HISTONE ACETYLATION IS REQUIRED TO MAINTAIN THE UNFOLDED NUCLEOSOME STRUCTURE ASSOCIATED WITH TRANSCRIBING DNA

Nucleosomes associated with transcribing chromatin of mammalian cells have an unfolded structure in which the normally buried cysteinyl-thio l group of histone H3 is exposed. In this study we analyzed transcript ionally active/competent DNA-enriched chromatin fractions from chicken mature and immature erythrocytes for the presence of thiol-reactive n ucleosomes using organo-mercury-agarose column chromatography and hydr oxylapatite dissociation chromatography of chromatin fractions labeled with [H-3]iodoacetate. In mature and immature erythrocytes, the activ e DNA-enriched chromatin fractions are associated with histones that a re rapidly highly acetylated and rapidly deacetylated. When histone de acetylation was prevented by incubating cells with histone deacetylase inhibitors, sodium butyrate or trichostatin A, thiol-reactive H3 of u nfolded nucleosomes was detected in the soluble chromatin and nuclear skeleton-associated chromatin of immature, but not mature, erythrocyte s. We did not find thiol-reactive nucleosomes in active DNA-enriched c hromatin fractions of untreated immature erythrocytes that had How lev els of highly acetylated histones HS and H4 or in chromatin of immatur e cells incubated with inhibitors of transcription elongation. This st udy shows that transcription elongation is required to form, and histo ne acetylation is needed to maintain, the unfolded structure of transc ribing nucleosomes.