Long-Range Chromosome Organization in <em>E. coli</em>: A Site-Specific System Isolates the Ter Macrodomain

The organization of the Escherichia coli chromosome into a ring composed of four macrodomains and two less-structured regions influences the segregation of sister chromatids and the mobility of chromosomal DNA. The structuring of the terminus region (Ter) into a macrodomain relies on the interaction of the protein MatP with a 13-bp target called matS repeated 23 times in the 800-kb-long domain. Here, by using a new method that allows the transposition of any chromosomal segment at a defined position on the genetic map, we reveal a site-specific system that restricts to the Ter region a constraining process that reduces DNA mobility and delays loci segregation. Remarkably, the constraining process is regulated during the cell cycle and occurs only when the Ter MD is associated with the division machinery at mid-cell. The change of DNA properties does not rely on the presence of a trans-acting mechanism but rather involves a cis-effect acting at a long distance from the Ter region. Two specific 12-bp sequences located in the flanking Left and Right macrodomains and a newly identified protein designated YfbV conserved with MatP through evolution are required to impede the spreading of the constraining process to the rest of the chromosome. Our results unravel a site-specific system required to restrict to the Ter region the consequences of anchoring the Ter MD to the division machinery.大肠杆菌（Escherichia coli）的染色体被组装为包含四个宏结构域（macrodomain）与两个低结构化区域的环状结构，该结构可调控姐妹染色单体的分离与染色体DNA的流动性。终点区域（Ter）组装为宏结构域的过程，依赖于MatP蛋白（MatP）与该800kb长区域内重复23次的13bp靶位点matS之间的特异性结合。本研究借助一种可将任意染色体片段定向转座至遗传图谱指定位置的新方法，揭示了一套位点特异性系统：该系统可将限制DNA流动性并延迟位点分离的约束过程限定于终点区域（Ter）内。值得注意的是，该约束过程受细胞周期精准调控，且仅当终点宏结构域（Ter MD）锚定在细胞中部的分裂装置上时才会被激活。DNA特性的改变并非依赖于反式作用机制，而是源自距终点区域（Ter）较远位置的顺式作用效应。位于侧翼左、右宏结构域内的两段特异性12bp序列，以及一种与MatP蛋白协同进化的新鉴定蛋白YfbV（YfbV），是阻碍约束过程扩散至染色体其余区域的必要条件。本研究结果阐明了一套位点特异性系统，该系统可将终点宏结构域锚定至分裂装置所产生的效应限定于终点区域（Ter）内。