Chlamydomonas reinhardtii is a unicellular green alga with a size of about 10 micrometers and is found all over the world. It serves as a simple model organism for the investigation of photosynthesis and processing of light signals. With the help of two flagella it reacts to the light conditions of the environment (phototaxis). It uses a biological clock for daily time keeping, which is synchronized by light (circadian rhythm). To investigate the entrainment of this rhythm Chlamydomonas has already been sent to space on the Space Shuttle.

Photoreceptors

Typical for a member of the plant family, Chlamydomonas contains phototropin und cryptochrome. However, it is lacking the whole family of the red light sensitive phytochromes. Additionally, rhodopsins and cryptochromes with similarity to animal receptors are found in the green alga and even a class of rhodopsins with homology to archaebacterial receptors. The latter include the channelrhodopsins, which are responsible for phototaxis. Phototropin plays a role in the process of switching from vegetative to sexual reproduction. The function of the other receptors is not known yet.

The green alga offers the opportunity to study the mechanism of almost all common photoreceptors within a comparatively simple organism. For our biophysical investigations, we overproduce the photoreceptors or parts thereof in larger amounts in E. coli (such as the cryptochrome, see J. Biol. Chem. 2007). From similarities or differences in the properties of the Chlamydomonas photoreceptors as compared to those of higher plants, one can draw conclusions on the relevant processes and observe evolutionary developments.

Biotechnology and Medicine

-Genetically modified Chlamydomonas cells produce hydrogen or isoprene upon illumination under suitable conditions. They are candidates for a direct, environmentally sound conversion of solar energy into fuel.
-The photoreceptor channel rhodopsin from Chlamydomonas is applied to artificially activate nerve cells by light. This approach is used to study a whole variety of signaling processes in different organisms.
- The mechanism behind one of the most common human genetic diseases, polycystic kidney disease, and many other diseases has been investigated in the model system Chlamydomonas. The flagella of the alga have much in common with the primary cilium, which plays an important role in most of our cells.

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