The effect of CH3HgOH on the thermal stability of calf thymus chromatin in Na2SO4 (10 mᴍ Na+, pH 6.8) has been studied spectrophotometrically by heat denaturation. From changes observed in the integral and differential melting profiles, gathered via absorbance measurements at 260 nm as a function of CH3HgOH concentration, it is concluded that methylmercury at low concentrations (1-10μᴍ), and prior to its denaturating the double-helix of the chromosomal DNA, interferes with the binding of DNA by

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... histones H3 and H4 which is followed, at methyl- mercury concentrations between 10 and 32 μᴍ, also by its interfering with the complexing of DNA by the less basic histones H2A and H2B. DNA denaturation, in absence of the histones, proceeds only at methylmercury concentrations above 32 μᴍ under the experimental conditions given.