BRCA1-BARD1 combines multiple chromatin recognition modules to bridge nascent nucleosomes

Chromatin association of the BRCA1-BARD1 heterodimer is critical to promote homologous recombination repair of DNA double-strand breaks (DSBs) in S/G2. How the BRCA1-BARD1 complex interacts with chromatin that contains both damage induced histone H2A ubiquitin and inhibitory H4AK20 methylation is not fully understood. We characterised BRCA1-BARD1 binding and enzymatic activity to an array of mono- and di-nucleosome substrates using biochemical, structural, and single molecule imaging approaches. We find that the BRCA1-BARD1 complex preferentially interacts and modifies di-nucleosomes over mono-nucleosomes, allowing integration of H2A Lys-15 ubiquitylation signals with other chromatin modifications and features. Using high speed-AFM to provide real-time visualization of BRCA1-BARD1 complex recognising chromatin, we show a highly dynamic complex that bridges two nucleosomes and associates with the DNA linker region. Bridging is aided by multivalent cross-nucleosome interactions that enhance BRCA1-BARD1 E3 ubiqiutin ligase catalytic activity. Multivalent interactions across nucleosomes explains how BRCA1-BARD1 can recognize chromatin that retains partial di-methylation at H4 Lys-20 (H4K20me2), a parental histone mark that blocks BRCA1-BARD1 interaction with nucleosomes, to promote its enzymatic and DNA repair activities. ### Competing Interest Statement The authors have declared no competing interest.