Biology Department, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
Nucleic Acids Res. 2021 May 7;49(8):4586-4598. doi: 10.1093/nar/gkab229.
The nucleolus is the site of ribosome biosynthesis encompassing the ribosomal DNA (rDNA) locus in a phase separated state within the nucleus. In budding yeast, we find the rDNA locus and Cdc14, a protein phosphatase that co-localizes with the rDNA, behave like a condensate formed by polymer-polymer phase separation, while ribonucleoproteins behave like a condensate formed by liquid-liquid phase separation. The compaction of the rDNA and Cdc14's nucleolar distribution are dependent on the concentration of DNA cross-linkers. In contrast, ribonucleoprotein nucleolar distribution is independent of the concentration of DNA cross-linkers and resembles droplets in vivo upon replacement of the endogenous rDNA locus with high-copy plasmids. When ribosomal RNA is transcribed from the plasmids by Pol II, the rDNA-binding proteins and ribonucleoprotein signals are weakly correlated, but upon repression of transcription, ribonucleoproteins form a single, stable droplet that excludes rDNA-binding proteins from its center. Degradation of RNA-DNA hybrid structures, known as R-loops, by overexpression of RNase H1 results in the physical exclusion of the rDNA locus from the nucleolar center. Thus, the rDNA locus is a polymer-polymer phase separated condensate that relies on transcription and physical contact with RNA transcripts to remain encapsulated within the nucleolus.
核仁是核糖体生物合成的场所,包含核内相分离状态下的核糖体 DNA(rDNA)基因座。在 budding yeast 中,我们发现 rDNA 基因座和 Cdc14(一种与 rDNA 共定位的蛋白磷酸酶)的行为类似于聚合物-聚合物相分离形成的凝聚物,而核糖核蛋白则类似于液体-液体相分离形成的凝聚物。rDNA 和 Cdc14 的核仁分布的压缩依赖于 DNA 交联剂的浓度。相比之下,核糖核蛋白的核仁分布与 DNA 交联剂的浓度无关,并且在用高拷贝质粒替换内源性 rDNA 基因座后,在体内类似于液滴。当 Pol II 从质粒转录核糖体 RNA 时,rDNA 结合蛋白和核糖核蛋白信号之间的相关性较弱,但在转录被抑制后,核糖核蛋白形成一个单一、稳定的液滴,将 rDNA 结合蛋白排除在其中心之外。通过过度表达 RNase H1 降解 RNA-DNA 杂交结构(称为 R 环)会导致 rDNA 基因座从核仁中心的物理排斥。因此,rDNA 基因座是一种聚合物-聚合物相分离的凝聚物,它依赖于转录和与 RNA 转录本的物理接触来保持封装在核仁内。