Background
Background
The human Mediator complex is a large, multi-subunit protein assembly that exists in 2 major forms: a core complex that contains 26 subunits and is 1.2 MDa in size, and a CDK8-Mediator complex that contains 29 subunits (25 shared with the core Mediator complex) and is approximately 1.8 MDa in size. The core complex (typically referred to as Mediator) binds the RNA polymerase II (pol II) enzyme and generally serves to activate gene expression; by contrast, the CDK8-Mediator complex cannot bind pol II. Mediator appears to be required for regulating expression of most RNA polymerase II (pol II) transcripts, which include protein-coding and most non-coding RNA genes. Mediator and pol II function within the Pre-Initiation Complex (PIC), which consists of Mediator, pol II, TFIIA, TFIIB, TFIID, TFIIE, TFIIF, and TFIIH and is approximately 4.0 MDa in size. Mediator serves as a central scaffold within the PIC and helps regulate pol II activity in ways that remain poorly understood. Mediator is also generally targeted by sequence-specific, DNA-binding transcription factors (TFs) that work to control gene expression programs in response to developmental or environmental cues. At a basic level, Mediator functions by relaying signals from TFs directly to the pol II enzyme, thereby facilitating TF-dependent regulation of gene expression. Thus, Mediator is essential for converting biological inputs (communicated by TFs) to physiological responses (via changes in gene expression).
Whereas Mediator is present from yeast to human, the amino acid sequence of each individual Mediator subunit has diverged considerably throughout evolution. In fact, of all the components of the general transcription machinery (TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH, Mediator, and pol II), Mediator has diverged in sequence most significantly. Beyond poor sequence conservation, many structural and functional differences have been identified between human and yeast Mediator. Thus, despite many shared functions (e.g. pol II binding), it is important to understand that observations made with yeast Mediator complexes (e.g. location of CDK8 module binding or substrate specificity) do not necessarily translate to humans. Interestingly, the rapid evolution of Mediator subunits and sequences correlates with the vast expansion of DNA-binding TFs from yeast to humans. Given that Mediator is a general target of DNA-binding TFs and is required for TF-dependent transcription, Mediator may have evolved in part in response to increasingly diverse transcriptional regulatory factors.
Because proper regulation of transcription is so critical throughout development, and to prevent tumor formation and a host of other potential disorders, humans have evolved elaborate mechanisms by which the spatial and temporal patterns of gene expression are controlled. Nuclear architecture, chromosomal organization, and lineage-specific TFs help dictate gene expression patterns in specific cell types. In addition, a profusion of regulatory proteins and RNA elements work together in various capacities and contexts to help maintain proper transcriptional control. Regulatory proteins include sequence-specific DNA-binding TFs (a.k.a. activators), components of the general transcription machinery, various classes of cofactors and enzymes, non-coding RNAs, and a variety of chromatin remodeling/modifying enzymes. Despite the structural and functional diversity of this regulatory machinery, only a select few components—pol II, TFIIH, and Mediator—are considered global regulators of transcription; that is, they affect transcription of nearly all protein-coding and non-coding RNA genes. Comparatively little is known about Mediator because, in contrast to pol II or TFIIH, Mediator has been discovered in humans relatively recently.
Currently, it is believed that Mediator controls transcription by mediating signals between DNA-binding TFs and the core transcriptional machinery. This is based upon several experimental observations. First, Mediator is a general target of DNA-binding TFs (activators). Second, Mediator interacts extensively with pol II and forms a stable complex with the pol II enzyme. Because Mediator binds directly to pol II and is also a general target of DNA-binding TFs, Mediator appears to function as a molecular bridge that allows communication of regulatory signals between TFs and the pol II enzyme. In mammalian cells, this “bridging” function includes regulation of chromatin architecture, such as formation of enhancer-promoter loops. It is also evident that Mediator acts as a scaffold around which the human Pre-Initiation Complex (PIC) assembles. This central structural role for Mediator can at once explain its general requirement for transcription, its ability to regulate the function of other PIC factors, and its ability to regulate different stages of the transcription cycle (e.g. initiation and elongation). Our lab is working to identify the molecular mechanisms that underlie Mediator function, with the goal of identifying key molecular interfaces—and/or molecular probes to target these interfaces—that contribute to its broad regulatory influence on gene expression. Areas of interest range from chromatin architecture to transcription initiation and elongation, to mRNA processing. We are also actively studying factors that influence Mediator structure and activity, including TFs, non-coding RNAs, and the CDK8 module.
Lab publications for background information about Mediator:
Taatjes, DJ. The human Mediator complex: a versatile, genome-wide regulator of transcription. TiBS 2010, 35: 315 – 322.
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