When DNA is treated with Cu(II) and then heated, the melting temperature (T-m) of the DNA is dramatically decreased (8). The Cu(II) binds to the DNA in such a way as to destabilize the double helix and help to break the hydrogen bonds between the bases. When soluble chromatin is similarly treated with Cu(II) and heated, the T-m is unaffected. Apparently the Cu(II) cannot penetrate the chromatin structure and thus cannot initiate the DNA destabilization process. However, when H-1 histone is removed from the chromatin by affinity chromatography, subsequent treatment with Cu(II) does lead to a lowered T-m when the chromatin is heated. This T-m lowering is also achieved by two less drastic techniques that do not remove histone H-1, but decrease the affinity of the H-1 to the DNA: (1) a mild acetylation procedure that specifically modifies either 2 or 4 epsilon-amino groups of lysines on the H-1 histone, and (2) reaction with phosphate-binding divalent metal ions, e.g., Mg(II), Mn(II), or Co(II). Apparently, removal of H-1 or decreased affinity of H-1 for DNA increases the accessibility of the DNA to the Cu(II). This phenomenon suggests a very simple qualitative probe for the degree of structural change in chromatin produced by a change in the stability of the DNA-H-1 interaction.
