We have previously shown that the evolutionarily conserved COE (Collier, Olf, Ebf)-type transcription factor UNC-3 acts as a terminal selector and determines cholinergic motor neuron (MN) identity in multiple cholinergic MN classes (SAB, DA, DB, VA, VB, AS). UNC-3 directly controls the expression of both shared (e.g., acetylcholine pathway genes) and class-specific terminal identity genes (e.g., ion channels, neurotransmitter receptors). However,
unc-3 is expressed in all these MN classes, leading us to hypothesize the existence of repressor proteins that restrict the ability of UNC-3 to activate these class-specific genes more broadly. To test this hypothesis, we performed a forward genetic screen using the UNC-3 target gene
glr-4, which encodes a glutamate receptor subunit selectively expressed in SAB MNs. We found that
pbrm-1, the sole C. elegans ortholog of the evolutionarily conserved chromatin regulator BAF180, selectively prevents expression of a transgenic
glr-4 reporter in DA, VA, and AS classes, resulting in mixed MN identity. Similar results were obtained when we monitored endogenous
glr-4 expression via RNA fluorescent in situ hybridization and a reporter allele. Since PBRM-1/BAF180 is a subunit of PBAF, a chromatin remodeling complex of the SWI/SNF family, we reasoned that animals lacking gene activity for other PBAF subunits might display similar MN phenotypes. We indeed found that loss of
swsn-9 (C. elegans ortholog of human BRD7 and BRD9),
swsn-7 (C. elegans ortholog of human ARID2), and
phf-10 (ortholog of human PHF10) results in gain of
glr-4 expression in these three MN subtypes. Rescue and RNAi experiments using cholinergic MN-specific promoters (
cho-1 and
lin-39) further demonstrated that these four PBAF components, despite their ubiquitous expression, act cell-autonomously in post-mitotic MNs. Finally, we found that the transcription factors MAB-9/Tbx20 and UNC-4/UNCX represses
glr-4 expression in AS and DA/VA neurons, respectively. To account for the observed specificity of PBAF-mediated
glr-4 repression in select MN classes, we hypothesize that PBAF is recruited by MN class-specific transcription factors (e.g., MAB-9, UNC-4) to repress UNC-3 target genes. Altogether, we provide novel insights on the epigenetic mechanisms that generate neuronal diversity by uncovering a previously unrecognized, neuron-specific role for the PBAF chromatin-remodeling complex in selective repression of terminal selector target genes.