Li Junwei, Santoro Raffaella, Koberna Karel, Grummt Ingrid
Division of Molecular Biology of the Cell II, German Cancer Research Center, Heidelberg, Germany.
EMBO J. 2005 Jan 12;24(1):120-7. doi: 10.1038/sj.emboj.7600492. Epub 2004 Dec 2.
The ATP-dependent chromatin remodeling complex NoRC silences a fraction of mammalian ribosomal RNA genes (rDNA) by establishing heterochromatic structures at the rDNA promoter. Here we show that NoRC also plays a role in replication timing of rDNA. rDNA is replicated in a biphasic manner, active genes ( approximately 60%) replicating early and silent ones ( approximately 40%) replicating late in S-phase. The chromatin structure that marks active and silent rDNA repeats is propagated during cell division. To examine the function of NoRC in epigenetic inheritance and replication timing, we have monitored the chromatin structure, transcriptional activity and replication timing of rDNA in a cell line that moderately overexpresses NoRC. NoRC is exclusively associated with late-replicating rDNA arrays. Overexpression of NoRC silences rDNA transcription, reduces the size and number of nucleoli, impairs cell proliferation and resets replication timing from early to late. The results demonstrate that NoRC is an important determinant of replication timing and epigenetic marks are heritably maintained through DNA replication.
ATP 依赖的染色质重塑复合物 NoRC 通过在核糖体 RNA 基因（rDNA）启动子处建立异染色质结构，使一部分哺乳动物 rDNA 沉默。我们在此表明，NoRC 在 rDNA 的复制时间调控中也发挥作用。rDNA 以双相方式复制，活跃基因（约 60%）在 S 期早期复制，沉默基因（约 40%）在 S 期晚期复制。标记活跃和沉默 rDNA 重复序列的染色质结构在细胞分裂过程中得以传播。为了研究 NoRC 在表观遗传遗传和复制时间调控中的功能，我们监测了在适度过表达 NoRC 的细胞系中 rDNA 的染色质结构、转录活性和复制时间。NoRC 仅与晚期复制的 rDNA 阵列相关。NoRC 的过表达使 rDNA 转录沉默，减小核仁的大小并减少其数量，损害细胞增殖，并将复制时间从早期重置为晚期。结果表明，NoRC 是复制时间的重要决定因素，并且表观遗传标记通过 DNA 复制得以遗传维持。
