NUCLEOSOMAL HISTONES OF TRANSCRIPTIONALLY ACTIVE COMPETENT CHROMATIN PREFERENTIALLY EXCHANGE WITH NEWLY SYNTHESIZED HISTONES IN QUIESCENT CHICKEN ERYTHROCYTES

The incorporation of newly synthesized histones among various chromatin fractions isolated from non-replicating cell-cycle-phase-G0 chicken immature erythrocytes was investigated. We find that newly synthesized erythroid-specific histone H1 variant, H5, is incorporated randomly into chromatin. In contrast, newly synthesized nucleosomal histones H2A, H2A.Z, H2B, H3.3, and H4 are preferentially found in a fraction that is highly enriched in active/competent gene chromatin fragments and depleted in repressed gene chromatin. Moreover, ubiquitinated species of histones H2A and H2B and hyperacetylated species of H4 and H2B, which are complexed to active DNA, are labelled. These observations provide evidence that newly synthesized histones preferentially exchange with the nucleosomal histones of transcriptionally active/competent chromatin domains. The results of this study suggest that nucleosomes of active chromatin may be inherently less stable than bulk nucleosomes in vivo and have implications for chromatin remodelling.