Sternglanz R, DiNardo S, Voelkel K A, Nishimura Y, Hirota Y, Becherer K, Zumstein L, Wang J C
Proc Natl Acad Sci U S A. 1981 May;78(5):2747-51. doi: 10.1073/pnas.78.5.2747.
Mutations in top, the structural gene for Escherichia coli DNA topoisomerase I, have been identified and mapped at 28 min on the chromosome, near cysB. Strains carrying deletions of the top gene are viable. The top mutations, however, do exert pleiotropic effects on transcription and transposition. Mutants lacking DNA topoisomerase I have a more rapid rate of induction and a higher level of catabolite-sensitive enzymes including tryptophanase and beta-galactosidase. This general activation of transcription by top mutations can be attributed to an increase in the negative superhelicity of the DNA in vivo when the topoisomerase activity is abolished. The frequency of transposition of Tn5, a transposon carrying kanamycin resistance, is decreased by a factor of 40 or more in top mutants. A direct or indirect role of the topoisomerase in transposition is discussed. The transposition frequency of Tn3, however, is not dependent on top. Based on the studies of the E. coli top mutants, it appears that the supX gene, which was originally studied in Salmonella typhimurium [Dubnau, E. & Margolin, P. (1972) Mol. Gen. Genet. 117, 91-112] is likely to be the structural gene for DNA topoisomerase I.
已鉴定出大肠杆菌DNA拓扑异构酶I的结构基因top中的突变，并将其定位在染色体上28分钟处，靠近cysB。携带top基因缺失的菌株是可行的。然而，top突变确实对转录和转座产生多效性影响。缺乏DNA拓扑异构酶I的突变体具有更快的诱导速率和更高水平的分解代谢敏感酶，包括色氨酸酶和β-半乳糖苷酶。top突变引起的这种转录普遍激活可归因于当拓扑异构酶活性被消除时体内DNA负超螺旋的增加。携带卡那霉素抗性的转座子Tn5的转座频率在top突变体中降低了40倍或更多。讨论了拓扑异构酶在转座中的直接或间接作用。然而，Tn3的转座频率不依赖于top。基于对大肠杆菌top突变体的研究，最初在鼠伤寒沙门氏菌中研究的supX基因[Dubnau, E. & Margolin, P. (1972) Mol. Gen. Genet. 117, 91 - 112]似乎可能是DNA拓扑异构酶I的结构基因。
