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.保真DNA复制（faithful DNA replication）要求细胞配合细胞周期进程，对自身组蛋白池（histone pool）进行精细调控。依赖复制的组蛋白生物合成（replication-dependent histone biosynthesis）在细胞周期承诺（cell-cycle commitment）后以低水平启动，随后在G1/S期转换（G1/S transition）时出现爆发式表达，但目前尚不清楚细胞在DNA复制启动时究竟如何调控组蛋白生物合成的这一变化。本研究借助单细胞延时成像（single-cell timelapse imaging），阐明了细胞在细胞周期不同阶段调控组蛋白生成的机制。研究发现，在限制点（Restriction Point）处，细胞周期蛋白依赖性激酶2（CDK2）介导的NPAT磷酸化可触发组蛋白转录（histone transcription），从而在G1/S期边界精准诱导组蛋白mRNA（histone mRNA）的爆发式积累。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，细胞通过两种协同发挥作用的不同机制，严格配合细胞周期进程调控自身的组蛋白生成。受文件大小限制，正文配图的代表性原始图像已随附，所有完整原始图像均可应要求提供。