♦ See referenced article, J. Biol. Chem. 2012, 287, 35784–35794
The Polycomb group of proteins is critical in transcriptional regulation during development. Despite the importance of this group, not much is known about the mechanism by which its proteins turn off transcription. In this Paper of the Week, a team led by Michael Carey at UCLA studied how a recombinant form of Polycomb repressive complex 1 (PRC1), a complex involved in silencing the important Hox genes for development, interfered with the assembly of the RNA polymerase II preinitiation complex (PIC). Using an in vitro assay and searches through mouse embryonic stem cell genome databases, Carey and colleagues demonstrated that PRC1 blocked Mediator and other general transcription factors during assembly or made them dissociate from the preassembled PIC. The only transcription factor that could resist PRC1 appeared to be transcription factor IID (TFIID). The authors say that this study, along with previous reports from their group, suggests that transcriptional silencers, like PRC1, function by targeting critical steps in the assembly of the PIC.
TFIID but not Mediator resists PRC1.