The dif resolvase locus of the Escherichia coli chromosome can be replaced by a 33-bp sequence, but function depends on location.

The dif locus (deletion-induced filamentation) of Escherichia coli is a resolvase site, located in the terminus region of the chromosome, that reduces chromosome multimers to monomers. In strains in which this site has been deleted, a fraction of the cells is filamentous, has abnormal nucleoid structure, and exhibits elevated levels of the SOS repair system. We have demonstrated that a 33-bp sequence, which is sufficient for RecA-independent recombination and which shows similarity to the cer site of pColE1, suppresses the Dif phenotype when inserted in the terminus region. Flanking sequences were not required, since suppression occurred in strains in which dif as well as 12 kb or 173 kb of DNA had been deleted. However, location was important, and insertions at a site 118 kb away from the normal site did not suppress the Dif phenotype. These sites were otherwise still functional, and they exhibited wild-type levels of RecA-independent recombination with dif-containing plasmids and recombined with other chromosomal dif sites to cause deletions and inversions. It is proposed that the functions expressed by a dif site depend on chromosome location and structure, and analysis of these functions provides a way to examine the structure of the terminus region. IMAGES:

大肠杆菌（Escherichia coli）的dif位点（deletion-induced filamentation，缺失诱导丝状化位点）是一类解离酶位点，定位于染色体终点区域，可将染色体多聚体转化为单体。在该位点被敲除的菌株中，部分细胞会呈现丝状化表型、类核结构异常，且SOS修复系统（SOS repair system）的表达水平显著升高。本研究证实，一段长33 bp的序列可在插入染色体终点区域后抑制dif缺失表型；该序列足以介导不依赖RecA的重组，且与pColE1质粒的cer位点（cer site）具有序列相似性。侧翼序列并非该抑制效应的必需元件，因为即使在dif位点连同12 kb或173 kb的染色体DNA均被敲除的菌株中，该抑制作用仍可发生。但位点位置是关键因素：若将该序列插入距正常dif位点118 kb的区域，则无法产生抑制dif缺失表型的效果。这些异位插入的位点本身仍具备正常功能：它们与携带dif位点的质粒发生不依赖RecA重组的水平与野生型菌株一致，且可与染色体上其他dif位点发生重组，进而引发染色体片段的缺失与倒位。有研究提出，dif位点所介导的功能取决于其在染色体上的位置及染色体整体结构；对这些功能的分析可为解析染色体终点区域的结构提供有效研究途径。IMAGES: