Project Summary Eukaryotic RNA polymerase I (Pol I) transcribes ribosomal RNA, a key component of ribosomes. Pol I transcription accounts for the majority of the total RNA in cells, and its upregulation in human cells is a hallmark of cancer while its downregulation is a hallmark of several developmental disorders. Pol I transcription is understudied compared to transcription by Pol II and even Pol III. Our preliminary work suggests fundamental differences between Pol I and Pols II and III that are the basis for this proposal. Our broad long-term objectives are to determine the molecular mechanism of Pol I transcription and how its dysregulation leads to cancer and developmental disorders. There are major gaps in our understanding of (1) the structural organization and architecture of Pol I transcription complexes; (2) the mechanism for how Pol I initiation factors interact with rDNA, which encodes ribosomal RNA; and (3) the molecular function of several key Pol I transcription factors in the activation process. The first rationale for this work is that determining the mechanism and regulation of Pol I transcription will form the molecular basis for understanding how Pol I defects lead to human disease. Our central hypothesis is that Pol I factors use a unique mechanism to carry out transcription and their structure and function is different from the mechanisms governing Pol II and III transcription. The second rationale is that understanding the Pol I transcription mechanism at the most basic and fundamental levels will translate to a better understanding of the connection between Pol I and cancer, leading to new cancer therapeutic strategies. Our proposed research will use a conceptually and technically innovative cross-organismal and interdisciplinary approach that employs a combination of bioinformatic, computational, molecular, biochemical, genetic, genomic, proteomic, and structural methods in the yeast and human cells. Guided by strong preliminary studies, we will test two specific aims: (1) Determine the unique “coactivator” role of TATA-binding protein (TBP) in Pol I transcription, and (2) Determine the mechanism of Pol I transcription activation. To accomplish these aims, we will use well-established and complementary approaches to identify and map novel Pol I interactions in their native context. We will complement these studies with structural modeling in combination with molecular, genetic, and biochemical functional assays to identify Pol I factor functions conserved from yeast to humans. The proposed research is significant because it will lead to a detailed description of the Pol I transcription mechanism and will provide a conceptual framework for understanding the link between Pol I and human disease. Ultimately, this work will illuminate new avenues for diagnosis, potential interventions, and the development of therapies targeting these novel protein-protein and protein-DNA interactions.