Condensin-dependent chromatin condensation represses transcription globally during quiescence [ChIP-Seq]. Condensin-dependent chromatin condensation represses transcription globally during quiescence [ChIP-Seq]

Quiescence is a stress-resistant state in which cells reversibly exit the mitotic cell cycle and suspend most cellular processes. Quiescence is essential for stem cell maintenance and its misregulation is implicated in tumor formation. One of the conserved hallmarks of quiescent cells, from Saccharomyces cerevisiae to humans, is highly condensed chromatin. Here, we use Micro-C XL to map chromatin contacts at single-nucleosome resolution genome-wide to elucidate mechanisms and functions of condensed chromatin in quiescent S. cerevisiae cells. We describe previously uncharacterized chromatin domains on the order of 10-60 kilobases that in quiescent cells are formed by condensin-mediated chromatin loops. Conditional depletion of condensin prevents chromatin condensation during quiescence entry and leads to widespread transcriptional de-repression. We further demonstrate that condensin-dependent chromatin compaction is conserved in quiescent human fibroblasts. We propose that condensin-dependent condensation of chromatin represses transcription throughout the quiescent cell genome. Overall design: Examination of binding of condensin subunit Brn1 and RNA Polymerase II subunit Rpb3 in log and quiescent cells, and additionally of Rpb3 in quiescent cells in which transcription of the condensin subunit Smc4 has been shut down using an inducible tet-off system

细胞静止（Quiescence）是一种耐应激的细胞状态，此时细胞可逆性退出有丝分裂细胞周期，并暂停绝大多数细胞生理进程。细胞静止对于干细胞维持至关重要，其调控异常与肿瘤发生密切相关。从酿酒酵母（Saccharomyces cerevisiae）到人类，静止细胞的一个保守标志性特征是染色质高度凝集。本研究采用Micro-C XL技术（Micro-C XL），在全基因组范围内以单核小体分辨率绘制染色质互作图谱，以期阐明酿酒酵母静止细胞中凝集染色质的作用机制与生物学功能。我们发现了一类此前未被表征的、尺寸约10-60千碱基的染色质结构域，该类结构域在静止细胞中由凝缩素（condensin）介导的染色质环构成。对凝缩素进行条件性耗竭，会阻断静止态建立过程中的染色质凝集，并引发全基因组范围的转录去抑制现象。我们进一步证实，凝缩素依赖的染色质压实过程在人类静止成纤维细胞中同样具有保守性。我们提出假说：凝缩素介导的染色质凝集会在整个静止细胞基因组中发挥转录抑制作用。整体实验设计：检测对数生长期与静止期细胞中凝缩素亚基Brn1与RNA聚合酶II（RNA Polymerase II）亚基Rpb3的结合情况；此外，利用可诱导tet-off系统（tet-off system）关闭凝缩素亚基Smc4的转录后，检测此类静止细胞中的Rpb3结合水平。