Histone acetylation alters the capacity of the H1 histones to condense transcriptionally active/competent chromatin.

The relationship between histone acetylation and the capacity of H1 histones to cause the 0.15 M NaCl-induced aggregation/precipitation of transcriptionally active/competent gene chromatin fragments was investigated. Previous studies have shown that transcriptionally active/competent, but not repressed, gene chromatin polynucleosomes, which were isolated from chicken erythrocytes, remained soluble in 0.15 M NaCl after being reconstituted with H1 histones. This result suggested that some component of the active/competent gene nucleosome altered the capacity of the H1 histones to condense the chromatin fiber. Recently, Hebbes et al. (Hebbes, T.R., Thorne, A.W., and Crane-Robinson, C. (1988) EMBO J. 7, 1395-1402) demonstrated directly that active, but not repressed, gene chromatin of chicken erythroid cells contain high levels of acetylated histones. Here, we show that the solubility of active/competent gene chromatin fragments in 0.15 M NaCl is dependent on the level of acetylated histone species, with induction of hyperacetylation increasing the solubility of this gene chromatin. Also, we show that lowering the levels of the acetylated histone forms reduces the ability of the active/competent gene chromatin fragments to resist exogenously added H1-histone-induced 0.15 M NaCl aggregation/precipitation. These results suggest that histone acetylation alters the capacity of the H1 histones to form compact higher order chromatin structures such that active/competent gene chromatin is maintained in a less folded state than the bulk of chromatin.