Brd4 associates with mitotic chromosomes throughout early zebrafish embryogenesis.

Brd4 is a member of the BET (bromodomains and extraterminal) subfamily of bromodomain proteins that includes chromatin-modifying proteins and transcriptional regulators. Brd4 has a role in cell cycle progression, making it indispensable in mouse embryos and cultured cells. The N-terminal domain of Brd4 participates in a fusion oncogene. Brd4 associates with acetylated histones in chromatin, and this association persists during mitosis implicating Brd4 in epigenetic memory. Brd4 sequence, particularly the bromodomains and ET domain, is conserved in the zebrafish and Xenopus laevis proteins reported here. Brd4 is expressed and localized on mitotic chromosomes in early zebrafish embryos before and after the midblastula transition (MBT), indicating that the Brd4-chromosome association is a conserved property that is maintained even before zygotic transcription. The association of Brd4 with acetylated histones may also be conserved in early embryos as we found that histones H3 and H4 are already acetylated during pre-MBT stages.