Richardson S M, Higgins C F, Lilley D M
EMBO J. 1984 Aug;3(8):1745-52. doi: 10.1002/j.1460-2075.1984.tb02041.x.
We have elucidated the genetic control of DNA supercoiling in Salmonella typhimurium. The level of superhelix density is controlled by two classes of genes. The only member of the first class is topA, the structural gene for topoisomerase I. The second class, tos, (topoisomerase one suppressor) consists of at least two genes, one of which is linked to gyrA, the structural gene for the topoisomerase subunit of DNA gyrase. Deletions of topA result in oversupercoiling of plasmid DNA. These mutations do not require the acquisition of second-site compensatory mutations to allow cell growth, in contrast to the situation in Escherichia coli. However, tos mutations, unlinked to topA, have been isolated which reduce plasmid superhelix density. We conclude that the level of DNA supercoiling in S. typhimurium is a dynamic balance between the effects of the gene products of topA (relaxation) and tos (supercoiling) which act independently of each other. Using a variety of combinations of these mutations we have constructed a series of isogenic strains, each of which has a different but precisely defined level of plasmid supercoiling; the series as a whole provides a wide range of supercoiling both above and below the wild-type level.
我们已经阐明了鼠伤寒沙门氏菌中DNA超螺旋的遗传控制机制。超螺旋密度水平由两类基因控制。第一类的唯一成员是topA，即拓扑异构酶I的结构基因。第二类基因，tos（拓扑异构酶I抑制基因）至少由两个基因组成，其中一个与gyrA相连，gyrA是DNA旋转酶拓扑异构酶亚基的结构基因。topA的缺失会导致质粒DNA过度超螺旋。与大肠杆菌的情况不同，这些突变不需要获得第二位点补偿性突变来允许细胞生长。然而，已分离出与topA不连锁的tos突变，这些突变会降低质粒超螺旋密度。我们得出结论，鼠伤寒沙门氏菌中DNA超螺旋水平是topA（松弛）和tos（超螺旋）基因产物相互独立作用的动态平衡。利用这些突变的各种组合，我们构建了一系列同基因菌株，每个菌株都有不同但精确界定的质粒超螺旋水平；整个系列在野生型水平之上和之下都提供了广泛的超螺旋范围。
