Drosophila Mediator Complex Is Broadly Utilized by Diverse Gene-Specific Transcription Factors at Different Types of Core Promoters

Abstract

To decipher the mechanistic roles of Mediator proteins in regulating developmental specific gene expression and compare them to those of TATA-binding protein (TBP)-associated factors (TAFs), we isolated and analyzed a multiprotein complex containing Drosophila Mediator (dMediator) homologs. dMediator interacts with several sequence-specific transcription factors and basal transcription machinery and is critical for activated transcription in response to diverse transcriptional activators. The requirement for dMediator did not depend on a specific core promoter organization. By contrast, TAFs are preferentially utilized by promoters having a specific core element organization. Therefore, Mediator proteins are suggested to act as a pivotal coactivator that integrates promoter-specific activation signals to the basal transcription machinery.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 TAF II 145/130 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 ...

A subunit of the mediator complex regulates vertebrate neuronal development

“…Med12 belongs to a submodule that has been implicated in transcriptional repression in yeast (39) and transcriptional activation in Drosophila (40). Given that Med12 is found to interact with Sox9's transcriptional activation domain (17), and that both factors are required for the activation of lim1 and zash1a, we favor the hypothesis that Mot͞Med12 acts as a coactivator for Sox9 in regulating lim1 and zash1a expression.…”

A structural perspective of CTD function

“…The phosphorylated CTD does not associate with Mediator (Svejstrup et al 1997), and CTD phosphorylation during transcription initiation apparently breaks the Pol II-Mediator interaction, resulting in an elongating polymerase and a scaffold complex that remains at the promoter (Liu et al 2004). Consistent with the conservation of Mediator, mouse and Drosophila Mediator also bind the CTD (Jiang et al 1998;Park et al 2001). The human Mediator-like complex CRSP also interacts with the CTD and adopts a specific CTD-bound conformation .…”

TheDrosophilatrithorax group genetonalli(tna) interacts genetically with the Brahma remodeling complex and encodes an SP-RING finger protein

“…There is also some in vitro evidence to suggest that some specific Mediator subcomplexes act as transcriptional corepressors (Balciunas et al, 1999;Song and Carlson, 1998;Sun et al, 1998). In flies, the Mediator complex has been purified and its interactions with different promoters, sequence-specific transcription factors and basal transcription machinery has been characterized to some extent (Park et al, 2001). In addition, many subunits have been identified in the Drosophila genomic DNA sequence by their similarity to yeast or human Mediator subunits (Boube et al, 2000;Rachez and Freedman, 2001).…”