![]() In the fission yeast Schizosaccharomyces pombe, the mitotic cohesin complex is composed of two structure maintenance of chromosome (SMC) subunits, Psm1 and -3, and two non-SMC subunits, Rad21 and Psc3 ( Tomonaga et al., 2000). It has also been shown that Pds5/Spo76 has a role in meiosis because its loss results in defects in spore formation and SC integrity ( van Heemst et al., 1999 Zhang et al., 2005). cerevisiae: FISH analysis demonstrated that these molecules are required for ribosomal DNA (rDNA) condensation at metaphase of mitosis ( Guacci et al., 1997 Hartman et al., 2000). A role for Mcd1/Scc1 (the Rad21 homologue) and Pds5 in mitotic chromosome condensation has been suggested in S. ![]() Two Pds5s, Pds5A and Pds5B, have been identified in vertebrate cells ( Sumara et al., 2000 Losada et al., 2005). This protein has been studied in Sordaria macrospora (called Spo76), Aspergillus nidulans (called BimD), Saccharomyces cerevisiae, and Caenorhabditis elegans ( van Heemst et al., 1999, 2001 Hartman et al., 2000 Wang et al., 2003 Zhang et al., 2005). In addition to the core cohesin complex, a conserved protein known as Pds5, Spo76, or BimD is associated with cohesin and is implicated in sister chromatid cohesion and chromosome structure maintenance. However, the molecular mechanisms that underlie meiotic prophase chromosome compaction remain unclear. In many organisms, longitudinal compaction of the chromosome is observed along with chromosome axis formation during meiotic prophase ( Heng et al., 1996 Moens et al., 1998 Zickler and Kleckner, 1999 Kleckner et al., 2003). This chromosome axis later forms the axial/lateral element of the synaptonemal complex (SC Klein et al., 1999 Pasierbek et al., 2001 Pelttari et al., 2001 Eijpe et al., 2003). The meiotic cohesin complex not only mediates sister chromatid cohesion but also plays a critical role in assembling a proteinaceous chromosome axis as its major component ( Revenkova and Jessberger, 2006). To explain these observations, we propose that meiotic prophase chromosomes are organized as chromatin loops emanating from a Rec8-containing axis: the absence of Rec8 disrupts the axis, resulting in disorganized chromosomes, whereas reduced Rec8 loading results in a longitudinally compacted axis with fewer attachment points and longer chromatin loops.ĭuring the mitotic cell cycle, the regulated segregation of sister chromatids is achieved by the mitotic cohesin complex, and upon entering meiosis, additional meiosis-specific cohesin variants appear to choreograph meiosis-specific chromosomal events ( Nasmyth, 2001 Revenkova and Jessberger, 2006). Although this hypercompaction requires Rec8, binding of Rec8 to chromatin was reduced in the absence of Pds5, indicating that Pds5 promotes chromosome association of Rec8. Conversely, loss of the cohesin-associated protein Pds5 resulted in hypercompaction. In the absence of Rec8, chromosomes were decompacted relative to those of wild-type cells. ![]() By directly measuring chromosome compaction in living cells of the fission yeast Schizosaccharomyces pombe, we found an additional role for the meiotic cohesin in the compaction of chromosomes during meiotic prophase. The meiotic cohesin Rec8 is required for the stepwise segregation of chromosomes during the two rounds of meiotic division.
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