RESUMO
macroH2A (mH2A) is an unusual histone variant consisting of a histone H2A-like domain fused to a large nonhistone region. In this work, we show that histone mH2A represses p300- and Gal4-VP16-dependent polymerase II transcription, and we have dissected the mechanism by which this repression is realized. The repressive effect of mH2A is observed at the level of initiation but not at elongation of transcription, and mH2A interferes with p300-dependent histone acetylation. The nonhistone region of mH2A is responsible for both the repression of initiation of transcription and the inhibition of histone acetylation. In addition, the presence of this domain of mH2A within the nucleosome is able to block nucleosome remodeling and sliding of the histone octamer to neighboring DNA segments by the remodelers SWI/SNF and ACF. These data unambiguously identify mH2A as a strong transcriptional repressor and show that the repressive effect of mH2A is realized on at least two different transcription activation chromatin-dependent pathways: histone acetylation and nucleosome remodeling.
Assuntos
Proteínas de Ciclo Celular/antagonistas & inibidores , Histona Acetiltransferases/antagonistas & inibidores , Histonas/metabolismo , Nucleossomos/metabolismo , Proteínas Repressoras/metabolismo , Transativadores/antagonistas & inibidores , Fatores de Transcrição/antagonistas & inibidores , Transcrição Gênica , Acetilação , Animais , Proteínas de Ciclo Celular/metabolismo , DNA Polimerase II/metabolismo , Regulação para Baixo , Histona Acetiltransferases/metabolismo , Proteínas Nucleares/metabolismo , Nucleossomos/química , Estrutura Terciária de Proteína , Transativadores/metabolismo , Fatores de Transcrição/metabolismo , Xenopus laevis , Fatores de Transcrição de p300-CBPRESUMO
A histone variant H2ABbd was recently identified, but its function is totally unknown. Here we have studied the structural and functional properties of nucleosome and nucleosomal arrays reconstituted with this histone variant. We show that H2ABbd can replace the conventional H2A in the nucleosome, but this replacement results in alterations of the nucleosomal structure. The remodeling complexes SWI/SNF and ACF are unable to mobilize the variant H2ABbd nucleosome. However, SWI/SNF was able to increase restriction enzyme access to the variant nucleosome and assist the transfer of variant H2ABbd-H2B dimer to a tetrameric histone H3-H4 particle. In addition, the p300- and Gal4-VP16-activated transcription appeared to be more efficient for H2ABbd nucleosomal arrays than for conventional H2A arrays. The intriguing mechanisms by which H2ABbd affects both nucleosome remodeling and transcription are discussed.