Structure And Stabilization Of The Bacterial Nucleoid

大肠杆菌的基因组DNA存在于被称为类核的致密小体中。对类核的组织和结构知之甚少。通过多聚赖氨酸-亚精胺程序分离的致密的、特征形状的核在DNA荧光显微镜下可见。用尿素或胰酶处理使致密类核转变为部分膨胀的形式。在尿素溶液中发生的突变伴随着大多数DNA相关蛋白质的释放；致密和部分扩张形式之间的转换点不因蛋白质的损失而改变，从氯霉素作用后的细胞或从DPS、FIS或H-NS和StpA缺失的细胞中分离的核样也不改变。被剥离了大多数DNA相关蛋白的部分扩张形式在突然转变时可以被聚乙二醇8000有效地重新压缩，聚乙二醇8000是一种大分子挤压剂。相应地，DNA相关蛋白对细胞类核的相类行为的影响相对较小。部分扩增的形式在RNase作用下变得分散，表明RNA在维持部分扩增中起作用。尿素转变的变化表明，先前描述的类核致密过程可能具有人工基础，并对涉及裂解细胞DNA的重复局部结构的报道提出了质疑。这份材料已经准备好并提交出版。

The genomic DNA of Escherichia coli occurs in compact bodies known as nucleoids. Organization and structure of nucleoids are poorly understood. Compact, characteristically-shaped, nucleoids isolated by the polylysine-spermidine procedure were visualized by DNA fluorescence microscopy. Treatment with urea or trypsin converted compact nucleoids to partially expanded forms. The abrupt transition that occurred in urea solutions was accompanied by release of most DNA-associated proteins; the transition point between compact and partially expanded forms was not changed by the loss of the proteins nor was it changed in nucleoids isolated from cells after exposure to chloramphenicol or from cells in which Dps, Fis, or H-NS and StpA had been deleted. Partially expanded forms that had been stripped of most DNA-associated proteins were efficiently recompacted by polyethylene glycol 8000, a macromolecular crowding agent, in an abrupt transition. DNA-associated proteins are accordingly suggested to have relatively little effect on the phase-like behavior of the cellular nucleoid. Partially expanded forms became dispersed upon RNase exposure, indicating a role of RNA in maintaining the partial expansion. Changes in the urea transition indicate that a previously described procedure for nucleoid compaction may have an artifactual basis, and raise questions about reports of repetitive local structures involving the DNA of lysed cells. This material has been prepared and submitted for publication.