Mediator MED18 subunit plays a negative role in transcription via the CDK/cyclin module

Area of Science:

• Molecular Biology
• Gene Regulation
• Eukaryotic Transcription

Background:

• The Mediator complex is a conserved eukaryotic transcriptional co-activator.
• The head module, including hMED18, interacts with RNA polymerase II and general transcription factors.
• Understanding hMED18's role is crucial for elucidating Mediator's dual function in transcription.

Purpose of the Study:

• To investigate the function of the human Mediator complex head module subunit, hMED18.
• To determine hMED18's role in transcriptional activation and repression.
• To elucidate the mechanism underlying hMED18-mediated transcriptional regulation.

Main Methods:

• siRNA-mediated depletion of hMED18.
• Treatment with histone deacetylase (HDAC) inhibitors (trichostatin A and nicotinamide).
• Chromatin immunoprecipitation (ChIP) to assess protein binding at target gene promoters (RAM/FAM103A1 and DMT1/SLC11A2).

Main Results:

• hMED18 depletion led to increased transcription, indicating a repressive role for hMED18.
• hMED18-mediated repression was not dependent on HDAC activity.
• hMED18 depletion caused the loss of hMED18 and hMED1 from target gene promoters, while hCDK8 remained bound, suggesting a novel repression mechanism.
• This mechanism involves hMED18, hCDK8, and the middle module subunit hMED1.

Conclusions:

• hMED18 functions as a transcriptional repressor.
• A novel mechanism of transcriptional repression mediated by hMED18 and hCDK8 was identified.
• The study suggests a novel positive role for the free CDK/cyclin module in transcriptional activation.