Drosophila Mediator complex is broadly utilized by diverse gene-specific transcription factors at different types of core promoters

Autor: Byung Soo Gim, Jeong Ho Yoon, Jung Mo Kim, Ju-Gyeong Kang, Hye-Suk Kim, Jin Mo Park, Young Joon Kim
Rok vydání: 2001
Předmět:
Zdroj: Molecular and cellular biology. 21(7)
ISSN: 0270-7306
Popis: Precise regulation of gene expression is fundamentally required for a broad spectrum of developmental processes in multicellular organisms. Although distinct sequence-specific transcription factors are primarily responsible for this regulation, transcriptional coactivator-corepressor proteins also add a significant secondary layer to the regulation of gene expression. A number of coactivator complexes have been identified in eukaryotes, and their functions in gene activation at specific promoters have been analyzed, primarily in vitro. However, the mechanism by which these transcriptional coactivators regulate gene expression in living organisms is not well understood. Among eukaryotic transcriptional coactivators, two classes of proteins, Mediator proteins and TATA-binding protein (TBP)-associated factors (TAFs), are central to the process of transcriptional regulation. These proteins were isolated as multiprotein complexes composed of more than 10 polypeptides and associate with the basal transcription machinery (RNA polymerase II [Pol II] and TBP, respectively). Both complexes interact with transcriptional activators and are required for transcriptional activation in reconstituted in vitro systems (3, 8, 40). These facts suggest that Mediator and TAF complexes function as coactivators by relaying transcriptional activation signals from DNA-bound activators to the basal transcription machinery. However, it has not been clearly determined whether Mediator and TAF complexes contribute redundantly or distinctly to the transcriptional activation process, nor have their mechanistic roles in Pol II transcription been clearly deciphered. Although both Mediator and TAF complexes were initially identified from in vitro assays, recent genetic analyses in yeast suggest that TAFs do not function as general coactivators under physiological conditions. First, the depletion of various TFIID-specific TAFs does not have a significant effect on transcriptional activation of most genes in yeast (12, 28, 41). Second, yeast TAFII145/130 was shown to function as a core promoter selectivity factor rather than a general coactivator (36). These observations are in good agreement with earlier reports that certain TAFs, especially Drosophila TAFII40 (dTAFII40), dTAFII60, dTAFII150, and dTAFII250, directly recognize core promoter elements (7, 39) and thus are likely to act as promoter selectivity factors. The Mediator complex was first identified in yeast as a multiprotein complex (15, 16) containing functionally distinct modular subassemblies composed of subunits with similar genetic properties (19). Inactivation of individual Mediator proteins causes widely variable effects on yeast gene expression, ranging from deregulation of transcription of a small subset of genes to a genomewide transcriptional defect (12). A number of coactivator complexes related to yeast Mediator have been subsequently isolated in mammals (5, 9, 13, 29, 31, 33, 37). Like yeast Mediator, mammalian complexes play a key role in regulating Pol II transcription in vitro. However, their compositional and functional heterogeneity indicates that there has been a considerable increase in the functional diversity of Mediator complexes during evolution. In this study, we analyzed the function of the Drosophila Mediator (dMediator) complex in order to gain insight into the mechanism by which these coactivator complexes govern eukaryotic transcriptional activation during Drosophila development. As the first step toward this goal, we isolated a multiprotein complex containing Drosophila homologs of yeast and mammalian Mediator proteins. Our data shows that dMediator interacts physically with several sequence-specific transcription factors and basal transcription machinery and is critically required for transcriptional activation in response to diverse transcriptional activators in vitro. By contrast, the TAF complex is dispensable for transcription of TATA-containing promoters in the presence of dMediator but is required for transcription of TATA-less promoters that depend on an initiator element (Inr) and downstream promoter element (DPE). Our results suggest that dMediator functions as an essential coactivator complex that integrates diverse gene-specific regulatory signals at specific promoters, while TAF complex has its distinct role as a promoter selectivity factor required for the expression of genes with a specific type of core promoter.
Databáze: OpenAIRE
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