Ubiquitination of histones has been linked to the complex processes that regulate the activation of eukaryotic transcription. However, the cellular factors that interpose this histone modification during the processes of transcriptional activation are not well characterized. A biochemical approach identified the Drosophila coactivator TAF(II)250, the central subunit within the general transcription factor TFIID, as a histone-specific ubiquitin-activating/conjugating enzyme (ubac). TAF(II)250 mediates monoubiquitination of histone H1 in vitro. Point mutations within the putative ubac domain of TAF(II)250 abolished H1-specific ubiquitination in vitro. In the Drosophila embryo, inactivation of the TAF(II)250 ubac activity reduces the cellular level of monoubiquitinated histone H1 and the expression of genes targeted by the maternal activator Dorsal. Thus, coactivator-mediated ubiquitination of proteins within the transactivation pathway may contribute to the processes directing activation of eukaryotic transcription.
