Combinatorial action of temporally-segregated transcription factors

Combinatorial action of transcription factors(TFs) with partially-overlapping expression is a widespreadstrategyto generate novel gene-expression patternsand thuscellular diversity. Known mechanisms underlying combinatorialactivityrequireco-expression of TFswithin the same cell. Here, we describe themechanismby which twoTFsthat are never co-expressed, generate a new, intersectional expression pattern in C. elegans embryos:lineage-specific priming of a gene by a transiently-expressedTFgeneratesa uniqueintersectionwith a second TF acting on the same gene four cell divisions later;the second TF is expressed in multiple cells butonly activatestranscription in those where priming occurred.Early induction of active transcription is necessary and sufficient to establish a competent state that is maintained by broadly-expressed regulators in absence of theinitial trigger. We further uncover new cells diversified through this mechanism. Our findings define anovel mechanism forcombinatorial TF activity with important implications for generation of cell-type diversity in vivo and in vitro. Overall design: 6 ChIP-seq libraries and 32 ATAC-seq libraries from C. elegans embryos to assess TBX-37/38-bound genomic regions and their differential accessibility over time in sorted cells from the ABa and ABp lineages and mature ASEL/ASER neurons.