Effect of cations on the acetylation of chromatin in vitro

The effect of the differently charged cations Na+, Mg2+, spermidine and spermine on the acetylation of histones in vitro in soluble chromatin and in core particles was investigated. Up to a given concentration, which depends on the charge of the cation, all four cations activate the acetylation of these fractions. Above this critical concentration a gradual inhibition of the acetyltransferase activity occurs. Spermine, spermidine and Mg2+, but not Na+, affect the relative accessibilities of the acetyltransferase to the individual histones. As the concentration of these three polyvalent cations increases there is a gradual increase in the relative acetyl incorporation in histones H3 accompanied by a corresponding decrease in H4. Kinetic aspects of the system were also studied. A minor fraction of the soluble chromatin (mononucleosomes and dinucleosomes) which is preferentially digested by micrococcal nuclease is also acetylated preferentially. The role chromatin structure plays in determining the specificity of histone acetylation is discussed.