Winding of the DNA helix by divalent metal ions

Nucleic Acids Res. 1997 Oct 15;25(20):4067-71. doi: 10.1093/nar/25.20.4067.


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.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Calcium / pharmacology
  • Calcium Chloride / pharmacology
  • Cations, Divalent / pharmacology*
  • Chloroquine / pharmacology
  • Cobalt / pharmacology
  • DNA Topoisomerases, Type I / metabolism
  • DNA, Bacterial / chemistry
  • DNA, Superhelical / chemistry*
  • Electrophoresis, Agar Gel
  • Escherichia coli / genetics
  • Intercalating Agents / pharmacology
  • Manganese / pharmacology
  • Nucleic Acid Conformation*
  • Plasmids


  • Cations, Divalent
  • DNA, Bacterial
  • DNA, Superhelical
  • Intercalating Agents
  • Cobalt
  • Manganese
  • Chloroquine
  • DNA Topoisomerases, Type I
  • Calcium Chloride
  • Calcium