Structure of the Mediator head module

Abstract

The crystal structure of the Mediator head module from the fission yeast Schizosaccharomyces pombe is solved at 3.4 Å resolution. The Mediator complex has a central role in gene regulation, linking RNA polymerase II to its many transcriptional regulators. The structure of the budding yeast seven-subunit Mediator head module indicated three domains: the neck, fixed jaw and moveable jaw. Patrick Cramer and colleagues have determined the structure of the fission yeast head module at 3.4 Å resolution. The enhanced resolution allows refinement of interpretations of the initial structure and emphasizes the flexibility of the head module. The highly conserved structure is flexible, and the localization of known mutations permits the attribution of function to some regions of the molecule. Gene transcription by RNA polymerase (Pol) II requires the coactivator complex Mediator. Mediator connects transcriptional regulators and Pol II, and is linked to human disease1,2,3,4. Mediator from the yeast Saccharomyces cerevisiae has a molecular mass of 1.4 megadaltons and comprises 25 subunits that form the head, middle, tail and kinase modules5,6,7. The head module constitutes one-half of the essential Mediator core8, and comprises the conserved9 subunits Med6, Med8, Med11, Med17, Med18, Med20 and Med22. Recent X-ray analysis of the S. cerevisiae head module at 4.3 Å resolution led to a partial architectural model with three submodules called neck, fixed jaw and moveable jaw10. Here we determine de novo the crystal structure of the head module from the fission yeast Schizosaccharomyces pombe at 3.4 Å resolution. Structure solution was enabled by new structures of Med6 and the fixed jaw, and previous structures of the moveable jaw11 and part of the neck12, and required deletion of Med20. The S. pombe head module resembles the head of a crocodile with eight distinct elements, of which at least four are mobile. The fixed jaw comprises tooth and nose domains, whereas the neck submodule contains a helical spine and one limb, with shoulder, arm and finger elements. The arm and the essential shoulder contact other parts of Mediator. The jaws and a central joint are implicated in interactions with Pol II and its carboxy-terminal domain, and the joint is required for transcription in vitro. The S. pombe head module structure leads to a revised model of the S. cerevisiae module, reveals a high conservation and flexibility, explains known mutations, and provides the basis for unravelling a central mechanism of gene regulation.