Circular dichroism of micrococcal nuclease-treated calf thymus chromatin (soluble chromatin) in presence of CH3HgOH

Interaction of inorganic mercury salts with model and red cell membranes: importance of lipid binding sites

The effect of two mercury salts, HgCl2 and Hg(NO3)2, on the thermotropic properties of phosphatidylserine (PS) model membranes and sonicated rat red cell membranes was investigated by fluorescence polarization. Both Hg(II) salts abolished the phase transition and decreased the membrane fluidity by interacting with PS. Maximal effect was observed at a Hg/PS ratio of 2.5-5 for mercuric chloride and at 1.5 for the nitrate salt. For both mercury compounds, 10 mM NaCl protected model membranes from the effects of Hg(II). HgCl2 and Hg(NO3)2 also decreased the fluidity of rat red cell membranes. Maximal effect was observed for 0.4 mM HgCl2 and 0.6 mM Hg(NO3)2, with 0.0125 mg protein/ml. Addition of NaCl to the Hg(II)-red cell system decreased the Hg(II)-induced perturbation of the thermotropic properties. For both membrane systems, the effects observed with Hg(NO3)2 were greater than those with HgCl2, which can be accounted for by the absence of competition with chloride ions in samples containing Hg(NO3)2.[Cl-] governs the availability of Hg(II) by determining its chemical speciation: increasing [Cl-] generates HgCl3- and HgCl4(2-), which do not interact with lipid binding sites. These results indicate that besides protein thiol groups, Hg(II)-lipid binding sites play an important role in the interaction of Hg(II) with red cell membranes that is qualitatively different from Hg(II) binding to protein thiol groups.

Mercury-induced DNA polymorphism: probing the conformation of Hg(II)-DNA via staphylococcal nuclease digestion and circular dichroism measurements

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 less than r less than 0.12 (resembling DNA in C-form geometry) and assumes the spectral characteristics of a left-handed DNA double helix at 0.12 less than r less than or equal to 1.0. DNA cleavage proceeds at or near the rates exhibited by untreated DNA at 0 less than r less than 0.08. At Hg(II) levels of 0.08 less than r less than 0.5, the rate of DNA hydrolysis decreases monotonically with increasing Hg(II) concentrations, and at r greater than 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.(ABSTRACT TRUNCATED AT 250 WORDS)

Mercury-induced transitions between right-handed and putative left-handed forms of poly[d(A-T).d(A-T)] and poly[d(G-C).d(G-C)]

Poly[d(A-T).d(A-T)] and poly[d(G-C).d(G-C)], each dissolved in 0.1 M NaClO4, 5 mM cacodylic acid buffer, pH 6.8, experience inversion of their circular dichroism (CD) spectrum subsequent to the addition of Hg(ClO4)2. Let r identical to [Hg(ClO4)2]added/[DNA-P]. The spectrum of the right-handed form of poly[d(A-T).d(A-T)] turns into that of a seemingly left-handed structure at r greater than or equal to 0.05 while a similar transition is noted with poly[d(G-C).(G-C)] at r greater than or equal to 0.12. The spectral changes are highly cooperative in the long-wavelength region above 250 nm. At r = 1.0, the spectra of the two polymers are more or less mirror images of their CD at r = 0. While most CD bands experience red-shifts upon the addition of Hg(ClO4)2, there are some that are blue-shifted. The CD changes are totally reversible when Hg(II) is removed from the nucleic acids by the addition of a strong complexing agent such as NaCN. This demonstrates that mercury keeps all base pairs in register.

Effects of exposure of DNA to methyl mercury on its activity as a template-primer for DNA polymerases

A previous publication [Frenkel, Cain, and Chao, Biochem. Biophys. Res. Commun. 127, 849-856 (1985)] described the observation that double-stranded DNA which was briefly exposed to methyl mercury (MeHg) and purified to remove free methyl mercury was transcribed at a higher rate by RNA polymerase II from wheat germ. The specificity of this phenomenon has now been investigated by examining the activity of this MeHg-exposed DNA as a template-primer for DNA polymerases. DNA synthesis by the bacteriophage T4-induced DNA polymerase was higher with the MeHg-exposed DNA as a template-primer than with control DNA. In contrast, the rate of DNA synthesis by E. coli DNA polymerase I was lower with the MeHg-exposed DNA as template-primer. With both enzymes (as well as with RNA polymerase II), after denaturation of the MeHg-exposed and control DNAs the differences in template activity were either eliminated or markedly reduced. The enzymes are thus able to detect a MeHg-induced alteration in DNA. In contrast, circular dichroism, a physical method that is sensitive to conformational changes in DNA, did not detect any difference between the MeHg-exposed and control DNAs.