What is a transcriptional corepressor?

• A. A protein that binds RNA polymerase II and increases transcription.
• B. A protein that promotes histone acetylation to activate transcription.
• C. A protein that acts as a general transcription factor for RNA Pol I.
• D. A protein that interacts with transcription factors to suppress transcription, often recruiting histone deacetylases.

Answer: (D)

Transcriptional corepressors work by teaming up with transcription factors that are bound to DNA and repressing gene expression. They often bring in histone deacetylases, enzymes that remove acetyl groups from histone tails. This deacetylation makes chromatin tighter and less accessible, so RNA polymerase II can’t efficiently initiate transcription. That combination of blocking transcription factor activity and remodeling chromatin to a closed state is what defines a corepressor.

So, the best fit is a protein that interacts with transcription factors to suppress transcription, often recruiting histone deacetylases. The other ideas describe activating mechanisms or transcription factors not associated with repression: promoting RNA Pol II binding and transcription, promoting histone acetylation to activate transcription, or acting as a general factor for RNA Pol I, which isn’t about repressing Pol II–driven transcription.