Architecture of the RNA polymerase II-Mediator core initiation complex

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• The Mediator complex is a crucial co-activator that regulates transcription initiation by RNA polymerase II (Pol II).
• Understanding Mediator's structure and function is essential for deciphering gene expression control.

Purpose of the Study:

• To reconstitute an active 15-subunit core Mediator (cMed) complex from Saccharomyces cerevisiae.
• To determine the high-resolution cryo-electron microscopic structure of cMed bound to a core transcription initiation complex.

Main Methods:

• Reconstitution of the 15-subunit core Mediator complex.
• Cryo-electron microscopy (cryo-EM) to determine the structure at 9.7 Å resolution.
• Structural analysis of Mediator-Pol II-initiation complex interactions.

Main Results:

• The structure reveals cMed binding to Pol II near the Rpb4-Rpb7 stalk and the carboxy-terminal domain (CTD).
• Specific Mediator modules (head, middle, tail) interact with Pol II and transcription factors, stabilizing the initiation complex.
• The Med14 subunit acts as a structural 'beam' connecting different Mediator modules, potentially influencing Pol II conformation and CTD phosphorylation.

Conclusions:

• The reconstituted cMed structure provides atomic-level insights into Mediator's role in stabilizing the transcription initiation complex.
• The findings elucidate how Mediator interacts with Pol II and its associated factors to regulate transcription initiation and Pol II CTD phosphorylation.