Early embryos must rapidly generate large numbers of cells to form an organism. Many species accomplish this through a series of rapid, reductive, and transcriptionally silent cleavage divisions. Previous work has demonstrated that the number of divisions before both cell cycle elongation and zygotic genome activation (ZGA) is regulated by the ratio of nuclear content to cytoplasm (N/C). To understand how the N/C ratio affects the timing of ZGA, we directly assayed the behavior of several previously identified N/C-ratio-dependent genes using the MS2-MCP reporter system in living Drosophila embryos with altered ploidy and cell cycle durations. For every gene that we examined, we found that nascent RNA output per cycle is delayed in haploid embryos. Moreover, we found that the N/C ratio influences transcription through three separate modes of action. For some genes (knirps and snail) the effect of ploidy can be entirely accounted for by changes in cell cycle duration. However, for other genes (giant, bottleneck and fruhstart) the N/C ratio directly affects ZGA. For giant and bottleneck, the N/C ratio regulates the kinetics of transcription activation, while for fruhstart it controls the probability of transcription initiation. Our data demonstrate that the regulatory elements of N/C-ratio-dependent genes respond directly to the N/C ratio, through multiple modes of regulation, independent of interphase length.

中文翻译：

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核质比通过多种方式直接调节果蝇的合子转录

早期胚胎必须迅速产生大量细胞才能形成生物体。许多物种通过一系列快速的，还原性的和转录沉默的分裂分裂来实现这一目标。先前的研究表明，细胞周期延长和合子基因组激活（ZGA）之前的分裂数受核含量与细胞质的比率（N / C）调节。为了了解N / C比如何影响ZGA的时间，我们使用MS2-MCP报告系统直接检测了几个先前鉴定的N / C比率依赖性基因在果蝇胚胎中的倍性和细胞周期持续时间的变化。对于我们检查的每个基因，我们发现单倍体胚胎每个周期的新生RNA输出均被延迟。此外，我们发现N / C比通过三种独立的作用模式影响转录。对于某些基因（knirps和蜗牛），倍性的影响可以完全通过细胞周期持续时间的变化来解释。但是，对于其他基因（巨人，瓶颈和果蝇），N / C比直接影响ZGA。对于巨大的瓶颈，N / C比调节转录激活的动力学，而对于fruhstart，它控制转录起始的可能性。我们的数据表明，N / C比依赖性基因的调控元件通过多种调控模式直接响应N / C比，而与相间长度无关。N / C比调节转录激活的动力学，而对于果糖启动，它控制转录起始的可能性。我们的数据表明，N / C比依赖性基因的调控元件通过多种调控模式直接响应N / C比，而与相间长度无关。N / C比调节转录激活的动力学，而对于果糖启动，它控制转录起始的可能性。我们的数据表明，N / C比依赖性基因的调控元件通过多种调控模式直接响应N / C比，而与相间长度无关。