous chromatids (Mell et al 2008, this issue).…”