Distinct Mechanisms of Recognition of Phosphorylated RNAPII C-Terminal Domain by BRCT Repeats of the BRCA1–BARD1 Complex: Insights from Structural and Functional Analyses

Transcription competes with other DNA-dependent processes, such as DNA repair, for access to its substrate, DNA. However, the principles governing the interplay between these processes remain poorly understood. Evidence suggests that the BRCA1-BARD1 complex, a key player in the DNA damage response, may act as a mediator of this crosstalk. In this study, we investigated the molecular mechanism underpinning the interaction between RNA polymerase II (RNAPII) and the BRCA1-BARD1 complex, as well as its functional implications. Our findings reveal that the BRCT repeat of BRCA1 binds the Ser5-phosphorylated CTD of RNAPII, utilising a mechanism previously established for other BRCT ligands. Furthermore, we demonstrate that this interaction is critical for the organisation of RNAPII into condensates with liquid-like properties. Analysis of disease-associated variants within the BRCT repeats further supports the biological relevance of this condensation. Collectively, our results suggest that the BRCA1-BARD1 complex may coordinate transcription and DNA repair by facilitating the organisation of RNAPII into transcription factories.