Robustness of Circadian Clocks to Daylight Fluctuations: Hints from the Picoeucaryote Ostreococcus tauri

Abstract

The development of systemic approaches in biology has put emphasis on identifying genetic modules whose behavior can be modeled accurately so as to gain insight into their structure and function. However, most gene circuits in a cell are under control of external signals and thus, quantitative agreement between experimental data and a mathematical model is difficult. Circadian biology has been one notable exception: quantitative models of the internal clock that orchestrates biological processes over the 24-hour diurnal cycle have been constructed for a few organisms, from cyanobacteria to plants and mammals. In most cases, a complex architecture with interlocked feedback loops has been evidenced. Here we present the first modeling results for the circadian clock of the green unicellular alga Ostreococcus tauri. Two plant-like clock genes have been shown to play a central role in the Ostreococcus clock. We find that their expression time profiles can be accurately reproduced by a minimal model of a two-gene transcriptional feedback loop. Remarkably, best adjustment of data recorded under light/dark alternation is obtained when assuming that the oscillator is not coupled to the diurnal cycle. This suggests that coupling to light is confined to specific time intervals and has no dynamical effect when the oscillator is entrained by the diurnal cycle. This intringuing property may reflect a strategy to minimize the impact of fluctuations in daylight intensity on the core circadian oscillator, a type of perturbation that has been rarely considered when assessing the robustness of circadian clocks.

Tandem Repeats, High Copy Number and Remarkable Diel Expression Rhythm of Form II RuBisCO in Prorocentrum donghaiense (Dinophyceae)

“…As rhythms under circadian clock control usually would persist at least 2–3 days after the LD to DD shift [44], [45], we would expect that the diel rhythm of Pdrbc would persist for at least one day after the LD-DD shift. In our case, the cultures shifted to DD showed strong cell cycle arrest in the G1 phase (>96.8% cells in G1 phase throughout the sampling period), and the expression level of Pdrbc showed no remarkable changes.…”

Competition for Catalytic Resources Alters Biological Network Dynamics

“…For example, the limited lifetime of proteins, which, from a productive point of view, may be seen only as a deleterious side effect (because it hampers efficient production), becomes a crucial cornerstone for the global functioning of, e.g., a genetically regulated oscillator [8]: in this case, the dynamic decay of proteins becomes compulsory in order to let the system cycle back to its initial state. The importance of controlling such destructive processes has been recognized in the early days of synthetic biology, and proteins were explicitly modified to accelerate their enzymatic degradation [5].…”

Velocity response curves demonstrate the complexity of modeling entrainable clocks

“…We also know that for entrainment to be robust, the deadzone is critical (Pfeuty et al, 2011, 2012). However, if a model accurately captures all clock processes affected by light, and is merely missing a gate, then it should be possible to entrain it with a light/dark cycle that itself gates the light, as in the work of Thommen et al (2010). The gated light signal has a so-called skeleton photoperiod.…”