Leslie N R, Sherratt D J
Department of Genetics, University of Glasgow, UK.
EMBO J. 1995 Apr 3;14(7):1561-70. doi: 10.1002/j.1460-2075.1995.tb07142.x.
The replication terminus region of the Escherichia coli chromosome encodes a locus, dif, that is required for normal chromosome segregation at cell division. dif is a substrate for site-specific recombination catalysed by the related chromosomally encoded recombinases XerC and XerD. It has been proposed that this recombination converts chromosome multimers formed by homologous recombination back to monomers in order that they can be segregated prior to cell division. Strains mutant in dif, xerC or xerD share a characteristic phenotype, containing a variable fraction of filamentous cells with aberrantly positioned and sized nucleoids. We show that the only DNA sequences required for wild-type dif function in the terminus region of the chromosome are contained within 33 bp known to bind XerC and XerD and that putative active site residues of the Xer recombinases are required for normal chromosome segregation. We have also shown that recombination by the loxP/Cre system of bacteriophage P1 will suppress the phenotype of a dif deletion strain when loxP is inserted in the terminus region. Suppression of the dif deletion phenotype did not occur when either dif/Xer or loxP/Cre recombination acted at other positions in the chromosome close to oriC or within lacZ, indicating that site-specific recombination must occur within the replication terminus region in order to allow normal chromosome segregation.
大肠杆菌染色体的复制终止区编码一个位点dif，它是细胞分裂时正常染色体分离所必需的。dif是由相关的染色体编码重组酶XerC和XerD催化的位点特异性重组的底物。有人提出，这种重组将同源重组形成的染色体多聚体转化回单体，以便它们能够在细胞分裂前进行分离。dif、xerC或xerD突变的菌株具有共同的特征表型，含有不同比例的丝状细胞，其类核的位置和大小异常。我们表明，染色体末端区域野生型dif功能所需的唯一DNA序列包含在已知能结合XerC和XerD的33 bp内，并且Xer重组酶的假定活性位点残基是正常染色体分离所必需的。我们还表明，当loxP插入末端区域时，噬菌体P1的loxP/Cre系统的重组将抑制dif缺失菌株的表型。当dif/Xer或loxP/Cre重组作用于染色体上靠近oriC或lacZ内的其他位置时，dif缺失表型没有受到抑制，这表明位点特异性重组必须发生在复制终止区内，以便允许正常的染色体分离。
