Mata Martin Carmen, Sun Zhe, Zhou Yan Ning, Jin Ding Jun
Transcription Control Section, RNA Biology Laboratory, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Frederick, MD, United States.
Front Microbiol. 2018 Jun 5;9:1115. doi: 10.3389/fmicb.2018.01115. eCollection 2018.
In the fast-growing cells, RNA polymerase (RNAP) molecules are concentrated and form foci at clusters of ribosomal RNA (rRNA) operons resembling eukaryotic nucleolus. The bacterial nucleolus-like organization, spatially compartmentalized at the surface of the compact bacterial chromosome (nucleoid), serves as transcription factories for rRNA synthesis and ribosome biogenesis, which influences the organization of the nucleoid. Unlike wild type that has seven rRNA operons in the genome in a mutant that has six (Δ6) rRNA operons deleted in the genome, there are no apparent transcription foci and the nucleoid becomes uncompacted, indicating that formation of RNAP foci requires multiple copies of rRNA operons clustered in space and is critical for nucleoid compaction. It has not been determined, however, whether a multicopy plasmid-borne rRNA operon (p) could substitute the multiple chromosomal rRNA operons for the organization of the bacterial nucleolus-like structure in the mutants of Δ6 and Δ7 that has all seven rRNA operons deleted in the genome. We hypothesized that extrachromosomal nucleolus-like structures are similarly organized and functional from p in these mutants. In this report, using multicolor images of three-dimensional superresolution Structured Illumination Microscopy (3D-SIM), we determined the distributions of both RNAP and NusB that are a transcription factor involved in rRNA synthesis and ribosome biogenesis, p clustering, and nucleoid structure in these two mutants in response to environmental cues. Our results found that the extrachromosomal nucleolus-like organization tends to be spatially located at the poles of the mutant cells. In addition, formation of RNAP foci at the extrachromosomal nucleolus-like structure condenses the nucleoid, supporting the idea that active transcription at the nucleolus-like organization is a driving force in nucleoid compaction.
在快速生长的细胞中，RNA聚合酶（RNAP）分子集中并在类似于真核生物核仁的核糖体RNA（rRNA）操纵子簇处形成焦点。细菌的类核仁组织在致密的细菌染色体（类核）表面进行空间分隔，作为rRNA合成和核糖体生物发生的转录工厂，这会影响类核的组织。与基因组中有七个rRNA操纵子的野生型不同，在一个基因组中六个rRNA操纵子被删除的突变体（Δ6）中，没有明显的转录焦点，类核变得松散，这表明RNAP焦点的形成需要多个rRNA操纵子在空间上聚集，并且对类核压缩至关重要。然而，尚未确定多拷贝质粒携带的rRNA操纵子（p）是否可以替代多个染色体rRNA操纵子，以在基因组中所有七个rRNA操纵子都被删除的Δ6和Δ7突变体中组织细菌类核仁样结构。我们假设在这些突变体中，染色体外类核仁样结构从p开始具有相似的组织和功能。在本报告中，我们使用三维超分辨率结构光照显微镜（3D-SIM）的多色图像，确定了RNAP和NusB的分布，NusB是一种参与rRNA合成、核糖体生物发生、p聚集和类核结构的转录因子，这两个突变体中的这些分布是对环境线索的响应。我们的结果发现，染色体外类核仁样组织倾向于在突变细胞的两极空间定位。此外，在染色体外类核仁样结构处形成RNAP焦点会使类核凝聚，支持了类核仁样组织处的活跃转录是类核压缩驱动力的观点。
