During meiosis, homologues become juxtaposed and synapsed along their entire length. Mutations in the cohesin complex disrupt not only sister chromatid cohesion but also homologue pairing and synaptonemal complex formation. In this study, we report that Pds5, a cohesin-associated protein known to regulate sister chromatid cohesion, is required for homologue pairing and synapsis in budding yeast. Pds5 colocalizes with cohesin along the length of meiotic chromosomes. In the absence of Pds5, the meiotic cohesin subunit Rec8 remains bound to chromosomes with only minor defects in sister chromatid cohesion, but sister chromatids synapse instead of homologues. Double-strand breaks (DSBs) are formed but are not repaired efficiently. In addition, meiotic chromosomes undergo hypercondensation. When the mitotic cohesin subunit Mcd1 is substituted for Rec8 in Pds5-depleted cells, chromosomes still hypercondense, but synapsis of sister chromatids is abolished. These data suggest that Pds5 modulates the Rec8 activity to facilitate chromosome morphological changes required for homologue synapsis, DSB repair, and meiotic chromosome segregation.

Sister chromatid cohesion and interhomologue recombination are coordinated to promote the segregation of homologous chromosomes instead of sister chromatids at the first meiotic division. During meiotic prophase in Saccharomyces cerevisiae , the meiosis-specific cohesin Rec8p localizes along chromosome axes and mediates most of the cohesion. The mitotic cohesin Mcd1p/Scc1p localizes to discrete spots along chromosome arms, and its function is not clear. In cells lacking Tid1p, which is a member of the SWI2/SNF2 family of helicase-like proteins that are involved in chromatin remodeling, Mcd1p and Rec8p persist abnormally through both meiotic divisions, and chromosome segregation fails in the majority of cells. Genetic results indicate that the primary defect in these cells is a failure to resolve Mcd1p-mediated connections. Tid1p interacts with recombination enzymes Dmc1p and Rad51p and has an established role in recombination repair. We propose that Tid1p remodels Mcd1p-mediated cohesion early in meiotic prophase to facilitate interhomologue recombination and the subsequent segregation of homologous chromosomes.

Pds5p and the cohesin complex are required for sister chromatid cohesion and localize to the same chromosomal loci over the same cell cycle window. However, Pds5p and the cohesin complex likely have distinct roles in cohesion. We report that pds5 mutants establish cohesion, but during mitosis exhibit precocious sister dissociation. Thus, unlike the cohesin complex, which is required for cohesion establishment and maintenance, Pds5p is required only for maintenance. We identified SMT4 , which encodes a SUMO isopeptidase, as a high copy suppressor of both the temperature sensitivity and precocious sister dissociation of pds5 mutants. In contrast, SMT4 does not suppress temperature sensitivity of cohesin complex mutants. Pds5p is SUMO conjugated, with sumoylation peaking during mitosis. SMT4 overexpression reduces Pds5p sumoylation, whereas smt4 mutants have increased Pds5p sumoylation. smt4 mutants were previously shown to be defective in cohesion maintenance during mitosis. These data provide the first link between a protein required for cohesion, Pds5p, and sumoylation, and suggest that Pds5p sumoylation promotes the dissolution of cohesion.

We identified the chromosomal addresses of a cohesin subunit, Mcd1p, in vivo by chromatin immunoprecipitation coupled with high resolution PCR-based chromosomal walking. The mapping of new Mcd1p-binding sites (cohesin-associated regions [ CAR s]) in single-copy sequences of several chromosomes establish their spacing (∼9 kb), their sequestration to intergenic regions, and their association with AT-rich sequences as general genomic properties of CAR s. We show that cohesins are not excluded from telomere proximal regions, and the enrichment of cohesins at the centromere at mitosis reflects de novo loading. The average size of a CAR is 0.8–1.0 kb. They lie at the boundaries of transcriptionally silenced regions, suggesting they play a direct role in defining the silent chromatin domain. Finally, we identify CAR s in tandem (rDNA) and interspersed repetitive DNA (Ty2 and subtelomeric repeats). Each 9-kb rDNA repeat has a single CAR proximal to the 5S gene. Thus, the periodicity of CARs in single-copy regions and the rDNA repeats is conserved. The presence and spacing of CARs in repetitive DNA has important implications for genomic stability and chromosome packaging/condensation.

The PDS5 gene (precocious dissociation of sisters) was identified in a genetic screen designed to identify genes important for chromosome structure. PDS5 is an essential gene and homologues are found from yeast to humans. Pds5p function is important for viability from S phase through mitosis and localizes to chromosomes during this cell cycle window, which encompasses the times when sister chromatid cohesion exists. Pds5p is required to maintain cohesion at centromere proximal and distal sequences. These properties are identical to those of the four cohesion complex members Mcd1p/Scc1p, Smc1p, Smc3p, and Scc3p/Irr1p (Guacci, V., D. Koshland, and A. Strunnikov. 1997. Cell . 91:47–57; Michaelis, C., R. Ciosk, and K. Nasmyth. 1997. Cell . 91:35–45; Toth, A., R. Ciosk, F. Uhlmann, M. Galova, A. Schleiffer, and K. Nasmyth. 1999. Genes Dev . 13:307–319). Pds5p binds to centromeric and arm sequences bound by Mcd1p. Furthermore, Pds5p localization to chromosomes is dependent on Mcd1p. Thus, Pds5p, like the cohesin complex members, is a component of the molecular glue that mediates sister chromatid cohesion. However, Mcd1p localization to chromosomes is independent of Pds5p, which may reflect differences in their roles in cohesion. Finally, Pds5p is required for condensation as well as cohesion, which confirms the link between these processes revealed through analysis of Mcd1p (Guacci, V., D. Koshland, and A. Strunnikov. 1997. Cell . 91:47–57). Therefore, the link between cohesion and condensation is a general property of yeast chromosomes.