Functions of the co-activator CBP in transcription and in control of early Drosophila embryo development. Mattias Mannervik. Wenner-Gren Institute, Stockholm University, Stockholm, Sweden.

   The p300 and CBP co-activators are histone acetylases and central regulators of transcription in metazoans. The genomic occupancy of p300/CBP detected by ChIP-seq experiments can be used to identify transcriptional enhancers. Whether p300/CBP is preferentially involved in some gene regulatory networks is not known. We therefore compared the genome occupancy of Drosophila CBP (nejire) with that of 40 different transcription factors in early embryos. We found a striking overlap of CBP ChIP-seq peaks with regions bound by Dorsal. In mutant embryos where Dorsal fails to enter the nucleus, CBP peaks were less associated with Dorsal-binding regions, and instead best correlated with Dpp-signaling and Smad binding. Thus, two key processes in dorsal-ventral patterning, the Dorsal gene regulatory network and Dpp-signaling, overlap the genomic distribution of CBP most significantly, whereas anterior-posterior activators such as Bicoid and Caudal show little overlap. Perhaps CBP serves to coordinate the Dorsal and Dpp pathways in dorsal-ventral patterning. Surprisingly, although CBP occupancy in general correlates with gene activation, it can also be found at silent regions. At silent sites, CBP occupancy does not cause histone acetylation. One mechanism for preventing histone acetylation at these sites is methylation of H3K27 by the Polycomb complex PRC2. Interestingly, H3K27me3-repressed chromatin does not preclude CBP binding, resulting in a bivalent situation. The antagonism between H3K27ac and H3K27me3 indicates that CBP may be involved in switching between repressed and active chromatin states. We performed ChIP-seq of CBP in S2 cells, which showed that CBP occupancy depends of GAGA factor (GAF), and is preferentially found at promoters with a paused RNA polymerase. Using a CBP inhibitor, we show that CBP is required for pol II occupancy at GAF-bound paused promoters. Our studies show that there is a preference for some transcription factors over others in directing CBP to the genome, and that CBP is required for transcription from promoters with a paused polymerase.