Coordinate regulation of ribosomal protein gene transcription mediates embryonic secretory cell growth

Cell growth is well defined for the late (post-embryonic) stages of development, but evidence for early (embryonic) cell growth during post-mitotic morphogenesis is quite limited. Here, we identify early cell growth as a key characteristic of Drosophila embryonic salivary gland (SG) tubulogenesis. A BTB/POZ domain nuclear factor, Ribbon (Rib), mediates early cell growth without affecting endoreplication. Rib binds the transcription start site of nearly every SG-expressed ribosomal protein gene (RPG), and is required for full expression of all RPGs tested. Rib binding to RPG enhancers is, however, weak and not sequence-specific, suggesting that specificity is achieved through co-factor interactions. We demonstrate Rib’s ability to physically interact with each of the three known activators of RPG transcription. Surprisingly, Rib-dependent early cell growth in another tubular organ—the embryonic trachea—is not mediated by direct RPG transcription. Our results support a model for tissue-specific transcriptional regulation in early cell growth programs. ChIP-seq of Drosophila melanogaster embryos