Richardson S M, Higgins C F, Lilley D M
Department of Biochemistry, University, Dundee, UK.
EMBO J. 1988 Jun;7(6):1863-9. doi: 10.1002/j.1460-2075.1988.tb03019.x.
DNA supercoiling is an important, but relatively poorly understood factor which influences promoter function. leu-500 is a point mutation in the promoter of the leucine operon of Salmonella typhimurium which confers leucine auxotrophy. It can be phenotypically suppressed by mutations in the topA gene, which encodes topoisomerase I, implicating DNA supercoiling in the regulation of this promoter. We have demonstrated that phenotypic suppression of this mutant promoter is transcriptional, and that topA mutations restore function to the mutant promoter. Transcription from the leu-500 promoter was examined in a series of strains harbouring topA and tos (presumptive gyr) mutations, each of which exhibits a different level of in vivo plasmid supercoiling. Promoter function did not correlate with the level of supercoiling but rather with the presence or absence of a functional topA gene. Furthermore, when cloned onto a multicopy plasmid, the leu-500 promoter failed to function, even in a topA background. Thus, local rather than global changes in DNA topology are implicated in the activation of this promoter.
DNA超螺旋是一个重要但相对了解较少的影响启动子功能的因素。leu-500是鼠伤寒沙门氏菌亮氨酸操纵子启动子中的一个点突变，它导致亮氨酸营养缺陷。它可以被编码拓扑异构酶I的topA基因突变在表型上抑制，这表明DNA超螺旋参与了该启动子的调控。我们已经证明，这种突变启动子的表型抑制是转录性的，并且topA突变可恢复突变启动子的功能。在一系列携带topA和tos（假定的gyr）突变的菌株中检测了来自leu-500启动子的转录，每个菌株在体内质粒超螺旋水平上都表现出不同程度的差异。启动子功能与超螺旋水平无关，而是与功能性topA基因的存在与否有关。此外，当克隆到多拷贝质粒上时，leu-500启动子即使在topA背景下也无法发挥功能。因此，DNA拓扑结构的局部而非全局变化与该启动子的激活有关。
