Condensin II is recruited to promoters by pRb and regulates expression of divergently paired genes [RNA-seq]

In this study, use of a gene-targeted mouse model that is defective for pRb-condensin II interactions, Rb1L, has revealed instances where condensin II localization in interphase nuclei is dependent on pRb. Condensin II binding overlaps significantly with marks of active chromatin, and is enriched at the promoters of genes, including closely spaced divergent promoters. Interestingly, there are some bidirectional promoters where condensin II binds in an pRb-dependent manner and transcription of only one of the two genes is upregulated in Rb1L/L MEFs, suggesting the pRb-condensin II complex may be acting as a repressor or an insulator at distinct genomic locations to influence transcription. Chromatin conformations at these locations demonstrated that the pRb-condensin II complex may be responsible for maintaining more static, long-range interactions. In addition, pRb also recruits TFIIIC, another condensin II interactor, to many genomic loci. These recently discovered non-canonical transcriptional functions of the pRb-condensin II complex may represent an additional mechanism of pRb-mediated tumor suppression. Overall design: Total RNA from proliferating MEFs was isolated. Three Rb1+/+ and Rb1L/L MEF pairs were used for this analysis and expression between genotypes was compared.

本研究中，通过使用存在pRb与凝缩蛋白II（condensin II）相互作用缺陷的基因靶向小鼠模型Rb1L，揭示了间期细胞核内凝缩蛋白II的定位依赖于pRb的情形。凝缩蛋白II的结合位点与活性染色质标记存在显著重叠，且在基因启动子区域（包括紧密间隔的双向启动子）处富集。有趣的是，在部分双向启动子区域，凝缩蛋白II以pRb依赖的方式结合，且在Rb1L/L小鼠胚胎成纤维细胞（mouse embryonic fibroblasts, MEFs）中仅两个基因之一的转录被上调，这提示pRb-凝缩蛋白II复合物可能在不同基因组位点充当阻遏物或绝缘子，以调控转录。对这些位点的染色质构象分析表明，pRb-凝缩蛋白II复合物可能负责维持更稳定的长程染色质相互作用。此外，pRb还可将另一种凝缩蛋白II相互作用蛋白——转录因子IIIC（Transcription Factor II C, TFIIIC）招募至众多基因组位点。此次新发现的pRb-凝缩蛋白II复合物的非经典转录功能，可能代表了pRb介导的肿瘤抑制的另一重机制。实验设计：本研究分离了增殖态MEFs的总RNA，使用3对Rb1+/+与Rb1L/L MEFs进行分析，并比较了不同基因型细胞间的基因表达差异。