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X-ray-crystallography
Abstract:
Gene transcription by RNA polymerase (Pol) II requires the coactivator
complex Mediator. Mediator connects transcriptional regulators
and Pol II, and is linked to human disease1–4. 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 modules5–7. 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
4.3A˚ 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 3.4A˚ 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 knownmutations, and provides
the basis for unravelling a central mechanism of gene regulation.
