Structure And Stabilization Of The Bacterial Nucleoid

大肠杆菌的基因组DNA定位于一个或几个紧密的类核中。快速生长的细胞中的类核呈现复杂的形状;这些形状与DNA的潜在排列的关系具有结构上的意义，并且在基因定位和类核分配研究中具有潜在的重要性。为了帮助评估这种形状上的变化，通过DNA荧光显微镜观察细胞在溶液中重新定向或通过光学切片获得了单个类核的有限三维信息。这些技术也适用于肿胀的细胞或原生质球内扩大的核。由此产生的图像表明，许多明显的变化是由于从不同方向和不同焦平面成像的更规则的潜在的类核形状。通过将细胞暴露于氯霉素或嘌呤霉素，类核图像可以转化为紧凑的双峰形状，与预先存在的二分类核结构一致。孤立的类核和静止期细胞中的类核也呈现双峰形状，支持这样的结构。潜在的结构被认为是两个亚基，对应于新生的子染色体，由间隔区连接。亚基和间隔区似乎都变形以适应细胞或原生质球内可用的空间（“柔性双重”模型）。

The genomic DNA of Escherichia coli is localized in one or a few compact nucleoids. Nucleoids in rapidly grown cells appear in complex shapes; the relationship of these shapes to underlying arrangements of the DNA is of structural interest and of potential importance in gene localization and nucleoid partition studies. To help assess this variation in shape, limited three-dimensional information on individual nucleoids was obtained by DNA fluorescence microscopy of cells as they reoriented in solution, or by optical sectioning. These techniques were also applied to enlarged nucleoids within swollen cells or spheroplasts. The resulting images indicated that much of the apparent variation was due to imaging from different directions and at different focal planes of more regular underlying nucleoid shapes. Nucleoid images could be transformed into compact doublet-shapes by exposure of cells to chloramphenicol or puromycin, consistent with a preexisting bipartite nucleoid structure. Isolated nucleoids and nucleoids in stationary phase cells also assumed a doublet shape, supporting such a structure. The underlying structure is suggested to be two subunits, corresponding to the nascent daughter chromosomes, joined by a spacer. Both the subunits and the spacer appear to deform to accommodate the space available within cells or spheroplasts ("flexible doublet" model).