Effect of polyamines and cations on the in vitro methylation of histones.

Na+ (0.05-0.15 M) increases both the rate and extent of methylation of chromosomal bound histone H4, while spermidine markedly inhibits this reaction. The effects of spermidine could be mimicked by increasing the concentration of Mg2+ or Ca2+ to 5-10 mM. At the concentrations listed above, these cations have no significant effect on the methylation of free or chromosomal bound histone H3, nor do they affect the rate r extent of methylation of soluble histone H4. Apparently, the accessibility of histone H4 to the methyltransferase is influenced by chromatin structure. Increasing concentrations of Na+ alter the conformation of chromatin (DNA) in such a way as to expose lysing residues in the N-terminal region of histone H4 to the methyltransferase, whereas Mg2+ or spermidine acts in an opposite manner.