Walkenhorst H M, Hemschemeier S K, Eichenlaub R
Lehrstuhl Gentechnologie/Mikrobiologie, Fakultät für Biologie, Universität Bielefeld, Germany.
Microbiol Res. 1995 Nov;150(4):347-61. doi: 10.1016/S0944-5013(11)80016-9.
The nucleotide sequence of a 6.8-kb chromosomal subfragment of plasmid pHW100 complementing an Escherichia coli modC (chlD) mutant has been determined. This DNA region encodes the genes of a high-affinity uptake system for molybdate arranged in an operon with the genes modABCD. Since the modA product has a signal peptide at the N-terminus it probably is the periplasmic binding-protein for molybdate. The products of modB (chlJ) and modC (chlD) have been described earlier as the inner membrane protein and the ATP-binding protein of the molybdate transport system, respectively. At present, there is no information on possible functions of the fourth gene of the operon, modD. Upstream of the mod operon, two other gene loci, termed modR and an open reading frame ORF6 could be identified. ModR shares homology with a molybdenum-pterin binding protein of Clostridium pasteurianum. ORF6 has extensive homology to ModC and other nucleotide-binding proteins of E. coli. Insertional inactivation of modR and ORF6 using a gentamicin resistance gene cartridge has no effect on molybdoenzyme activities, indicating that none of the two gene products is essential for molybdate uptake or molybdenum cofactor synthesis. However, by using a plasmid carrying a modA-lacZ gene fusion we observed that inactivation of modR releases repression of the mod operon independent of the molybdate concentration in the medium. This indicates that modR is a component of the mod operon regulation or the repressor itself.
已确定质粒pHW100中一个6.8kb染色体亚片段的核苷酸序列，该片段可互补大肠杆菌modC（chlD）突变体。此DNA区域编码一个钼酸盐高亲和力摄取系统的基因，这些基因与modABCD基因排列在一个操纵子中。由于modA产物在N端有一个信号肽，它可能是钼酸盐的周质结合蛋白。modB（chlJ）和modC（chlD）的产物先前已分别被描述为钼酸盐转运系统的内膜蛋白和ATP结合蛋白。目前，关于操纵子的第四个基因modD的可能功能尚无信息。在mod操纵子的上游，可以鉴定出另外两个基因位点，分别称为modR和一个开放阅读框ORF6。ModR与巴氏梭菌的一种钼蝶呤结合蛋白具有同源性。ORF6与ModC和大肠杆菌的其他核苷酸结合蛋白具有广泛的同源性。使用庆大霉素抗性基因盒对modR和ORF6进行插入失活，对钼酶活性没有影响，这表明这两种基因产物对于钼酸盐摄取或钼辅因子合成均非必需。然而，通过使用携带modA - lacZ基因融合体的质粒，我们观察到modR的失活可解除mod操纵子的阻遏，且与培养基中钼酸盐的浓度无关。这表明modR是mod操纵子调控的一个组成部分或阻遏物本身。
