The Role of Yeast Mediator in Transcriptional Regulation

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Mediator is a global transcriptional co-regulatory protein complex that plays a central role in the activation and repression of RNA polymerase II transcribed genes. The highly conserved composition and function of Mediator, as well as the intricacies of its mechanism, make Saccharomyces cerevisiae an ideal model for incisive molecular studies. The current paradigm for Mediator function is that it serves as a direct intermediary between DNA-bound activators and the general transcription machinery. However, yeast genetic studies have revealed two important functional properties of the complex that are not compatible with this simple model. The first is Mediator's ability to repress transcription of a large number of genes via a possible link to chromatin structure. The second is the additional co-activator complexes and specific core promoter sequences required for Mediator function in activated transcription. Although ably studied by yeast genetic techniques over the past 10 to 15 years, very little is known about the direct mechanisms used by Mediator to facilitate these functions. Our proposed research meets a critical need by using biochemical experiments to reveal the mechanistic basis of these novel functional properties of Mediator. The long-term goal of our research program is to obtain a complete molecular understanding of the 'communication' between activation or repression signals and the core transcription machinery in eukaryotic cells. Our proposed studies of yeast Mediator center on the use of purified proteins and nucleic acids to elucidate mechanisms. They also marshal the substantial genetic, genomic and proteomic resources in yeast to understand these mechanisms in a physiological context. Our central hypothesis is that Mediator facilitates repression and co-operates with other co-activators through mechanisms that are distinct from serving as a direct intermediary between activators and RNA Polymerase II. The specific aims of the current research project will answer two key questions. First, how are Mediator-facilitated and chromatin-based transcription repression linked? We propose biochemical and genetic experiments to elucidate a mechanism of repression that involves direct interactions between Mediator and chromatin. Second, how do SAGA, TFIID, and core promoter sequence affect Mediator activated transcription? We have designed biochemical experiments to determine how and if Mediator directly works with different co-activators to stimulate high levels of activated transcription in a cooperative manner. These studies are essential to understand the full range of transcriptional mechanisms used to regulate cellular and developmental processes.