Pruss G J
J Mol Biol. 1985 Sep 5;185(1):51-63. doi: 10.1016/0022-2836(85)90182-2.
Plasmid pBR322 DNA isolated from Salmonella typhimurium supX (topoisomerase I) mutants exhibits a novel supercoiling distribution characterized by extreme heterogeneity in linking number and the presence of highly negatively supercoiled topoisomers. The most negatively supercoiled topoisomers isolated from one supX mutant have more than twice the wild-type level of supercoiling; the distribution as a whole has a median superhelix density about 1.3 times that of wild type. Surprisingly, the supercoiling distribution of plasmid pUC9 DNA isolated from supX mutants differs from that of pBR322. Escherichia coli topoisomerase I mutants have been shown to acquire compensatory mutations that reduce bacterial chromosome supercoiling to below the wild-type level even in the absence of topoisomerase I. We find that such a compensatory mutation in an E. coli topoisomerase I deletion mutant does not reduce pBR322 DNA supercoiling to a level below that of wild type. Thus, the effects of topoisomerase mutations on supercoiling depend on the replicon.
从鼠伤寒沙门氏菌supX（拓扑异构酶I）突变体中分离出的质粒pBR322 DNA呈现出一种新型的超螺旋分布，其特征在于连接数的极端异质性以及高度负超螺旋拓扑异构体的存在。从一个supX突变体中分离出的最负超螺旋拓扑异构体的超螺旋程度是野生型水平的两倍多；整体分布的中值超螺旋密度约为野生型的1.3倍。令人惊讶的是，从supX突变体中分离出的质粒pUC9 DNA的超螺旋分布与pBR322不同。已证明大肠杆菌拓扑异构酶I突变体获得了补偿性突变，即使在没有拓扑异构酶I的情况下，这些突变也会将细菌染色体超螺旋降低到野生型水平以下。我们发现，大肠杆菌拓扑异构酶I缺失突变体中的这种补偿性突变不会将pBR322 DNA超螺旋降低到野生型以下的水平。因此，拓扑异构酶突变对超螺旋的影响取决于复制子。
