RESUMO
More than a dozen trithorax group (trxG) proteins are involved in activation of Drosophila HOX genes. How they act coordinately to integrate signals from distantly located enhancers is not fully understood. The female sterile (1) homeotic (fs(1)h) gene is one of the trxG genes that is most critical for Ultrabithorax (Ubx) activation. We show that one of the two double-bromodomain proteins encoded by fs(1)h acts as an essential factor in the Ubx proximal promoter. First, overexpression of the small isoform FSH-S, but not the larger one, can induce ectopic expression of HOX genes and cause body malformation. Second, FSH-S can stimulate Ubx promoter in cultured cells through a critical proximal region in a bromodomain-dependent manner. Third, purified FSH-S can bind specifically to a motif within this region that was previously known as the ZESTE site. The physiological relevance of FSH-S is ascertained using transgenic embryos containing a modified Ubx proximal promoter and chromatin immunoprecipitation. In addition, we show that FSH-S is involved in phosphorylation of itself and other regulatory factors. We suggest that FSH-S acts as a critical component of a regulatory circuitry mediating long-range effects of distant enhancers.
Assuntos
Proteínas de Drosophila/química , Proteínas de Drosophila/metabolismo , Drosophila melanogaster/genética , Genes Homeobox , Proteínas de Homeodomínio/genética , Regiões Promotoras Genéticas/genética , Fatores de Transcrição/genética , Animais , Sequência de Bases , Proteínas de Ligação a DNA/metabolismo , Proteínas de Drosophila/genética , Proteínas de Drosophila/isolamento & purificação , Embrião não Mamífero/metabolismo , Proteínas de Homeodomínio/metabolismo , Dados de Sequência Molecular , Ligação Proteica , Estrutura Terciária de Proteína , Elementos de Resposta/genética , Fatores de Transcrição/metabolismo , Ativação Transcricional/genéticaRESUMO
The Polycomb (Pc) group (Pc-G) of repressors is essential for transcriptional silencing of homeotic genes that determine the axial development of metazoan animals. It is generally believed that the multimeric complexes formed by these proteins nucleate certain chromatin structures to silence promoter activity upon binding to Pc-G response elements (PRE). Little is known, however, about the molecular mechanism involved in sequence-specific binding of these complexes. Here, we show that an immunoaffinity-purified Pc protein complex contains a DNA binding activity specific to the (GA)n motif in a PRE from the bithoraxoid region. We found that this activity can be attributed primarily to the large protein isoform encoded by pipsqueak (psq) instead of to the well-characterized GAGA factor. The functional relevance of psq to the silencing mechanism is strongly supported by its synergistic interactions with a subset of Pc-G that cause misexpression of homeotic genes.
