Structure And Stabilization Of The Bacterial Nucleoid

大肠杆菌的基因组DNA定位于一个或几个致密的类核中。快速生长的细胞中的类核呈现出复杂的形状；这些形状与DNA潜在排列的关系具有结构意义，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的变化，通过DNA荧光显微镜对细胞在溶液中重新定向或通过光学切片获得了关于单个核团的有限三维信息。这些技术也被应用于肿胀的细胞或球体内的扩大的核团。由此产生的图像表明，很大程度上的明显差异是由于来自不同方向的成像，以及在更规则的底层类核形状的不同焦平面上的成像。通过将细胞暴露在氯霉素或嘌呤霉素中，类核图像可以转变为紧凑的二重态，这与先前存在的二部分类核结构一致。分离的类核和静止期细胞中的类核也呈二重体形状，支持这种结构。潜在的结构被认为是两个亚基，对应于新生的子代染色体，由间隔区连接。亚基和间隔区似乎都发生变形，以适应细胞或球体内可用的空间(“柔性双重体”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).