Exposing native calf thymus DNA to increasing concentrations of Hg(ClO4)2 not only produces dramatic changes in its circular dichroism (CD) but results also in the decrease, and ultimate cessation, of endonucleolytic DNA cleavage by staphylococcal nuclease. Let r = [moles of added Hg(II)]/[mole of DNA base]: the conservative CD spectrum of the DNA B-form becomes nonconservative in appearance at 0.01 < r < 0.12 (resembling DNA in C-form geometry) and assumes the spectral characteristics of a left-handed DNA double helix at 0.12 < r ⩽ 1.0. DNA cleavage proceeds at or near the rates exhibited by untreated DNA at 0 < r < 0.08. At Hg(II) levels of 0.08 < r < 0.5, the rate of DNA hydrolysis decreases monotonically with increasing Hg(II) concentrations, and at r > 0.4, DNA cleavage ceases. Both the CD changes and the changes in the rate of DNA digestion are totally reversible upon the removal of Hg(II), at least up to r = 1.0, demonstrating that Hg(II) keeps all base pairs in register. For comparison purposes, native calf thymus DNA was also treated with methylmercury [CH3Hg(II)], an agent known to disrupt the secondary structure of DNA. The treatment yielded single-stranded methylmercurated DNA with preserved right-handed helix screwness. In addition, this DNA is digested by staphylococcal nuclease much more rapidly than double-stranded control DNA. Lastly, neither the CD nor the cleavage rate changes are reversible upon the removal of methylmercury. We interpret the Hg(II)-induced alterations in the CD of native calf thymus DNA, and the hydrolysis rate changes observed with staphylococcal nuclease, to indicate that Hg(II) either produces in DNA reversible B ↔ Z transitions, passing transiently through C-like conformations, or generates non-B-conformational structures of presumably left-handed geometry. © 1990, American Chemical Society. All rights reserved.
