在真核生物的细胞分裂过程中，DNA表现为一厘米长短的细丝，彼此缠结成有特定形状的染色体。当DNA复制时，两份DNA被环状蛋白复合物（被称为Cohesin 和 Condensin）结合在一起。这两种物质被认为每种环绕两个DNA链，形成一个网络的节点，该网络将两份DNA保持在一起，直到它们接收到让其分开形成不同子细胞的信号。现在，对芽殖酵母染色体上Cohesin点所做的一项高分辨率分析显示了这些节点的位置。那些蛋白环不是紧紧粘在DNA上，而是被在将DNA转录成mRNA过程中所涉及的其他蛋白复合物往前推动。
Cohesin relocation from sites of chromosomal loading to places of convergent transcription
Sister chromatids, the products of eukaryotic DNA replication, are held together by the chromosomal cohesin complex after their synthesis. This allows the spindle in mitosis to recognize pairs of replication products for segregation into opposite directions. Cohesin forms large protein rings that may bind DNA strands by encircling them, but the characterization of cohesin binding to chromosomes in vivo has remained vague. We have performed high resolution analysis of cohesin association along budding yeast chromosomes III–VI. Cohesin localizes almost exclusively between genes that are transcribed in converging directions. We find that active transcription positions cohesin at these sites, not the underlying DNA sequence. Cohesin is initially loaded onto chromosomes at separate places, marked by the Scc2/Scc4 cohesin loading complex, from where it appears to slide to its more permanent locations. But even after sister chromatid cohesion is established, changes in transcription lead to repositioning of cohesin. Thus the sites of cohesin binding and therefore probably sister chromatid cohesion, a key architectural feature of mitotic chromosomes, display surprising flexibility. Cohesin localization to places of convergent transcription is conserved in fission yeast, suggesting that it is a common feature of eukaryotic chromosomes.
Figure 1 Cohesin localizes to convergent intergene regions along budding yeast chromosome VI. Cells containing Smc3-Flag3 or Scc1-HA6 were arrested in metaphase by nocodazole treatment and processed for ChIP against the epitope tagged subunits. Enrichment in the immunoprecipitated fraction relative to a whole genome DNA sample is shown along the length of the chromosome. Each bar represents the average of 16 oligonucleotide probes within adjacent 300 bp windows. The y-axis scale is log2. Dark grey signals represent significant binding, as detailed in the Supplementary MIAME Document. Blue bars above and below the midline are genes transcribed respectively from left to right, and from right to left. The purple oval represents the centromere. Origins of replication are depicted in red, tRNA genes in yellow, and a Ty2 transposon in green.
Figure 2 Cohesin is moved towards the 3'-end of genes by their transcription. a, Localization of Scc1-HA6 around STE2 on chromosome VI in a nocodazole-arrested wild-type strain (top), and after deletion of the STE2 promoter (bottom). b, Localization of Scc1-HA6 around MSH4, close to centromere VI, in mitotic cells (top), and of Rec8-Flag3 in meiosis (bottom) when MSH4 is induced. c, Localization of Scc1-HA6 in nocodazole-arrested cells before (top) and 15 min after heat-shock (bottom). Regions around HSP30 and SRO9 on chromosome III, induced and repressed after heat-shock respectively, are shown. See Fig. 1 legend for key to colour coded chromosomal features. Red dashed lines flank the regions described in the text.
Figure 3 Cohesin loading at, and movement away from, sites of Scc2/Scc4 binding. a, Localization of Scc4-HA6 and Scc2-HA6 along chromosome VI in hydroxyurea-arrested cells. See Fig. 1 legend for meaning of colour coded chromosomal features. b, Transcriptional activity along chromosome VI in wild-type logarithmically growing cells. The signal intensity reflects the abundance of transcripts, as an approximation of transcriptional activity. Broken bars exceed 10,000; see Supplementary Fig. S5 for full values .c, Detail of the localization on the left arm of chromosome VI, from top to bottom, of Scc2-HA6 in hydroxyurea-arrested (HU) cells, Scc1-HA6 in scc2-4 mutant cells released from G1 into hydroxyurea block at restrictive temperature, Scc1-HA6 in wild-type cells 10 min, and 30 min following release from G1 at 16 °C, and Scc1-HA6 in hydroxyurea-arrested wild-type cells. The green and red dashed lines flank sites usually seen bound by Scc2 and Scc1, respectively. See Fig. 1 legend for meaning of colour coded chromosomal features.
Figure 4 Cohesin localization to convergent intergenic regions is conserved in fission yeast. ChIP was performed against the cohesin subunit Rad21-HA3 from logarithmically growing cells. A 240-kb-long stretch from the right arm of chromosome 2 is shown. The prefix 'SPBC' has been omitted from the systematic gene names.
