Cockayne syndrome is a congenital disease with impaired DNA repair in actively transcribed genes. Affected children show developmental abnormalities and signs of premature aging. Cockayne syndrome is caused by mutations in the Cockayne syndrome complementation group A (CSA) and B (CSB) genes. While CSB encodes a SWI/SNF ATPase that likely assists the stalled RNA polymerase in overcoming lesions, CSA’s detailed role in repair has so far remained elusive. CSA is part of the DDB1-CSA-Cul4-Rbx1 E3 ubiquitin ligase complex. The available data suggest that CSA may function as a substrate adaptor for ubiquitination by the DDB1-Cul4-Rbx1 ligase.I will pursue a novel structure-driven proteomic approach to identify the substrate epitope recognized by the DDB1-CSA-Cul4-Rbx1 E3 ubiquitin ligase. These experiments aim to provide the important missing signal required for recruitment of the ligase complex to sites of stalled RNA polII complexes. Importantly, the CSA ligase, as it arrives at sites of DNA damage, is inactive. Our comprehensive structural and biochemical efforts will thus include the mechanisms of regulation of the CSA ubiquitin ligase activity by activators and inhibitors following recruitment to the repair site.MoBa-CS will not only improve our understanding of CSA’s molecular role in Cockayne syndrome, but also reveal CSA’s mode of action within the essential transcription coupled repair pathway. Understanding the cellular signals overseeing transcription coupled repair will provide important insights into how the pathway is integrated within the overall DNA damage response circuitry of the cell. ERC funding would enable us to pursue an interdisciplinary proteomic and structure based approach in examining DDB1-CSA function.