Abstract
Transcriptional feedback loops are key to circadian clock function in many organisms. Current models of the Arabidopsis circadian network consist of several coupled feedback loops composed almost exclusively of transcriptional repressors. Indeed, a central regulatory mechanism is the repression of evening-phased clock genes via the binding of morning-phased Myb-like repressors to evening element (EE) promoter motifs. We now demonstrate that a related Myb-like protein, REVEILLE8 (RVE8), is a direct transcriptional activator of EE-containing clock and output genes. Loss of RVE8 and its close homologs causes a delay and reduction in levels of evening-phased clock gene transcripts and significant lengthening of clock pace. Our data suggest a substantially revised model of the circadian oscillator, with a clock-regulated activator essential both for clock progression and control of clock outputs. Further, our work suggests that the plant clock consists of a highly interconnected, complex regulatory network rather than of coupled morning and evening feedback loops. DOI:http://dx.doi.org/10.7554/eLife.00473.001.
Highlights
Circadian clocks are widespread in nature, presumably because they help diverse organisms prepare for predictable day/night cycles
glucocorticoid receptor (GR) fusion proteins are held in the cytoplasm unless the synthetic ligand for GR, dexamethasone (DEX), is applied, which allows the chimeric factor to move into the nucleus (Picard et al, 1988)
We examined the ability of DEX-inducible RVE8-GR to activate expression of a known RVE8 target, the evening-phased clock gene PRR5 (Rawat et al, 2011)
Summary
Circadian clocks are widespread in nature, presumably because they help diverse organisms prepare for predictable day/night cycles. In Arabidopsis, the first-identified clock genes function in a double negative feedback loop, with two morning-phased Myb-like transcription factors, CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY), repressing expression of an evening-phased pseudo-response regulator, TIMING OF CAB EXPRESSION 1 (TOC1 or PRR1), which in turn represses expression of CCA1 and LHY (Schaffer et al, 1998; Wang and Tobin, 1998; Strayer et al, 2000; Alabadi et al, 2001; Gendron et al, 2012; Huang et al, 2012; Pokhilko et al, 2012). CCA1 and LHY promote the expression of PRR7 and 9, two day-phased genes, and are in turn repressed by these PRRs and their homolog PRR5, forming another negative feedback circuit (Farre et al, 2005; Nakamichi et al, 2010).
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