Escolar L, de Lorenzo V, Pérez-Martín J
Centro Nacional de Biotecnología, Campus de Cantoblanco, Madrid, Spain.
Mol Microbiol. 1997 Nov;26(4):799-808. doi: 10.1046/j.1365-2958.1997.6211987.x.
The mechanism of transcriptional repression of the aerobactin operon of Escherichia coli by the Fe2+-responsive Fur (ferric uptake regulation) protein has been investigated. In the presence of a divalent metal, such as Mn2+, the Fur protein sequentially occupies two defined sites at the aerobactin promoter region, followed by a looser occupation of upstream DNA sequences. However, binding to the primary target site suffices for the entire repression effect. Comparison of transcription patterns generated with run-off experiments in the presence and absence of heparin showed that access of the RNA polymerase to the principal -35/-10 hexamers of the promoter region was fully prevented by Fur-Mn2+ bound to its primary site. Similarly, promoter-bound RNA polymerase could not be competed out from the DNA even in the presence of a large Fur-Mn2+ excess, although the repressor could immediately bind its target sequence at the region as soon as RNA polymerase moved away from the promoter during transcription. The high affinities of either protein for the promoter produce, in practice, a first-come, first-served effect that helps the system to respond instantly to changes in the iron status of the cells.
对铁离子响应性Fur（铁摄取调节）蛋白抑制大肠杆菌气杆菌素操纵子转录的机制进行了研究。在存在二价金属（如Mn2+）的情况下，Fur蛋白依次占据气杆菌素启动子区域的两个特定位点，随后对上游DNA序列的占据较为松散。然而，与主要靶位点的结合就足以产生整个抑制效应。在有无肝素的情况下进行的径流实验所产生的转录模式比较表明，结合到其主要位点的Fur-Mn2+完全阻止了RNA聚合酶接近启动子区域的主要-35/-10六聚体。同样，即使存在大量过量的Fur-Mn2+，与启动子结合的RNA聚合酶也不能从DNA上被竞争掉，尽管在转录过程中RNA聚合酶一旦离开启动子，阻遏物就能立即结合到该区域的靶序列上。实际上，这两种蛋白对启动子的高亲和力产生了一种先到先得的效应，有助于该系统对细胞铁状态的变化立即做出反应。
