MRC London Institute of Medical Sciences (LMS), Du Cane Road, London W12 0NN, UK; Institute of Clinical Sciences (ICS), Faculty of Medicine, Imperial College London, Du Cane Road, London W12 0NN, UK.
Department of Mathematics, Faculty of Natural Sciences, Imperial College London, London SW7 2AZ, UK.
Curr Biol. 2020 Apr 6;30(7):1217-1230.e7. doi: 10.1016/j.cub.2020.01.053. Epub 2020 Feb 13.
Cell size varies during the cell cycle and in response to external stimuli. This requires the tight coordination, or "scaling," of mRNA and protein quantities with the cell volume in order to maintain biomolecule concentrations and cell density. Evidence in cell populations and single cells indicates that scaling relies on the coordination of mRNA transcription rates with cell size. Here, we use a combination of single-molecule fluorescence in situ hybridization (smFISH), time-lapse microscopy, and mathematical modeling in single fission yeast cells to uncover the precise molecular mechanisms that control transcription rates scaling with cell size. Linear scaling of mRNA quantities is apparent in single fission yeast cells during a normal cell cycle. Transcription of both constitutive and periodic genes is a Poisson process with transcription rates scaling with cell size and without evidence for transcriptional off states. Modeling and experimental data indicate that scaling relies on the coordination of RNA polymerase II (RNAPII) transcription initiation rates with cell size and that RNAPII is a limiting factor. We show using real-time quantitative imaging that size increase is accompanied by a rapid concentration-independent recruitment of RNAPII onto chromatin. Finally, we find that, in multinucleated cells, scaling is set at the level of single nuclei and not the entire cell, making the nucleus a determinant of scaling. Integrating our observations in a mechanistic model of RNAPII-mediated transcription, we propose that scaling of gene expression with cell size is the consequence of competition between genes for limiting RNAPII.
细胞大小在细胞周期中以及对外界刺激的反应中会发生变化。这就需要严格协调（“缩放”）mRNA 和蛋白质的数量与细胞体积之间的关系，以维持生物分子浓度和细胞密度。细胞群体和单细胞的证据表明，缩放依赖于 mRNA 转录率与细胞大小的协调。在这里，我们在单个裂殖酵母细胞中结合使用单分子荧光原位杂交 (smFISH)、延时显微镜和数学建模，以揭示控制转录率与细胞大小缩放的精确分子机制。在正常的细胞周期中，单个裂殖酵母细胞中的 mRNA 数量呈现线性缩放。组成型和周期性基因的转录都是泊松过程，转录率与细胞大小成比例，没有转录关闭状态的证据。模型和实验数据表明，缩放依赖于 RNA 聚合酶 II (RNAPII) 转录起始率与细胞大小的协调，并且 RNAPII 是一个限制因素。我们使用实时定量成像显示，大小增加伴随着 RNAPII 在染色质上的快速、浓度独立的募集。最后，我们发现，在多核细胞中，缩放是在单个核的水平上设定的，而不是整个细胞，这使得核成为缩放的决定因素。我们将对 RNAPII 介导的转录的机制模型中的观察结果进行整合，提出细胞大小与基因表达的缩放是由基因之间对有限的 RNAPII 的竞争所导致的。
