Funnell B E, Gagnier L
Department of Molecular and Medical Genetics, University of Toronto, Ontario, Canada.
J Bacteriol. 1995 May;177(9):2381-6. doi: 10.1128/jb.177.9.2381-2386.1995.
The partition system of the low-copy-number plasmid/prophage of bacteriophage P1 encodes two proteins, ParA and ParB, and contains a DNA site called parS. ParB and the Escherichia coli protein IHF bind to parS to form the partition complex, in which parS is wrapped around ParB and IHF in a precise three-dimensional conformation. Partition can be thought of as a positioning reaction; the plasmid-encoded components ensure that at least one copy of the plasmid is positioned within each new daughter cell. We have used an E. coli chromosomal partition mutant to test whether this positioning is mediated by direct plasmid-chromosomal attachment, for example, by pairing of the partition complex that forms at parS with a bacterial attachment site. The E. coli MukB protein is required for proper chromosomal positioning, so that mukB mutants generate some cells without chromosomes (anucleate cells) at each cell division. We analyzed the plasmid distribution in nucleate and anucleate mukB cells. We found that P1 plasmids are stable in mukB mutants and that they partition into both nucleate and anucleate cells. This indicates that the P1 partition complex is not used to pair plasmids with the host chromosome and that P1 plasmids must be responsible for their own proper cellular localization, presumably through host-plasmid protein-protein interactions.
噬菌体P1的低拷贝数质粒/原噬菌体的分配系统编码两种蛋白质，ParA和ParB，并包含一个称为parS的DNA位点。ParB和大肠杆菌蛋白IHF与parS结合形成分配复合物，其中parS以精确的三维构象包裹在ParB和IHF周围。分配可以被认为是一种定位反应；质粒编码的成分确保质粒的至少一个拷贝定位在每个新的子细胞内。我们使用了一个大肠杆菌染色体分配突变体来测试这种定位是否由直接的质粒-染色体附着介导，例如，通过在parS处形成的分配复合物与细菌附着位点配对。大肠杆菌MukB蛋白是正确染色体定位所必需的，因此mukB突变体在每次细胞分裂时会产生一些没有染色体的细胞（无核细胞）。我们分析了有核和无核mukB细胞中的质粒分布。我们发现P1质粒在mukB突变体中是稳定的，并且它们会分配到有核和无核细胞中。这表明P1分配复合物不用于将质粒与宿主染色体配对，并且P1质粒必须负责它们自身在细胞内的正确定位，大概是通过宿主-质粒蛋白-蛋白相互作用。
