Architecture of the mediator head module

Abstract

Mediator is a key regulator of eukaryotic transcription 1 , connecting activators and repressors bound to regulatory DNA elements with RNA polymerase II (Pol II) 1-4 . In the yeast S. cerevisiae, Mediator comprises 25 subunits with a total mass over 1 MDa 5,6 , and is organized into three modules, termed Head, Middle/Arm and Tail 7-9 . Our understanding of Mediator assembly and its role in regulating transcription has been impeded to date by limited structural information. Here, we report the crystal structure of the essential Mediator Head module (seven subunits, 223 kDa) at 4.3 Å resolution. Our structure reveals three distinct domains with the integrity of the complex centered on a bundle of ten helices from five different Head subunits. An intricate pattern of interactions within this helical bundle ensures stable assembly of the Head subunits, and provides the binding sites for general transcription factors (GTFs) and Pol II. Our structural and functional data suggest the Head module to juxtapose TFIIH and the carboxyl-terminal domain (CTD) of the

Structure of the Mediator head module

“…The head module constitutes one-half of the essential Mediator core 8 , and comprises the conserved 9 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 jaw 10 . Here we determine de novo the crystal structure of the head module from the fission yeast Schizosaccharomyces pombe at 3.4 Å resolution.…”

A structural perspective on Mediator function

“…Recombinant head module was obtained after co-expression of the subunits in insect cells, and was first studied by EM at low resolution [19]. Recently, a 7-subunit partial backbone model of the head module was derived by X-ray crystallography at 4.3 Å resolution [20 ]. This model comprises 60% of the total head module, revealing the topology and interaction network between its subunits.…”

“…This model comprises 60% of the total head module, revealing the topology and interaction network between its subunits. The atomic model of Med18/Med20 [15] was placed into the medium-resolution electron density [20 ]. The recent atomic structure of the Med11/Med22 heterodimer [16 ] can now also be docked into the head module density.…”

“…All currently available crystal and NMR structures (see Table 3) are shown indicating the respective locations within Mediator. In addition, the crystallographic backbone model of the complete S. cerevisiae head module is depicted [20 ].…”

“…Current Opinion in Cell Biology EM reconstructions (black mesh) of Mediator from human (left, [41 ]) and yeast (right, [20 ]) with Pol II in a similar orientation (orange, Rpb4/7 in pink).…”