Wade J T, Belyaeva T A, Hyde E I, Busby S J
School of Biosciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
Mol Microbiol. 2000 Apr;36(1):223-9. doi: 10.1046/j.1365-2958.2000.01850.x.
The Escherichia coli MelR protein is a transcription activator that, in the presence of melibiose, activates expression of the melAB operon by binding to four sites located just upstream of the melAB promoter. MelR is encoded by the melR gene, which is expressed from a divergent transcript that starts 237 bp upstream of the melAB promoter transcript start point. In a recent study, we have identified a fifth DNA site for MelR that overlaps the melR promoter transcript start and -10 region. Here we show that MelR binding to this site can downregulate expression from the melR promoter; thus, MelR autoregulates its own expression. Optimal repression of the melR promoter is observed in the absence of melibiose and requires one of the four other DNA sites for MelR at the melAB promoter. The two MelR binding sites required for this optimal repression are separated by 177 bp. We suggest that, in the absence of melibiose, MelR forms a loop between these two sites. We argue that, in the presence of melibiose, this loop is broken as the melAB promoter is activated. However, in the presence of melibiose, the melR promoter can still be partially repressed by MelR binding to the site that overlaps the transcript start and -10 region. Parallels with the Escherichia coli araC-araBAD regulatory region are discussed.
大肠杆菌MelR蛋白是一种转录激活因子，在蜜二糖存在的情况下，它通过与位于melAB启动子上游的四个位点结合来激活melAB操纵子的表达。MelR由melR基因编码，该基因从一个不同的转录本表达，该转录本起始于melAB启动子转录起点上游237 bp处。在最近的一项研究中，我们鉴定出了MelR的第五个DNA位点，它与melR启动子转录起点和 -10区域重叠。在这里我们表明，MelR与该位点的结合可以下调melR启动子的表达；因此，MelR对其自身的表达进行自我调节。在没有蜜二糖的情况下观察到melR启动子的最佳抑制，并且需要melAB启动子上MelR的其他四个DNA位点之一。这种最佳抑制所需的两个MelR结合位点相隔177 bp。我们认为，在没有蜜二糖的情况下，MelR在这两个位点之间形成一个环。我们认为，在蜜二糖存在的情况下，随着melAB启动子被激活，这个环会被打破。然而，在蜜二糖存在的情况下，melR启动子仍然可以通过MelR与重叠转录起点和 -10区域的位点结合而被部分抑制。文中还讨论了与大肠杆菌araC-araBAD调控区域的相似之处。
