Elucidation of the mechanism of nucleosomal structural change mediated by histone modifications

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Nucleosomes around the promoter region are disassembled for transcription in response to various signals, such as acetylation and methylation of histones. Although the interactions between histone-acetylation- recognizing bromodomains and factors involved in nucleosome disassembly have been reported, no structural basis connecting histone modifications and nucleosome disassembly has been obtained. We determined at 3.3 A resolution the crystal structure of histone chaperone cell cycle gene 1 (CCG1) interacting factor A/antisilencing function 1 (CIA/ASF1) in complex with the double bromodomain in the CCG1/TAF1/TAF(II)250 subunit of transcription factor IID. Structural, biochemical, and biological studies suggested that interaction between double bromodomain and CIA/ASF1 is required for their colocalization, histone eviction, and pol II entry at active promoter regions. Furthermore, the present crystal structure has characteristics that can connect histone acetylation and CIA/ASF1-mediated histone eviction. These findings suggest that the molecular complex between CIA/ASF1 and the double bromodomain plays a key role in site-specific histone eviction at active promoter regions. The model we propose in this study is the initial structure-based model of the biological signaling fromhistone modifications to structural change of the nucleosome (hi-MOST model).