# Mechanisms of Circadian Repression

> **NIH NIH R01** · TEXAS A&M UNIVERSITY · 2020 · $288,829

## Abstract

PROJECT SUMMARY
Daily rhythms in animal behavior, physiology and metabolism are driven by cell-autonomous circadian clocks
that are synchronized by environmental cycles, but maintain ~24h rhythms even in their absence. These clocks
keep time and control overt rhythms via transcriptional feedback loops. Because clock dysfunction negatively
impacts human health and well being, it is imperative to define the mechanisms that drive rhythmic
transcription. The goal of this proposal is to understand how a critical event controlling rhythmic transcription,
i.e. feedback repression, is achieved using two complementary model systems, the monarch butterfly and
Drosophila. In animals, two feedback loop paradigms having orthologous components can be discerned: A
Drosophila-like (dl) paradigm in which CLOCK (CLK) activates and PERIOD (PER) represses transcription,
and a mammal-like paradigm (ml) in which BMAL1 activates and PER-CRYPTOCHROME (PER-CRY)
complexes repress transcription. Monarch butterflies possess an ml clock, but unlike mammals, monarchs
carry single copies of both circadian activators and repressors, thus making it an attractive model to dissect
circadian mechanisms relevant to mammals. Common features of dl and ml clocks are that PER initiates
transcriptional repression `on-DNA' by binding CLK complexes present on E-box regulatory elements, and that
the subsequent removal of repressor-activator complexes from E-boxes initiates an `off-DNA' repression phase
that is maintained for many hours until activators bind E-boxes to reactivate transcription. How PER initiates
on-DNA repression and how off-DNA repression is initiated and maintained remains poorly understood. Our
preliminary data demonstrate that PER-dependent repression in monarch requires a conserved region
encoded by CLK exon 19 (hereafter e19ar), that Drosophila CLOCKWORK ORANGE (CWO) binds E-boxes in
antiphase to CLK-CYC, and that loss of cwo increases trough levels of CLK-CYC binding. Based on published
and preliminary data, we will pursue two aims to determine 1) how PER initiates on-DNA repression of CLK-
BMAL1 and CLK-CYC transcription with a focus on CLKe19ar, and 2) how PER and CWO collaborate to
maintain off-DNA transcriptional repression and promote CLK-CYC/CLK-BMAL1 transcription. Successful
completion of these aims will provide mechanistic insight into how circadian repression determines the phase,
period and amplitude of transcriptional rhythms. Ultimately, such knowledge may be broadly applied for
diagnosis and treatment of many diseases and ailments associated with clock dysfunction.

## Key facts

- **NIH application ID:** 9980957
- **Project number:** 5R01GM124617-04
- **Recipient organization:** TEXAS A&M UNIVERSITY
- **Principal Investigator:** PAUL E HARDIN
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2020
- **Award amount:** $288,829
- **Award type:** 5
- **Project period:** 2017-08-11 → 2022-07-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/9980957

## Citation

> US National Institutes of Health, RePORTER application 9980957, Mechanisms of Circadian Repression (5R01GM124617-04). Retrieved via AI Analytics 2026-05-24 from https://api.ai-analytics.org/grant/nih/9980957. Licensed CC0.

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