The physiology of the circadian (daily) clock has been well studied in the unicellular eukaryote Chlamydomonas reinhardtii. Circadian rhythms of phototaxis, chemotaxis, cell division, UV sensitivity, and adherence to glass have been characterized in this green alga. Circadian phototaxis was even shown to operate in outer space! The related phenomenon of photoperiodic time measurement of germination has been demonstrated. The C. reinhardtii system now offers genetic and proteomic opportunities that make it an excellent unicellular eukaryotic model organism to study the circadian clock at all levels of organization. Several clock-controlled genes have been identified as well as a clock-controlled RNA-binding protein that acts on circadian output. A computer-based search in C. reinhardtii for components of the circadian system that are similar to those from other model species has shown that some phototransduction components and especially kinases and phosphatases are well conserved in this green alga, while their target proteins appear to be different. The first functional proteomic approaches have discovered novel components of the circadian system, including a protein disulfide isomerase and a tetratricopeptide repeat protein.

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