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Arabidopsis circadian clock protein, TOC1, is a DNA-binding transcription factor.

ABSTRACT: The first described feedback loop of the Arabidopsis circadian clock is based on reciprocal regulation between Timing of CAB Expression 1 (TOC1) and Circadian Clock-associated 1 (CCA1)/late elongated hypocotyl (LHY). CCA1 and LHY are Myb transcription factors that bind directly to the TOC1 promoter to negatively regulate its expression. Conversely, the activity of TOC1 has remained less well characterized. Genetic data support that TOC1 is necessary for the reactivation of CCA1/LHY, but there is little description of its biochemical function. Here we show that TOC1 occupies specific genomic regions in the CCA1 and LHY promoters. Purified TOC1 binds directly to DNA through its CCT domain, which is similar to known DNA-binding domains. Chemical induction and transient overexpression of TOC1 in Arabidopsis seedlings cause repression of CCA1/LHY expression, demonstrating that TOC1 can repress direct targets, and mutation or deletion of the CCT domain prevents this repression showing that DNA-binding is necessary for TOC1 action. Furthermore, we use the Gal4/UAS system in Arabidopsis to show that TOC1 acts as a general transcriptional repressor, and that repression activity is in the pseudoreceiver domain of the protein. To identify the genes regulated by TOC1 on a genomic scale, we couple TOC1 chemical induction with microarray analysis and identify previously unexplored potential TOC1 targets and output pathways. Taken together, these results define a biochemical action for the core clock protein TOC1 and refine our perspective on how plant clocks function.

SUBMITTER: Gendron JM

PROVIDER: S-EPMC3286946 | biostudies-literature | 2012 Feb

REPOSITORIES: biostudies-literature

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Arabidopsis circadian clock protein, TOC1, is a DNA-binding transcription factor.

Gendron Joshua M JM Pruneda-Paz José L JL Doherty Colleen J CJ Gross Andrew M AM Kang S Earl SE Kay Steve A SA

Proceedings of the National Academy of Sciences of the United States of America 20120206 8

The first described feedback loop of the Arabidopsis circadian clock is based on reciprocal regulation between Timing of CAB Expression 1 (TOC1) and Circadian Clock-associated 1 (CCA1)/late elongated hypocotyl (LHY). CCA1 and LHY are Myb transcription factors that bind directly to the TOC1 promoter to negatively regulate its expression. Conversely, the activity of TOC1 has remained less well characterized. Genetic data support that TOC1 is necessary for the reactivation of CCA1/LHY, but there is ...[more]

PMID: 22315425

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Project description:BACKGROUND:24-hour biological clocks are intimately connected to the cellular signalling network, which complicates the analysis of clock mechanisms. The transcriptional regulator TOC1 (TIMING OF CAB EXPRESSION 1) is a founding component of the gene circuit in the plant circadian clock. Recent results show that TOC1 suppresses transcription of multiple target genes within the clock circuit, far beyond its previously-described regulation of the morning transcription factors LHY (LATE ELONGATED HYPOCOTYL) and CCA1 (CIRCADIAN CLOCK ASSOCIATED 1). It is unclear how this pervasive effect of TOC1 affects the dynamics of the clock and its outputs. TOC1 also appears to function in a nested feedback loop that includes signalling by the plant hormone Abscisic Acid (ABA), which is upregulated by abiotic stresses, such as drought. ABA treatments both alter TOC1 levels and affect the clock's timing behaviour. Conversely, the clock rhythmically modulates physiological processes induced by ABA, such as the closing of stomata in the leaf epidermis. In order to understand the dynamics of the clock and its outputs under changing environmental conditions, the reciprocal interactions between the clock and other signalling pathways must be integrated. RESULTS:We extended the mathematical model of the plant clock gene circuit by incorporating the repression of multiple clock genes by TOC1, observed experimentally. The revised model more accurately matches the data on the clock's molecular profiles and timing behaviour, explaining the clock's responses in TOC1 over-expression and toc1 mutant plants. A simplified representation of ABA signalling allowed us to investigate the interactions of ABA and circadian pathways. Increased ABA levels lengthen the free-running period of the clock, consistent with the experimental data. Adding stomatal closure to the model, as a key ABA- and clock-regulated downstream process allowed to describe TOC1 effects on the rhythmic gating of stomatal closure. CONCLUSIONS:The integrated model of the circadian clock circuit and ABA-regulated environmental sensing allowed us to explain multiple experimental observations on the timing and stomatal responses to genetic and environmental perturbations. These results crystallise a new role of TOC1 as an environmental sensor, which both affects the pace of the central oscillator and modulates the kinetics of downstream processes.

Dataset Information

Arabidopsis circadian clock protein, TOC1, is a DNA-binding transcription factor.

Publications

Arabidopsis circadian clock protein, TOC1, is a DNA-binding transcription factor.

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