Histone accessibility determined by lysine-specific acetylation in chicken erythrocyte nuclei

N-Hydroxysulfosuccinimidyl [3H]acetate was synthesized and, following the determination of the optimal reaction conditions, was used to acetylate histones in chicken erythrocyte nuclei at 4 degrees C, pH 8. The histones were extracted from the labelled nuclei and the distribution of the acetyl groups determined from the amount of tritiated acetate in isolated peptides. The relative degree of acetylation of molecules was H1 1.0, H5 0.81, H2B 0.48, H2A 0.24, H3 0.24, H4 0.16. Histone H1 is the most exposed histone followed by H5. The core histones are much less accessible to chemical modification than the linker histones by a factor of 4-5. Histones H2A, H2B and H5 appear to be labelled at random along the entire polypeptide chain, while histones H3 and H4 are labelled almost exclusively in the first 30 residues from the N terminus. Control and acetylated chicken erythrocyte nuclei were digested with DNase I and the resulting DNA hybridized to globin and ovalbumin cDNAs. Acetylation, at 14 molecules acetate/core nucleosome or 20 molecules acetate/chromatosome, increased the DNase I sensitivity of the ovalbumin gene to that of the globin sequences in the control sample, while the globin sequences became even more nuclease-sensitive. Our results suggest that increased sensitivity of chromatin towards nuclease digestion might be due to increased solubility of the chromatin fibre.