Structural basis of a SWI/SNF-associated transcription pre-initiation complex.

SWI/SNF forms a giant 7-megadalton complex with the RNA polymerase II pre-initiation complex and Mediator, including a nucleosome, revealing its integral role in transcription initiation.

• Why it matters: SWI/SNF was long thought to act independently of transcription machinery; this work addresses how it physically integrates with PIC and Mediator to enable promoter activation.
• What they did: Using high-resolution cryo-EM structures of PIC-Mediator-SWI/SNF and PIC-Mediator-SWI/SNF-nucleosome complexes, plus cross-linking and mass spectrometry, the authors map assembly and interactions.
• The result: The structures show SWI/SNF enhances initiation by stabilizing PIC organization and concertedly remodeling the +1 nucleosome, suggesting coordinated action of Snf2 and Ssl2 during transcription initiation.

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