Global early replication disrupts gene expression and chromatin conformation in a single cell cycle: MNAse-Seq mapping. Global early replication disrupts gene expression and chromatin conformation in a single cell cycle: MNAse-Seq mapping

The early embryonic divisions of many organisms, including fish, flies and frogs are characterised by a very rapid S-phase caused by high rates of replication initiation. In somatic cells, S-phase is much longer due to both a reduction in the total number of initiation events and the imposition of a temporal order of origin activation. The physiological importance of changes in the rate and timing of replication initiation in S-phase remains unclear. Here we assess the importance of the temporal control of replication initiation using a conditional system in budding yeast to drive the early replication of all origins in a single cell cycle. We show that global early replication disrupts the expression of over a quarter of all genes. By deleting individual origins we show that delaying replication is sufficient to restore normal gene expression, directly establishing replication timing control in this regulation. Global early replication disrupts nucleosome positioning and transcription factor binding during S-phase, suggesting that the rate of S-phase is important to regulate the chromatin landscape. Together these data provide new insight into the role of a temporal order of origin firing for coordinating replication, gene expression and chromatin establishment as occurs in the early embryo. Overall design: Analysis of nucleosome landscape using MNase-Seq upon a global advance in replication timing caused by overexpression of six limiting initiation factors (SSDDCS)

包括鱼类、果蝇和青蛙在内的诸多生物的早期胚胎分裂，均以极快的S期（S-phase）为特征，该S期由高频率的复制起始事件所驱动。在体细胞中，S期则长得多，这既源于总复制起始事件数量的减少，也源于复制起点激活的时序秩序的确立。S期中复制起始速率与时序变化的生理学重要性仍不明晰。本研究借助酿酒酵母（budding yeast）中的条件性系统，驱动单个细胞周期内所有复制起点的提前复制，以此评估复制起始时序调控的重要性。我们发现，全局提前复制会扰乱超过四分之一的基因的表达。通过敲除单个复制起点，我们证实延迟复制足以恢复正常基因表达，直接确立了复制时序调控在该调控过程中的核心作用。全局提前复制会在S期扰乱核小体定位与转录因子结合，这表明S期的速率对调控染色质景观至关重要。综上，这些数据为复制起点激活的时序秩序在协调复制、基因表达与染色质构建（正如早期胚胎中所发生的那样）中的作用提供了新见解。整体设计：通过过表达六种限制性复制起始因子（SSDDCS）引发复制时序的全局提前，借此利用微球菌核酸酶测序（MNase-Seq）分析核小体景观。