Transcriptional versus non-transcriptional clocks: a case study in Ostreococcus

Circadian rhythms are ubiquitous on earth from cyanobacteria to land plants and animals. Circadian clocks are synchronized to the day/night cycle by environmental factors such as light and temperature. In eukaryotes, clocks rely on complex gene regulatory networks involving transcriptional regulation but also post-transcriptional and post-translational regulations. In multicellular organisms clocks are found at multiple levels from cells to organs and whole organisms, making the study of clock mechanisms more complex. In recent years the picoalga Ostreococcus has emerged as a new circadian model organism thanks to its reduced gene redundancy and its minimalist cellular organization. A simplified version of the "green" plant clock, involving the master clock genes TOC1 and CCA1, has been revealed when the functional genomics and mathematical model approaches were combined. Specific photoreceptors such as a blue light sensing LOV histidine kinase mediate light input to the Ostreococcus clock. Non-transcriptional redox rhythms have also been identified recently in Ostreococcus and human red blood cells. This review highlights the progress made recently in the understanding of circadian clock architecture and function in Ostreococcus in the context of the marine environment.