EFFECTS OF HISTONE ACETYLATION, UBIQUITINATION AND VARIANTS ON NUCLEOSOME STABILITY

The properties of the nucleosomes of a salt-soluble, transcriptionally active gene-enriched fraction of chicken erythrocyte chromatin were e valuated by hydroxyapatite dissociation chromatography. We have demons trated previously that the salt-soluble, transcriptionally active gene -enriched polynucleosomes are enriched in dynamically acetylated and u biquitinated histones, and in an atypical U-shaped nucleosome that pos sessed about 20% less protein than a typical nucleosome. Further, newl y synthesized histones H2A and H2B exchange preferentially with the nu cleosomal histones H2A and H2B of this salt-soluble chromatin fraction . Analysis of the histones eluting from the hydroxyapatite-bound chrom atin demonstrated that hyperacetylated and ubiquitinated (u), includin g multiubiquitinated, H2A-H2B.1 dimers dissociated at lower concentrat ions of NaCl than unmodified dimers or dimers with histone variants H2 A.Z and/or H2B.2. Cross-linking studies revealed that at least 50% of uH2B.1 was paired with uH2A. uH2A-uH2B.1 dimers dissociated at lower N aCl concentrations than H2A-uH2B.1 dimers. Hyperacetylated histone (H3 -H4), tetramers also eluted at lower concentrations of NaCl than unmod ified tetramers. Our results support the idea that acetylation and ubi quitination of histones H2A and H2B.1 increase the lability of H2A-H2B .1 dimers in transcriptionally active nucleosomes. In contrast, our ob servations suggest that histone variants H2A.Z and H2B.2. stabilize th e association of the H2A-H2B dimer in nucleosomes. The elevated labili ty of the H2A-H2B dimer may facilitate processes such as the exchange of these dimers with newly synthesized histones, the elongation proces s of transcription and transcription factor binding.