CREB-binding protein (CBP)/p300 and RNA polymerase II colocalize in transcriptionally active domains in the nucleus

The spatial organization of transcription-associated proteins is an importa nt control mechanism of eukaryotic gene expression. Here we analyzed the nu clear distribution of the transcriptional coactivators CREB-binding protein (CBP)/p300 in situ by confocal laser scanning microscopy, and in vivo comp lex formation by coimmunoprecipitation. A subpopulation of CBP and p300 is targeted to active sites of transcription and partially colocalizes with hy per- and hypophosphorylated RNA polymerase II (pol II) in discrete regions of variable size throughout the nucleus. However, the coactivators were fou nd in tight association with hypophosphorylated, but not hyperphosphorylate d pol II. Transcriptional inhibition induced a relocation of CBP/p300 and p ol II into speckles. Moreover, double and triple immunofluorescence analyse s revealed the presence of CBP, p300, and pol II in a subset of promyelocyt ic leukemia (PML) bodies. Our results provide evidence for a dynamic spacia l link between coactivators of transcription and the basal transcription ma chinery in discrete nuclear domains dependent upon the transcriptional acti vity of the cell. The identification of pol II in CBP/PML-containing nuclea r bodies supports the idea that transcription takes place at PML bodies.