Structure of DNA in denaturing solvents: I. Bacteriophage PM2 DNA in aqueous sodium perchlorate

Abstract

The sedimentation velocity behavior of both closed (I) and nicked (II) circular bacteriophage PM2 DNA's has been determined in aqueous NaClO 4 solutions ranging from 0.98 to 7.2 m. The addition of the salt results in a progressive, non-cooperative unwinding of the DNA duplex to the extent of 3.9% at 7.2 m-NaClO 4, as determined by comparison with the sedimentation velocity/dye titration results of Upholt, Gray & Vinograd (1971), The salt-induced unwinding is complemented and extended by the intercalation of ethidium bromide, and the shape of the sedimentation velocity/dye titration curve in 0.98 m-NaClO 4 is similar to that in 2.85 m-CsCl ( Upholt et al., 1971) . The sedimentation coefficient ratio S I S II has the same dependence upon NaClO 4 concentration at 5 °C and at 20 °C, except for an effective translation to 0.8 m higher salt concentrations at the lower temperature. This shift is consistent with the known effects of temperature upon the duplex rotation angle ( Wang, 1969). The unwinding of the DNA duplex in the presence of increasing NaClO 4 concentrations is to be contrasted with the much smaller overwinding which occurs upon the addition of NaCl or CsCl at 20 °C ( Wang, 1969). The NaClO 4-induced unwinding takes place at temperatures well below the onset of the melting transition, and the specific salt effect does not appear to have a large temperature coefficient. The results indicate that substantial angular alteration of the duplex may be induced by a change in solution environment in the absence of transition to the coiled form of DNA.