Clocks in the green lineage: comparative functional analysis of the circadian architecture of the picoeukaryote ostreococcus.
Biological rhythms that allow organisms to adapt to the solar cycle are generated by endogenous circadian clocks. In higher plants, many clock components have been identified and cellular rhythmicity is thought to be driven by a complex transcriptional feedback circuitry. In the small genome of the green unicellular alga Ostreococcus tauri, two of the master clock genes Timing of Cab expression1 (TOC1) and Circadian Clock-Associated1 (CCA1) appear to be conserved, but others like Gigantea or Early-Flowering4 are lacking. Stably transformed luciferase reporter lines and tools for gene functional analysis were therefore developed to characterize clock gene function in this simple eukaryotic system. This approach revealed several features that are comparable to those in higher plants, including the circadian regulation of TOC1, CCA1, and the output gene Chlorophyll a/b Binding under constant light, the relative phases of TOC1/CCA1 expression under light/dark cycles, arrhythmic overexpression phenotypes under constant light, the binding of CCA1 to a conserved evening element in the TOC1 promoter, as well as the requirement of the evening element for circadian regulation of TOC1 promoter activity. Functional analysis supports TOC1 playing a central role in the clock, but repression of CCA1 had no effect on clock function in constant light, arguing against a simple TOC1 /CCA1 one-loop clock in Ostreococcus. The emergence of functional genomics in a simple green cell with a small genome may facilitate increased understanding of how complex cellular processes such as the circadian clock have evolved in plants.
Corellou
Florence
F
University Pierre and Marie Curie Paris 06, Laboratoire d'Océanographie Microbiene, Observatoire Océanologique, F-66651 Banyuls/Mer, France. corellou@obs-banyuls.fr
Schwartz
Christian
C
Motta
Jean-Paul
JP
Djouani-Tahri
El Batoul
el B
Sanchez
Frédéric
F
Bouget
François-Yves
FY
D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism; D056926:CLOCK Proteins / Q000235:genetics* / Q000378:metabolism*; D000460:Chlorophyta / Q000235:genetics* / Q000378:metabolism* / Q000528:radiation effects; D002940:Circadian Rhythm / Q000235:genetics*; D019143:Evolution, Molecular; D018506:Gene Expression Regulation, Plant / Q000235:genetics*; D018745:Genome, Plant / Q000235:genetics; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000235:genetics / Q000378:metabolism; D008954:Models, Biological; D008969:Molecular Sequence Data; D010775:Photic Stimulation; D045332:Photosystem II Protein Complex / Q000235:genetics / Q000378:metabolism; D010802:Phylogeny; D011401:Promoter Regions, Genetic / Q000235:genetics; D011485:Protein Binding / Q000235:genetics; D013045:Species Specificity; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic / Q000235:genetics
D029681:Arabidopsis Proteins; C490217:CAB1 protein, Arabidopsis; C114425:CCA1 protein, Arabidopsis; D045342:Light-Harvesting Protein Complexes; D045332:Photosystem II Protein Complex; C411822:TOC1 protein, Arabidopsis; D014157:Transcription Factors; D056926:CLOCK Proteins