title: Winding of the DNA helix by divalent metal ions.
abstract: When supercoiled pBR322 DNA was relaxed at 0 or 22 degrees C by topoisomerase I in the presence of the divalent cations Ca2+, Mn2+ or Co2+, the resulting distributions of topoisomers observed at 22 degrees C had positive supercoils, up to an average delta Lk value of +8.6 (for Ca2+at 0 degrees C), corresponding to an overwinding of the helix by 0.7 degrees/bp. An increase of the divalent cation concentration in the reaction mixture above 50 mM completely reversed the effect. When such ions were present in agarose electrophoresis gels, they caused a relaxation of positively supercoiled DNA molecules, and thus allowed a separation of strongly positively supercoiled topoisomers. The effect of divalent cations on DNA adds a useful tool for the study of DNA topoisomers, for the generation as well as separation of positively supercoiled DNA molecules.
authors.affiliation: Molecular and Cell Biology Programs, FO3.1, University of Texas at Dallas, Box 830688, Richardson, TX 75083-0688, USA.
authors.forename: Y C
authors.initials: YC
authors.lastname: Xu
authors.forename: H
authors.initials: H
authors.lastname: Bremer
mesh_terms: D002118:Calcium / Q000494:pharmacology; D002122:Calcium Chloride / Q000494:pharmacology; D002413:Cations, Divalent / Q000494:pharmacology*; D002738:Chloroquine / Q000494:pharmacology; D003035:Cobalt / Q000494:pharmacology; D004264:DNA Topoisomerases, Type I / Q000378:metabolism; D004269:DNA, Bacterial / Q000737:chemistry; D004278:DNA, Superhelical / Q000737:chemistry*; D004587:Electrophoresis, Agar Gel; D004926:Escherichia coli / Q000235:genetics; D007364:Intercalating Agents / Q000494:pharmacology; D008345:Manganese / Q000494:pharmacology; D009690:Nucleic Acid Conformation; D010957:Plasmids
chemical_list: D002413:Cations, Divalent; D004269:DNA, Bacterial; D004278:DNA, Superhelical; D007364:Intercalating Agents; D003035:Cobalt; D008345:Manganese; D002738:Chloroquine; D004264:DNA Topoisomerases, Type I; D002122:Calcium Chloride; D002118:Calcium