Different accessibilities in chromatin to histone acetylase.

The addition of millimolar concentrations of n-butyrate to tissue culture media has been shown to cause a dramatic buildup of the acetylated l -aminolysine forms of cellular histones, without causing cell death (Riggs, M. G., Whittaker, R. G., Neumann, J. R., and Ingram, V. M. (1977) Nature 268,462-464). We find that n-butyrate acts by strongly inhibiting histone deacetylation, without any apparent effect on the acetylation reaction, per se. In a rat hepatoma cell line (HTC cells), the presence of high concentrations of n-butyrate causes the in viva half-life of the normally labile histone acetyl groups to be extended over 150-fold to more than 24 h. All histones naturally acetylated in uiuo (H2a, H2b, H3, and H4) are affected, and deacetylation is inhibited regardless of the number of acetyl groups present on any particular histone. n-Butyrate can also be shown to be an effective in vitro inhibitor of partially purified histone deacetylating enzymes (Ki 60 pM, noncompetitive). This in vitro inhibition shows the same fatty acid specificity as the in viva inhibition of deacetylation, and its magnitude is sufficient to account for the full in viva effect. An intriguing finding is that a substantial subclass of each acetylatable histone remains totally unacetylated, even at the highest concentrations of n-butyrate. At the other extreme, we show that a readily acetylated class of histones responds to n-butyrate treatment much more rapidly, and to a larger extent, than do the bulk histones. More generally, the manner in which the acetylated histones accumulate suggests the existence of special nucleosome environments within the cell, which differ markedly in their accessibility to the histone acetylase enzyme(s).