Timeless noncoding DNA contains cell-type preferential enhancers important for proper Drosophila circadian regulation.

Abstract

To address the contribution of transcriptional regulation to <i>Drosophila</i> clock gene expression and to behavior, we generated a series of CRISPR-mediated deletions within two regions of the circadian gene <i>timeless</i> (<i>tim</i>), an intronic E-box region and an upstream E-box region that are both recognized by the key transcription factor Clock (Clk) and its heterodimeric partner Cycle. The upstream deletions but not an intronic deletion dramatically impact <i>tim</i> expression in fly heads; the biggest upstream deletion reduces peak RNA levels and <i>tim</i> RNA cycling amplitude to about 15% of normal, and there are similar effects on <i>tim</i> protein (TIM). The cycling amplitude of other clock genes is also strongly reduced, in these cases due to increases in trough levels. These data underscore the important contribution of the upstream E-box enhancer region to <i>tim</i> expression and of TIM to clock gene transcriptional repression in fly heads. Surprisingly, <i>tim</i> expression in clock neurons is only modestly affected by the biggest upstream deletion and is similarly affected by a deletion of the intronic E-box region. This distinction between clock neurons and glia is paralleled by a dramatically enhanced accessibility of the intronic enhancer region within clock neurons. This distinctive feature of <i>tim</i> chromatin was revealed by ATAC-seq (assay for transposase-accessible chromatin with sequencing) assays of purified neurons and glia as well as of fly heads. The enhanced cell type-specific accessibility of the intronic enhancer region explains the resilience of clock neuron <i>tim</i> expression and circadian behavior to deletion of the otherwise more prominent upstream <i>tim</i> E-box region.