PROJECT SUMMARY/ABSTRACT The mitochondrial protein sulfite oxidase (SUOX) has surprisingly emerged as a potential regulator of ribosome biogenesis in a genome-wide screen. Ribosome biogenesis, which occurs in the nucleolus of eukaryotic cells, is a highly regulated process essential for cell function. Despite its importance and relevance to diseases such as cancer and ribosomopathies, the regulation of ribosome biogenesis in human cells is not fully understood. To identify novel regulators of this process, the Baserga laboratory pioneered a genome-wide siRNA screen using the number of nucleoli per nucleus as an endpoint. MCF10A breast epithelial cells have an average nucleolar number of 2-3, and a decrease to one indicates aberrant ribosome biogenesis. SUOX is a mitochondrial protein that has unexpectedly surfaced as a hit from this screen. SUOX oxidizes toxic cellular sulfite to sulfate, and some SUOX variants are known to cause the severe, fatal developmental disease Isolated Sulfite Oxidase Deficiency (ISOD). Validation shows that SUOX depletion reduces nucleolar number and ribosomal RNA (rRNA) biogenesis. I have collected further data demonstrating ribosome biogenesis disruption upon SUOX depletion, and proteomics data strongly supporting a role for SUOX in making ribosomes. The similarity of the presentation of ISOD to that of known ribosomopathies, along with our intriguing preliminary results, has raised compelling questions about the involvement of ribosome biogenesis in ISOD pathogenesis. Drilling down to the mechanistic level, our preliminary metabolomics data reveal that SUOX depletion causes a decrease in the methyl donor required for the nucleolar methyltransferase fibrillarin. Fibrillarin methylates an rDNA-specific histone and rRNA, regulating rDNA transcription and rRNA processing, respectively. To date, no work has investigated the role of SUOX in ribosome biogenesis nor of ribosome biogenesis in ISOD pathogenesis. In Aim 1, I will establish the precise role of SUOX in ribosome biogenesis, taking advantage of established assays measuring multiple stages of the process. I will describe effects of SUOX depletion on rRNA methylation using the recently developed RibOxi-seq method, and histone methylation using established antibodies. I will further validate our results using the auxin-inducible degron version 2 (AID2) system. In Aim 2, I will determine the effects of disease-associated SUOX variants on human ribosome biogenesis by rescuing the defects that occur upon siRNA-mediated SUOX depletion with translationally silent and disease-associated loss-of-function SUOX variants. The experiments proposed will clearly define a role for SUOX in human ribosome biogenesis and describe the effects of known disease- causing variants on this essential cellular process. Our unique approach based on an unbiased screen for nucleolar function, combined with both well-established and novel methodology to study ribosome biogenesis, gives us the opp...