Cyclin E/CDK2 and feedback from soluble histone protein regulate the S phase burst of histone biosynthesisCyclin E/CDK2 and feedback from soluble histone protein regulate the S phase burst of histone biosynthesis

Faithful DNA replication requires that cells fine-tune their histone pool in coordination with cell-cycle progression. Replication-dependent histone biosynthesis is initiated at a low level upon cell-cycle commitment, followed by a burst at the G1/S transition, but it remains unclear how exactly the cell regulates this change in histone biosynthesis as DNA replication begins. Here, we use single-cell timelapse imaging to elucidate the mechanisms by which cells modulate histone production during different phases of the cell cycle. We find that CDK2-mediated phosphorylation of NPAT at the Restriction Point triggers histone transcription, which results in a burst of histone mRNA precisely at the G1/S phase boundary. Excess soluble histone protein further modulates histone abundance by promoting the degradation of histone mRNA for the duration of S phase. Thus, cells regulate their histone production in strict coordination with cell-cycle progression by two distinct mechanisms acting in concert. Due to size limitations, representative raw images for main text figures have been included with all raw images available upon request.忠实的DNA复制需要细胞与细胞周期进程协同调控其组蛋白池（histone pool）。依赖复制的组蛋白生物合成在细胞做出周期承诺后以低水平启动，随后在G1/S转换期（G1/S transition）出现爆发式表达，但目前学界尚未明确细胞究竟如何在DNA复制启动阶段精准调控组蛋白生物合成的动态变化。本研究借助单细胞延时成像技术，阐明了细胞在细胞周期不同阶段调控组蛋白生成的具体机制。我们发现，细胞周期蛋白依赖性激酶2（CDK2）介导的限制点（Restriction Point）处NPAT的磷酸化可触发组蛋白转录，使组蛋白mRNA（histone mRNA）的表达爆发精准发生于G1/S期边界。过量的可溶性组蛋白可通过促进S期（S phase）内组蛋白mRNA的降解，进一步调控组蛋白的丰度。综上，细胞通过两种协同作用的不同机制，实现组蛋白生成与细胞周期进程的严格协同调控。受篇幅限制，本文已附上正文图表的代表性原始图像，所有原始图像均可应要求提供。