Ubiquitinated histone H2B is preferentially located in transcriptionally active chromatin

Using an anti-ubiquitin antibody in Western blotting experiments, we detected polyubiquitinated species of histones H2A, H2A.Z, and H2B in histone preparations of bovine thymus, chicken erythrocyte, and Tetrahymena macro- and micronuclei. Histone H2A had the greatest level of polyubiquitinated species, with tetra- to hexaubiquitinated forms of this histone being observed. The fraction of bovine thymus and chicken erythrocyte chromatin enriched in transcriptionally active gene sequences was enriched in mono- and polyubiquitinated species of histones H2A, H2B, and H2A.Z, especially in the ubiquitinated forms of histone H2B. Histones H2A and H2B were ubiquitinated in the transcriptionally active Tetrahymena macronucleus, with monoubiquitinated (u) H2B being the predominant ubiquitinated histone species. Ubiquitinated forms of histones H2A and H2B were found in transcriptionally inert micronuclei, but at lower levels than seen in macronuclear histones. Also, the level of micronuclear uH2A was greater than that of uH2B which may be from macronuclei that contaminate the preparation. These results indicate that the mono- and polyubiquitinated species of histone H2B are preferentially located in transcriptionally active chromatin regions. Ubiquitinated histone H2A is located in both expressed and repressed chromatin domains, but expressed chromatin is enriched in mono- and polyubiquitinated forms of this histone. These observations are consistent with the hypothesis that ubiquitinated histones have a role maintaining the structure of transcriptionally active chromatin.