In Vivo Studies of Ribosomal RNA Transcriptional Regulation within a Drosophila Stem Cell Lineage

As the first step of ribosome biogenesis, RNA polymerase I-directed ribosomal RNA gene (rRNA) transcription is critical for cell growth, proliferation and cell survival. Upregulated rRNA levels have been observed in many types of cancers. However, the extent to which rRNA transcription is differentially regulated in cells within the same lineage during differentiation in vivo and how changes in rRNA levels affect cell fate determination remains unclear. Here I present the discovery and characterization of a novel Drosophila RNA polymerase I transcriptional regulator Under-developed (Udd). The initial udd1 mutation was discovered in a sterility screen, and was further revealed to disrupt the expression of a gene CG18316. The udd null phenotype was recessive embryonic lethal, and both udd mutant phenotypes were rescued by a transgene carrying the CG18316 ORF. As a nucleolar protein, Udd colocalized with nascent rRNAs and was enriched in the rRNA gene promoter. Disruption of Udd decreased pre-rRNA levels. Moreover, Udd interacted with another two nucleolar proteins which are potential homologs of mammalian rRNA transcription initiation factors, the knockdown of which affected nucleolar expression of Udd and exhibited similar phenotypes to udd mutants. I further observed that the level of rRNA transcription was correlated with the differentiation state of germ cells in Drosophila ovaries. The pre-rRNA level was high in germline stem cells (GSCs), then decreased in early differentiating cysts, and again increased in the later differentiated germ cells. This difference was also demonstrated in undifferentiated ovaries (bam mutants) before and after introduced differentiation. More intriguingly, increasing rRNA synthesis by Tif-IA overexpression led to a mild expansion of GSCs, while downregulation of Pol I transcription in the undifferentiated ovaries filled with GSC-like cells resulted in multicellular cyst formation. Additionally, I observed another cell fate change, an eye-to-antennal transformation, when rRNA transcription was reduced in the undifferentiated eye primordia. These results suggest that rRNA transcription is closely related to differentiation and development, and the modulation of rRNA synthesis could be a part of the differentiation process.