RESUMEN
Acetylation of histones H4 and H3 targeted to promoters/enhancers is linked to the activation of transcription, whereas widespread, long range acetylation of the same histones has been linked to the requirement for open chromatin at transcriptionally active loci and regions of V(D)J recombination. Using affinity-purified polyclonal antibodies to tetra/tri-acetylated histone H2B in chromatin immunoprecipitation (ChIP) assays with mononucleosomes from 15-day chicken embryo erythrocytes, a high resolution distribution of H2B acetylation has been determined and compared with that of H4 and H3 at the same genes/loci. At the beta-globin locus, the H2B acetylation is high throughout and in general mirrors that of H3 and H4, consistent with the observation of co-precipitation of hyperacetylated H4 together with the hyperacetylated H2B. In contrast, at the weakly expressed genes glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and Gas41 (housekeeping) and carbonic anhydrase (tissue specific), very little or no hyperacetylated H2B was found despite the presence of acetylated H4 and H3 at their promoters and proximal transcribed sequences. At the inactive lysozyme and ovalbumin genes essentially no acetylation of H2B, H3, or H4 was observed. Acetylation of H2B appears to be principally a feature of only the most actively transcribed genes/loci.