The meiotic cohesin Rec8 is required for the stepwise segregation of chromosomes during the two rounds of meiotic division. 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. In the absence of Rec8, chromosomes were decompacted relative to those of wild-type cells. Conversely, loss of the cohesin-associated protein Pds5 resulted in hypercompaction. 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. 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.

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Three new cyclolanostanol xylosides were isolated from a batch of commercial Cimicifuga Rhizome, cimicifugoside H-1 (1), C35H52O9, mp 260-262 degrees C, [alpha]D -43.5 degrees, cimicifugoside H-2 (2), C35H54O10, mp 227-229 degrees C, [alpha]D -38.8 degrees, and cimicifugoside H-5 (3), C35H52O10, mp 262-264 degrees C, [alpha]D -22.9 degrees, together with known glycosides, actein and 27-deoxyactein. Their structures were determined on the basis of chemical and spectrometric evidence including an X-ray crystallographic analysis. The structure of cimicifugoside H-1 (1) was established as (20R,24R)-24,25-epoxy-11 beta-hydroxy-3-beta-(beta-D- xylopyranosyloxy)-9,19-cyclolanost-7-ene-16,23-dione. Cimicifugoside H-5 (3) is the 15-hydroxylated derivative of 1. Since 1 changed into cimicifugoside H-2 (2) on treatment with p-toluenesulfonic acid, 2 has a 24R,25-diol structure derived from 1 by opening its epoxy ring.