Department of Cell Biology, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
J Cell Biol. 2009 Nov 30;187(5):623-35. doi: 10.1083/jcb.200905144.
In the mammalian genome, early- and late-replicating domains are often separated by temporal transition regions (TTRs) with novel properties and unknown functions. We identified a TTR in the mouse immunoglobulin heavy chain (Igh) locus, which contains replication origins that are silent in embryonic stem cells but activated during B cell development. To investigate which factors contribute to origin activation during B cell development, we systematically modified the genetic and epigenetic status of the endogenous Igh TTR and used a single-molecule approach to analyze DNA replication. Introduction of a transcription unit into the Igh TTR, activation of gene transcription, and enhancement of local histone modifications characteristic of active chromatin did not lead to origin activation. Moreover, very few replication initiation events were observed when two ectopic replication origin sequences were inserted into the TTR. These findings indicate that the Igh TTR represents a repressive compartment that inhibits replication initiation, thus maintaining the boundaries between early and late replication domains.
在哺乳动物基因组中，早期复制域和晚期复制域通常由具有新特性和未知功能的时间过渡区（TTR）隔开。我们在小鼠免疫球蛋白重链（Igh）基因座中鉴定出一个 TTR，其中包含在胚胎干细胞中沉默但在 B 细胞发育过程中激活的复制起点。为了研究在 B 细胞发育过程中哪些因素有助于原点激活，我们系统地修饰了内源性 Igh TTR 的遗传和表观遗传状态，并使用单分子方法分析了 DNA 复制。在 Igh TTR 中引入转录单元、激活基因转录以及增强局部组蛋白修饰特征的活性染色质并没有导致原点激活。此外，当将两个异位复制起点序列插入 TTR 时，观察到很少有复制起始事件。这些发现表明，Igh TTR 代表一个抑制区，抑制复制起始，从而维持早期和晚期复制域之间的边界。
