HP1 (heterochromatin protein 1)-family proteins are widely conserved among eukaryotes, with the exception of budding yeast. They all contain a chromodomain that binds directly to di- or tri-methylated histone H3 lysine 9 (H3K9me2 or H3K9me3), an established marker of silenced chromatin. In addition, the artificial recruitment of these proteins to promoter regions results in gene silencing. Therefore, it is likely that HP1 proteins can repress transcription.
In vitro transcription experiments using chromatin templates have revealed that human HP1 proteins (HP1α, β, and γ) repress transcription by preventing the binding of TFIIB, TFIID, TFIIE, TFIIH, RNAPII, and specific subunits of the Mediator to the promoter (Smallwood et al. 2008). Remarkably, these HP1 proteins can also evict TFIID, RNAPII, and specific subunits of the Mediator, but not TFIIB, TFIIE, or TFIIH, from the preassembled PIC. Given that the HP1 proteins do...