Drosophila GAGA factor is required for full activation of the dE2f1-Yki/Sd transcriptional program

The Hippo signaling pathway regulates organ size by controlling the activity of the transcriptional co-activator Yorkie (Yki). Yki is recruited to its target genes by DNA-binding proteins such as Scalloped (Sd). In addition, transcription factor dE2f1, of the Retinoblastoma (Rb) pathway, cooperates with Yki/Sd to synergistically activate a set of common cell cycle target genes. However, little is known about other factors that ensure the proper transcriptional output of Hippo signaling. In this report we identified the chromatin protein GAGA factor (GAF), which is encoded by the Trithorax-like (Trl) gene, as a novel and critical partner in transcriptional regulation by Yki/Sd and dE2f1. We show that GAF is required for the full activation of target genes by dE2f1 and Yki/Sd; while ablation of GAF compromises both normal and inappropriate cell proliferation driven by Yki and dE2f1 in multiple tissues. The importance of GAF is further supported by strong genetic interactions between GAF and the Rb and Hippo pathways. Additionally, we show that GAF directly interacts with RBF, a Drosophila pRB homolog, and partially co-localizes with RBF on polytene chromosomes. Collectively, our data provide a novel connection between a chromatin-binding protein and a transcriptional program governed by the Hippo and Rb pathways.