Widespread association of Polycomb complex-deposited histone H2A monoubiquitylation with enhancers and neuronal gene regulation.

Disruption of histone H2A monoubiquitylation (H2AK119ub) causes neurodevelopmental disorders through poorly understood mechanisms. Polycomb complex-deposited H2AK119ub and trimethylation of histone H3 at lysine 27 (H3K27me3) can cooperatively maintain gene repression. However, the extent to which H2AK119ub functions independently of H3K27me3 to repress or potentiate gene expression, and the evolutionary conservation of these roles, remains unclear. Here, we address the interplay among Polycomb-deposited marks and chromatin states in Caenorhabditis elegans embryos. We find that H2AK119ub distribution is distinct from and largely dispensable for H3K27me3 patterns. Unexpectedly, H2AK119ub is enriched at predicted enhancers with developmentally dynamic changes in accessibility, including in neurons. More than two-thirds of well-documented axon guidance genes and neuronal identity transcription factors are associated with H2AK119ub. Many of the genes differentially expressed in H2AK119ub-deficient animals are involved in neuronal differentiation and signaling and feature H2AK119ub-enriched promoters and enhancers, including a subset that is H3K27me3 repressed. We uncover a likely conserved yet underappreciated dual role for H2AK119ub at enhancers and H3K27me3-repressed chromatin, with implications for nervous system development.