Abstract
Circadian rhythms provide organisms with an adaptive advantage, allowing them to regulate physiological and developmental events so that they occur at the most appropriate time of day. In plants, as in other eukaryotes, multiple transcriptional feedback loops are central to clock function. In one such feedback loop, the Myb-like transcription factors CCA1 and LHY directly repress expression of the pseudoresponse regulator TOC1 by binding to an evening element (EE) in the TOC1 promoter. Another key regulatory circuit involves CCA1 and LHY and the TOC1 homologs PRR5, PRR7, and PRR9. Purification of EE–binding proteins from plant extracts followed by mass spectrometry led to the identification of RVE8, a homolog of CCA1 and LHY. Similar to these well-known clock genes, expression of RVE8 is circadian-regulated with a dawn phase of expression, and RVE8 binds specifically to the EE. However, whereas cca1 and lhy mutants have short period phenotypes and overexpression of either gene causes arrhythmia, rve8 mutants have long-period and RVE8-OX plants have short-period phenotypes. Light input to the clock is normal in rve8, but temperature compensation (a hallmark of circadian rhythms) is perturbed. RVE8 binds to the promoters of both TOC1 and PRR5 in the subjective afternoon, but surprisingly only PRR5 expression is perturbed by overexpression of RVE8. Together, our data indicate that RVE8 promotes expression of a subset of EE–containing clock genes towards the end of the subjective day and forms a negative feedback loop with PRR5. Thus RVE8 and its homologs CCA1 and LHY function close to the circadian oscillator but act via distinct molecular mechanisms.
Highlights
We live on a world with prominent and predictable daily changes in the environment
We have recently shown that a protein related to CLOCK ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY), REVEILLE 1 (RVE1), binds to the EE
Purified proteins were subjected to LC-MS/MS and peptide sequences compared to the Arabidopsis proteome
Summary
We live on a world with prominent and predictable daily changes in the environment. To better anticipate and mitigate these changes, most organisms possess circadian clocks, internal timers that generate roughly 24-hour rhythms in physiology even in the absence of environmental cues. Processes influenced by the circadian clock include once-in-a-lifetime developmental events such as the eclosion of insects from their pupal cases or the transition of plants from vegetative to reproductive growth, to daily events such as changes in activity levels in animals or the opening and closing of flowers [1,2]. Given this diversity of clock outputs, it is not surprising that the circadian system influences expression of a substantial fraction of the genome, with 30% of plant genes and 10% of mammalian genes estimated to be circadian regulated [3,4,5,6]. Transcriptional feedback loops play a crucial role in the circadian oscillator, post-transcriptional events such as regulated protein degradation are essential for robust clock function [12]
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