Ribosome biogenesis is an essential process for cell growth as it enables cells to synthesise necessary proteins and accumulate biomass. RNA polymerase Ⅰ (PolⅠ) transcribes rDNA in the nucleolus to generate pre-rRNA (i.e. 28S, 18S, and 5.8S rRNAs) which form the rRNA backbone of ribosomes. In mammalian systems, UBF1 (Upstream Binding Factor 1) drives rDNA transcription, by converting r-chromatin to active euchromatin and recruiting SL-1 and Pol I complexes to the rDNA promoter. Although the major components of the pre-initiation complex are well conserved in eukaryotes, a functional orthologue of UBF1 in Drosophila has not been reported. As rDNA transcription is an ancient process common to all organisms, we hypothesized that a UBF1 orthologue exists in D. melanogaster. BLASTp searches using human UBF1 (hUBF1) or the S. cerevisiae orthologue, Hmo1, revealed that Dsp1 is the closest relative of both hUBF1 and Hmo1 in D. melanogaster. We aimed to test whether Dsp1 controls rDNA transcription, and therefore nucleolar architecture and cell growth in Drosophila salivary gland cells, which have a relatively large nucleolus. Dsp1 showed nuclear and nucleolar localization consistent with roles in both Pol I and Pol II-dependent transcription. A time course of either Dsp1 or hUBF1 overexpression revealed a rapid disruption to nucleolar architecture, with swelling at earlier timepoints followed by collapse of polytene chromosome organization. Therefore, our results suggest that Dsp1 could function analogously to hUBF1. We are currently determining whether the dynamic changes in nucleolar structure caused by increased Dsp1 of hUBF1 are associated with altered rDNA transcription, structure and activity of rDNA repeats. We will also investigate whether Dsp1 depletion alters rDNA transcription, cell and tissue growth. Thus, we will gain insight into the mechanisms controlling rDNA architecture in Drosophila and establish a genetic model to investigate factors influencing rDNA structure and function during development.