Díaz-Mireles Edit, Wexler Margaret, Todd Jonathan D, Bellini Dominico, Johnston Andrew W B, Sawers R Gary
School of Biological Science, University of East Anglia, Norwich NR4 7TJ, UK.
Department of Molecular Microbiology, John Innes Centre, Colney Lane, Norwich NR4 7UH, UK.
Microbiology (Reading). 2005 Dec;151(Pt 12):4071-4078. doi: 10.1099/mic.0.28342-0.
The manganese uptake regulator Mur of Rhizobium leguminosarum is a close homologue of the global iron regulatory protein Fur. Mur represses the sitABCD operon, which encodes a Mn2+ transport system, specifically in response to Mn2+ but not Fe2+. In previous work the authors mapped the 5' ends of two sit operon transcripts, termed TS1 and TS2, which were co-ordinately regulated by Mn2+-Mur, but this paper now shows that only TS1 is a primary transcript. DNase I protection analyses showed that purified Mur bound, with similar affinity, to two sites in the regulatory region of sitABCD, but only when Mn2+ was present in the reaction buffer. These Mn2+-Mur-binding sites, termed MRS1 and MRS2 (Mur-responsive sequence), were closely related in sequence to each other and were separated by 16 bp, spanning the transcription initiation site TS1. The extent of the protected DNA was 34 and 31 bp for MRS1 and MRS2, respectively, which is in accord with other members of the Fur family. The DNA sequences recognized by Mn2+-Mur are wholly different from conventional Fur boxes, but some similarities to a recognition sequence for the Fur regulator from Bradyrhizobium japonicum were noted. Transcription analysis of the R. leguminosarum mur gene showed its expression to be independent of Mn2+-Mur. Thus, Mur is a sequence-specific DNA-binding protein that responds in vitro to manganese, and thus can occlude RNA polymerase access to the sitABCD promoter. Moreover, Mur recognizes a DNA sequence atypical for the Fur superfamily and, like Fur from B. japonicum, defines a new subclass of Fur-like transcriptional regulators.
豆科根瘤菌的锰摄取调节蛋白Mur是全球铁调节蛋白Fur的紧密同源物。Mur特异性地响应Mn2+而非Fe2+，抑制编码Mn2+转运系统的sitABCD操纵子。在先前的研究中，作者绘制了两个sit操纵子转录本（称为TS1和TS2）的5'末端图谱，它们受Mn2+-Mur的协同调节，但本文现在表明只有TS1是初级转录本。DNase I保护分析表明，纯化的Mur以相似的亲和力结合到sitABCD调节区域的两个位点，但仅当反应缓冲液中存在Mn2+时才会结合。这些Mn2+-Mur结合位点，称为MRS1和MRS2（Mur反应序列），在序列上彼此密切相关，相隔16 bp，跨越转录起始位点TS1。MRS1和MRS2的受保护DNA长度分别为34和31 bp，这与Fur家族的其他成员一致。Mn2+-Mur识别的DNA序列与传统的Fur框完全不同，但注意到与日本慢生根瘤菌的Fur调节因子的识别序列有一些相似之处。对豆科根瘤菌mur基因的转录分析表明其表达独立于Mn2+-Mur。因此，Mur是一种序列特异性DNA结合蛋白，在体外对锰有反应，因此可以阻止RNA聚合酶进入sitABCD启动子。此外，Mur识别Fur超家族非典型的DNA序列，并且与日本慢生根瘤菌的Fur一样，定义了一类新的Fur样转录调节因子。
