Cooperative structural transition of PM2 DNA at high ionic strength and its dependence on DNA damages

Abstract

Pohl and Jovin¹ have shown that poly (dG–dC) · poly (dG–dC) in solution undergoes a cooperative transition at 2.5 M NaCl at neutral pH. The two forms of the polymer are structurally different as indicated by a change in ethidium bromide intercalation and in circular dichroism (CD)^1,2. Wang et al.³ and Davies and Zimmerman⁴ have suggested that the low salt form of the polymer might correspond to the B form of DNA (right-handed helix) and the high salt form to the Z form of DNA (left-handed helix). We describe here a salt-induced transition of supercoiled PM2 DNA from a form which passes through nitrocellulose filters to a form which is retained by these filters. The transition occurs between 2.5 and 3.5 M NaCl. The dependence of the apparent equilibrium constant on the salt concentration indicates a cooperative transition. Irradiation of DNA with UV light or alkylation with N-acetoxyacetylaminofluorene (AAAF) shifts the transition to lower salt concentrations. The transition can also be observed with linear DNA but requires a much higher salt concentration.