text: pRb acts as a recruiter that allows for the binding of proteins that alter chromatin structure onto the site E2F-regulated promoters. Access to these E2F-regulated promoters by transcriptional factors is blocked by the formation of nucleosomes and their further packing into chromatin. Nucleosome formation is regulated by post-translational modifications to histone tails. Acetylation leads to the disruption of nucleosome structure. Proteins called histone acetyltransferases (HATs) are responsible for acetylating histones and thus facilitating the association of transcription factors on DNA promoters. Deacetylation, on the other hand, leads to nucleosome formation and thus makes it more difficult for transcription factors to sit on promoters. Histone deacetylases (HDACs) are the proteins responsible for facilitating nucleosome formation and are therefore associated with transcriptional repressors proteins.
  pRb interacts with the histone deacetylases HDAC1 and HDAC3. pRb binds to HDAC1 in its pocket domain in a region that is independent to its E2F-binding site. pRb recruitment of histone deacetylases leads to the repression of genes at E2F-regulated promoters due to nucleosome formation. Some genes activated during the G1/S transition such as cyclin E are repressed by HDAC during early to mid-G1 phase. This suggests that HDAC-assisted repression of cell cycle progression genes is crucial for the ability of pRb to arrest cells in G1. To further add to this point, the HDAC-pRb complex is shown to be disrupted by cyclin D/Cdk4 which levels increase and peak during the late G1 phase.