New insights into the circadian clock in Chlamydomonas.

The unicellular green alga Chlamydomonas reinhardtii has long been used in research on circadian rhythm. Various circadian rhythms in behavior and physiology including phototaxis, chemotaxis, the cell division cycle, UV sensitivity, stickiness to glass (changes in properties of the cell surface), and starch content are observed in this alga. Soon after the isolation of clock mutants in Drosophila melanogaster and Neurospora crassa, clock mutants were also isolated in Chlamydomonas. Whereas genes responsible (clock genes) in Drosophila and Neurospora mutants have been identified and these organisms have become important models for understanding the circadian clock, Chlamydomonas clock genes have not been identified and the molecular basis of the algal clock was unclear until a few years ago. Recently, reverse and forward genetic studies revealed several clock genes in Chlamydomonas. These studies unveiled that the Chlamydomonas clock consists of both original and plant-like components. Further study of the Chlamydomonas circadian clock should provide new insights into the evolution of plant clocks. Furthermore, the identification of its clock genes makes Chlamydomonas a new model for molecular studies of the circadian oscillatory mechanisms in eukaryotes. Its simple cellular architecture may provide significant advantages for a comprehensive understanding of the intracellular temporal organization, especially between the nucleus and chloroplast.