Competitive recruitment of CBP and Rb-HDAC regulates UBF acetylation and ribosomal transcription.

RNA polymerase I (PolI) transcription is activated by the HMG box architectural factor UBF, which loops approximately 140 bp of DNA into the enhancesome, necessitating major chromatin remodeling. Here we show that the acetyltransferase CBP is recruited to and acetylates UBF both in vitro and in vivo. CBP activates PolI transcription in vivo through its acetyltransferase domain and acetylation of UBF facilitates transcription derepression and activation in vitro. CBP activation and Rb suppression of ribosomal transcription by recruitment to UBF are mutually exclusive, regulating in vivo PolI transcription through an acetylation-deacetylation "flip-flop." Thus, PolI transcription is regulated by protein acetylation, and the competitive recruitment of CBP and Rb.