Terminal selectors organize postmitotic chromatin accessibility for acquisition of serotonergic identity

SUMMARY Maturation of transcriptomes encoding unique neuronal identities requires selective accessibility of transcription factors to cis-regulatory sequences in nucleosome- embedded postmitotic chromatin. Yet the mechanisms controlling postmitotic neuronal chromatin accessibility are poorly understood. We used ATAC-seq, ChIPmentation, and single-cell analyses to show that heterogeneous chromatin landscapes are established early and reveal the regulatory programs driving subtype identities of Pet1 -lineage neurons that generate serotonin (5-HT) neurons. Distal enhancer accessibility is highly dynamic as Pet1 neurons mature, suggesting the existence of regulatory factors that reorganize postmitotic neuronal chromatin. We find that Pet1 and Lmx1b control chromatin accessibility to select Pet1 -lineage specific enhancers for 5-HT neurotransmission and synaptogenesis. Additionally, these factors are required to maintain chromatin accessibility during early maturation suggesting that postmitotic neuronal open chromatin is unstable and requires continuous regulatory input. Together our findings reveal postmitotic transcription factors that reorganize accessible chromatin for neuron specialization.