Author: Tang Cristina
Institution: Biochemistry
Date: April 2002
Under the bright sunlight, tiny, green creatures move gracefully in their pond of water. With two long antennae' leading the way, these creatures seem to be swimming about randomly, looking as if they are trying to position themselves in the right place to receive signals from outer space. Despite their appearances, these small creatures are not the classical science-fiction inhabitants of Mars, or "E.T." lost on Earth. They have lived on Earth for millions of years. They are Chlamydomonas.
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Chlamydomonas reinhardtii are unicellular eukaryotes that use two flagella for motility. They can be easily grown at room temperature on agar plates or in liquid culture. Though not as well known as fruit flies or mice, they are some of the most popular organisms in laboratory science today. Traits that make Chlamydomonas a popular organism for research include a well-defined cell cycle and a single set of chromosomes. In diploid organisms, which have two sets of chromosomes, a normal gene can sometimes mask a mutation. For example, a person with a gene defective for the utilization of the sugar fructose may not show fructose intolerance because he/she still has a good' copy of the gene. But since Chlamydomonas, which is haploid, only has a single copy of each gene, mutations cannot be masked. This substantially facilitates the isolation of Chlamydomonas that have mutations on interesting genes, such as those that have human counterparts or genes encoding a protein with a critical function in the cell. It is not surprising then that in recent years the value of Chlamydomonas in research has soared rapidly and has gone from obscure algae taxonomy books to the papers published in today's most prestigious scientific journals.
Dr. Lynne Quarmby, organizer of this year's International Conference on the Cell and Molecular Biology of Chlamydomonas in Vancouver, Canada, has investigated several features of this organism, such as deflagellation and centrosome function, for nearly 15 years. In her eyes, "chlamys," as she calls them, are truly wonderful organisms for their usefulness in the study of many biological problems. As a flagellated, haploid organism, Chlamydomonas is especially suited for the study of basal bodies - cylindrical organelles - and of flagellar assembly and function. The walls of basal bodies are composed of nine sets of triplet microtubules, and are involved in the formation of cilia and flagella. Defects in the proper assembly and movement of human cilia and/or flagella have been linked to several human diseases. For example, the cilia found on the linings of the respiratory tract are responsible for removing the dust and dirt from the air when we breathe. Complications in the respiratory system may arise if these cilia are absent or immobile. Scientists therefore believe it is very important to understand the formation, assembly, and normal function of basal bodies, cilia, and flagella.
Studies of Chlamydomonas with defects in flagellar assembly or movement have generated a wealth of information about basal bodies and cilia/flagella by identifying key components of these organelles.The protein, d-tubulin, for example, has been discovered in Chlamydomonas that produce a defective d-tubulin due to mutations in the UNI3 gene. Whereas wild-type Chlamydomonas are biflagellated, Chlamydomonas with defective d-tubulin grow zero or only one flagella. These and other observations allowed researchers to conclude that in normal Chlamydomonas, the d-tubulin protein was required for the normal assembly of basal bodies, the organelles that are essential for flagellar formation.
In addition, the recent discovery of a non-neuronal transport system in Chlamydomonas by the Rosenbaum Lab at Yale University has further contributed to our understanding of flagellar assembly. This transport pathway uses a protein called kinesin II to carry key flagellar components from the cell body where they are synthesized to the ends of the flagellum for assembly. Therefore, defects in kinesin II impair the proper assembly of cilia and flagella which affect their growth and movement. Furthermore, kinesin II has also been found in non-motile cilia present in the photoreceptor or light sensing cells of our eyes where they transport a substantial amount of material between the two segments of the cell through the so-called connecting cilium. Failure to transport the molecules across the cilium leads to death of the photoreceptor cell. Chlamydomonas studies have not yet produced cures for any of the diseases linked to defective cilia or flagella, such as polycystic kidney disease or male infertility caused by sperm flagellar motility defects. However, these studies have given us a better insight into the causes of these illnesses.
Chlamydomonas are also good for surprising us with novel biological phenomena, which takes them beyond merely being useful for studying fundamental biological processes. It has been found, for example, that they can be used to generate hydrogen from light, water, and basic nutrients. Normally, the production of hydrogen gas in Chlamydomonas is limited in the presence of oxygen. It has been discovered, however, that Chlamydomonas can produce significant amounts of hydrogen if they are starved from sulfur for two days. Further research in this area is of interest because it holds the possibility of generating large quantities of hydrogen, which is a renewable fuel, from cheap and abundant sources such as light and water.
To review research on Chlamydomonas presented in 2000 in the Netherlands, please refer to http://quarmby.ca/chlamy2002/review.pdf.
Suggested Reading
Dutcher, S., and E.C. Trabuco. "The UNI3 Gene is required for assembly of basal bodies of Chlamydomonas and encodes d-tubulin, a new member of the tubulin superfamily." Mol. Biol. Cell, 9(6) (1998), 1293-1308.
Rosenbaum, J.L., et.al "Intraflagellar transport: The eyes have it." J. Cell Biol. 144(1999), 385-388.
Beech, P.L., et.al "Localization of kinesin superfamily proteins to the connecting cilium of fish photoreceptors." J. Cell Sci. 109(1996), 889-897.
Muresan, V., et.al "The kinesin Motor KIF3A is a component of the presynaptic ribbon in vertebrate photoreceptors." J. Neurosc. 19(1999), 1027-1037.
Sung, C. H., et.al "A rhodopsin gene mutation responsible for autosomal dominant retinitis pigmentosa results in a protein that is defective in localization to the photoreceptor outer segment." J. Neurosci. 14(1994), 5818-5833.
Gfeller, R. P., et.al "Fermentative metabolism of Chlamydomonas reinhardtii: I. Analysis of fermentative products from starch in dark and light." Plant Physiol. 75(1984), 212-218.
Melis, A., et.al "Sustained photobiological hydrogen gas production upon reversible inactivation of oxygen evolution in the green alga Chlamydomonas reinhardtii." Plant Physiol. 122(2000), 127-136.