Methylmercury(II)-induced histone perturbations in nuclei isolated from calf thymus

Intact nuclei isolated from calf thymus were first incubated for 2 hr at 37 degree C with CH sub(3)HgOH, ranging in concentration from 0.087 mu m (pM 7.06) to 8.7 mM (pM 2.06) (pM identical with - log (CH sub(3)HgOH) sub(added)), followed by a 1-hr treatment at the same temperature with 0.1% formaldehyde as contact-site cross-linking agent. Histones, isolated subsequently from the nuclei, were subjected to electrophoresis in acid-urea gels and their migration pattern was evaluated with the help of an integrating gel scanner. Using information available from the literature, the three cross-linked species X-1, X-2, and X-3 have been tentatively identified as representing the histone dimers H2A-H2B, alpha -H2B-H4, and beta -H2B-H4, respectively. The disappearance of the cross-linked forms does not take place gradually but commences within a narrow organomercurial concentration range. The authors believe this to be an indication of conformational changes taking place in the histone "core" of chromatin at pM < 4.5 under the experimental conditions give. Whether acting via conformational alterations or chemical blockage, either event will affect the biological activity of the constituent chromosomal DNA, and the demonstration that CH sub(3)HgOH interferes with contact-site cross-linking could be the first clue to the mechanism of its genotoxicity at the molecular level.