Abstract
Circadian clocks orchestrate ~24-hour cycles in gene expression, behavior, and physiology across most organisms(1). Our recent study has revealed a striking spatial organization in the nucleus during the repression phase: core clock proteins such as Drosophila PERIOD (PER) are organized into distinct nuclear foci close to the inner nuclear envelope of clock neurons, and clock-regulated genes are similarly positioned in the nucleus(2). However, the functional relevance of this subnuclear organization is unknown. Here, we investigate how the spatial organization of clock proteins and chromatin regulates circadian gene repression. Given that PER partners with TIMELESS (TIM) to enact transcriptional repression, we first investigated whether TIM is also a component of these nuclear foci. Using CRISPR-Cas9 to endogenously tag TIM with mNeonGreen, we performed high-resolution live imaging in Drosophila clock neurons. We found that TIM forms nuclear foci during the repression phase that co-localize with PER condensates, whereas TIM remains diffuse in the cytoplasm of per (01) null mutants. To probe the spatial relationship between these condensates and clock-regulated genes, we combined protein imaging with fluorescence in situ hybridization (FISH) which revealed that PER/TIM condensates were positioned adjacent to, but did not overlap with, clock gene loci during the repression phase. These results suggest that core clock proteins are spatially sequestered into repressive condensates away from chromatin, providing a new framework for understanding how nuclear architecture and phase separation together orchestrate rhythmic gene expression.