Level of ubiquitinated histone H2B in chromatin is coupled to ongoing transcription.

The relationship between transcription and ubiquitination of the histones was investigated. Previous studies have shown that ubiquitinated (u) histone H2B and, to a lesser extent, mono- and polyubiquitinated histone H2A are enriched in transcriptionally active gene-enriched chromatin fractions. Here, we show that treatment of T-47D-5 human breast cancer cells with actinomycin D (10 micrograms/mL) or 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole, inhibitors of heterogeneous nuclear RNA synthesis, selectively reduced the level of uH2B, but not uH2A, uH2A.Z, or polyubiquitinated H2A, in chromatin. Treatment of the cells with low levels (0.04 micrograms/mL) of actinomycin D slightly reduced the level of uH2B, suggesting that inhibition of ribosomal RNA synthesis does not have a profound effect on the level of uH2B in chromatin. The level of the ubiquitinated histones was not affected by treating the cells with inhibitors of DNA synthesis (sodium butyrate or aphidicolin), but heat-shock treatment resulted in the loss of all the monoubiquitinated histone species. These results demonstrate that maintenance of the levels of uH2B in chromatin is dependent upon ongoing transcription, particularly the synthesis of hnRNA. Thus, histone H2B would be ubiquitinated when the nucleosome was opened during transcription. Ubiquitination of histone H2B may impede nucleosome refolding, facilitating subsequent rounds of transcription.