The establishment of chromatin boundaries (transcriptional regulatory elements that counteract the spreading of silent chromatin) is essential for proper development. At these boundaries, the histone variant H3.3 replaces H3.1 in a process that, in Drosophila, involves the sequence-specific DNA-binding protein GAGA factor, the chromatin remodelling complex FACT, and the H3.3-specific chaperone HIRA. The H3.3 replacement is also likely to require an ATP-dependent remodelling factor and, on , Susumu Hirose and colleagues identify this factor. The researchers show that GAGA factor associates with the Polybromo-associated Brm (PBAP) remodelling complex, which consists of several Trithorax group proteins, and recruits it to chromatin boundaries in Drosophila. They further show that mutations in GAGA factor, Brm and Polybromo/Bap180 disrupt H3.3 replacement and boundary function in a synergistic manner. Moreover, PBAP is needed to generate a DNase-hypersensitive site at the d1 chromatin boundary and HIRA reverses this alteration. Based on these results, the researchers propose a model for H3.3 replacement at chromatin boundaries.
