Zhang Xiaolin, Zhang Shuyu, Meyer Richard J
Section of Molecular Genetics and Microbiology and The Institute for Molecular Biology, School of Biology, University of Texas at Austin, Austin, TX 78712, USA.
Nucleic Acids Res. 2003 Aug 15;31(16):4762-8. doi: 10.1093/nar/gkg687.
The assembly of plasmid-encoded proteins at a unique site (oriT) on the plasmid R1162, to form a complex called the relaxosome, is required for conjugative transfer of the plasmid and for negative regulation of neighboring promoters. Two-dimensional chloroquine gel electrophoresis was used to show that oriTs are physically coupled at the relaxosome. This interaction requires all the relaxosome proteins, which are assembled into a structure resulting in a decrease in the average linking number of the plasmid DNA in the cell. Molecules with higher superhelical densities are preferentially selected for assembly of the relaxosome. Genetic data obtained earlier indicate that the molecular coupling reported here is a 'handcuffing' reaction that contributes to the regulation of adjacent plasmid promoters. However, although these promoters affect the expression of the genes for replication, plasmid copy-control is regulated independently. This is the first time 'handcuffing' has been observed at an oriT, and its possible significance for transfer is discussed.
质粒R1162上独特位点（oriT）处质粒编码蛋白的组装，形成一种称为松弛体的复合物，是质粒接合转移和邻近启动子负调控所必需的。二维氯喹凝胶电泳用于表明oriT在松弛体处物理偶联。这种相互作用需要所有的松弛体蛋白，这些蛋白组装成一种结构，导致细胞中质粒DNA平均连环数减少。具有更高超螺旋密度的分子被优先选择用于松弛体的组装。早期获得的遗传数据表明，此处报道的分子偶联是一种“手铐”反应，有助于调节相邻质粒启动子。然而，尽管这些启动子影响复制基因的表达，但质粒拷贝控制是独立调节的。这是首次在oriT处观察到“手铐”现象，并讨论了其对转移的可能意义。
