Sedimentation and viscosity of bacteriophage T7 DNA in presence of CH3HgOH

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.

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

Exposing (soluble) calf thymus chromatin and, as reference, protein-free native calf thymus DNA (both in 0.01 M Na+, pH 6.8, 25 degrees C) to increasing concentrations of CH3HgOh produces cooperative transitions in their CD spectra. In the case of chromatin, and there especially at low concentrations of methylmercury, they are due to reactions affecting the relative orientation of the bases in the constituent DNA, without disrupting base-pairing. In the case of protein-free DNA, and with chromatin at higher methylmercury concentrations, the CD changes signal collapse of the DNA secondary structure. Primary data (molar ellipticities [theta], zero-ellipticity points, and rotational strengths R) are presented as a function of methylmercury concentration and wavelength. The results are discussed in relation to previous findings of this laboratory regarding methylmercury-DNA and methylmercury-chromatin interactions, and it is pointed out that the structural alterations observed with chromatin at low levels of methylmercury may very well be the primary events in a chain that is responsible for the teratogenic and clastogenic damages caused by organic mercury.

Exposure of DNA to methyl mercury results in an increase in the rate of its transcription by RNA polymerase II

Double-stranded DNA which was exposed to methyl mercury at concentrations of 1 mM and above, and purified by ethanol precipitation and dialysis, was transcribed at a higher rate by RNA polymerase II than was control DNA. The rate of transcription of single-stranded DNA was not affected by similar exposure to methyl mercury. The higher rate of transcription of methyl mercury-treated double-stranded DNA appears to result from a decreased Km of the enzyme for this DNA. This does not appear to result from extensive denaturation, nor from formation of a large number of single-stranded breaks in the DNA.