Histone acetylation is important for transcription and the correct timing of replication, and is mediated by histone acetyltransferases (HATs), including Gcn5, which, in yeast, is part of the Spt–Ada–Gcn5 acetyltransferase (SAGA) complex. Metazoans have two paralogs of Ada2: Ada2a, which forms the so-called ATAC complex with Gcn5, and Ada2b in SAGA. In addition, in Drosophila, two splice isoforms of Ada2b exist, Ada2b-PA and Ada2b-PB, but their exact functions are not well understood. In this study, Vikki Weake and colleagues (Torres-Zelada et al., 2019) report that the long isoform Ada2b-PB is part of Drosophila SAGA. Moreover, they identify a new Gcn5 complex termed CHAT: in addition to the short isoform Ada2b-PA, it contains Ada3, Sgf29 and Chiffon, the Drosophila ortholog of Dbf4, the activating subunit for the cell cycle kinase Cdc7. Interestingly, most of the CHAT complexes did not contain Cdc7, suggesting that there are distinct pools of Chiffon that interact with either Gcn5 or Cdc7. Furthermore, the authors also show that the C-terminal insect-specific domain of Chiffon directly binds to Gcn5 and is required for both histone H3 acetylation and viability, but not for replication, indicating that the activities of Chiffon in replication and histone acetylation can be genetically separated. This work thus describes the existence of a third Gcn5-containing HAT complex in flies that is not present in yeast or human cells, and further work in this system is needed to delineate its exact functions.
