title: Multiple light inputs to a simple clock circuit allow complex biological rhythms.
abstract: Circadian clocks are biological timekeepers that allow living cells to time their activity in anticipation of predictable environmental changes. Detailed understanding of the circadian network of higher plants, such as Arabidopsis thaliana, is hampered by the high number of partially redundant genes. However, the picoeukaryotic alga Ostreococcus tauri, which was recently shown to possess a small number of non-redundant clock genes, presents an attractive alternative target for detailed modelling of circadian clocks in the green lineage. Based on extensive time-series data from in vivo reporter gene assays, we developed a model of the Ostreococcus clock as a feedback loop between the genes TOC1 and CCA1. The model reproduces the dynamics of the transcriptional and translational reporters over a range of photoperiods. Surprisingly, the model is also able to predict the transient behaviour of the clock when the light conditions are altered. Despite the apparent simplicity of the clock circuit, it displays considerable complexity in its response to changing light conditions. Systematic screening of the effects of altered day length revealed a complex relationship between phase and photoperiod, which is also captured by the model. The complex light response is shown to stem from circadian gating of light-dependent mechanisms. This study provides insights into the contributions of light inputs to the Ostreococcus clock. The model suggests that a high number of light-dependent reactions are important for flexible timing in a circadian clock with only one feedback loop.
authors.affiliation: School of Biological Sciences, University of Edinburgh and Centre for Systems Biology at Edinburgh, Edinburgh EH93JD, UK.
authors.forename: Carl
authors.initials: C
authors.lastname: Troein
authors.forename: Florence
authors.initials: F
authors.lastname: Corellou
authors.forename: Laura E
authors.initials: LE
authors.lastname: Dixon
authors.forename: Gerben
authors.initials: G
authors.lastname: van Ooijen
authors.forename: John S
authors.initials: JS
authors.lastname: O'Neill
authors.forename: François-Yves
authors.initials: FY
authors.lastname: Bouget
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Millar
mesh_terms: D056926:CLOCK Proteins / Q000235:genetics / Q000528:radiation effects*; D000460:Chlorophyta / Q000235:genetics / Q000502:physiology / Q000528:radiation effects*; D057906:Circadian Clocks; D018506:Gene Expression Regulation, Plant; D008027:Light; D008954:Models, Biological; D017440:Photoperiod; D010940:Plant Proteins / Q000235:genetics / Q000528:radiation effects*; D014157:Transcription Factors / Q000378:metabolism
chemical_list: D010940:Plant Proteins; D014157:Transcription Factors; D056926:CLOCK Proteins