Phase separation directs ubiquitination of gene body nucleosomes

The conserved yeast E3 ligase Bre1 and its partner E2 Rad6 monoubiquitinate histone H2B across gene bodies during the transcription cycle. While processive ubiquitination might in principle arise from Bre1/Rad6 traveling with RNA polymerase II, we provide a different explanation. Here we implicate liquid-liquid phase separation as the underlying mechanism. Biochemical reconstitution shows that Bre1 binds the scaffold protein Lge1, whose intrinsically disordered region phase separates via dynamic, multivalent interactions. The resulting condensates comprise a core of Lge1 encapsulated by an outer catalytic shell of Bre1. This layered liquid recruits Rad6 and the nucleosomal substrate, accelerating H2B ubiquitination. In vivo, the condensate-forming region of Lge1 is required to ubiquitinate H2B in gene bodies beyond the +1 nucleosome. Our data suggest that layered condensates of histone modifying enzymes generate chromatin-associated reaction chambers with augmented catalytic activity along gene bodies. Equivalent processes may occur in human cells, causing neurological disease when impaired. Overall design: Enrichment of H2BK123ub and underlying H2B levels in WT and various LGE1 mutants were determined using highthroughput paired end sequencing using ChIP-exo 5.0. In addition, Genome wide binding profile of Lge1 in WT and bre1? backgrounds were determined using ChIP-exo 5.0.