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Tetrahymena are free-living protozoa that can also switch from commensalistic to pathogenic modes of survival. They are common in fresh-water. Tetrahymena species used as model organisms in biomedical research are T. thermophila and T. pyriformis.^1]

Main fields of biomedical research where Tetrahymena cells are used as models

t. Thermophila: a Model Organism in Experimental Biology

As a ciliated protozoan, Tetrahymena thermophila exhibits striking nuclear dimorphism: two types of cell nuclei, a large, somatic macronucleus and a small, germline micronucleus, exist in a single cell at the same time and carry out different functions with distinct cytological and biochemical properties. This unique versatility allows scientists to use Tetrahymena to identify several key factors regarding gene expression and genome integrity. In addition, Tetrahymena possesses hundreds of cilia and has complicated microtubule structures, making it also an ideal model to elucidate the diversity and functions of microtubule systems .

Since Tetrahymena can be easily cultured in a large quantity in the laboratory, for years it has been a great source for biochemical analysis of important enzymatic activities and for purification of sub-cellular components. In addition, advanced molecular genetic techniques have been developed, including DNA-mediated transformation, gene 'knock-out' and 'knock-in' by homologous recombination, epitope tagging and inducible/repressible gene expression, making it an excellent model to study the gene function in vivo. Recently, the whole macronuclear genome has been sequenced, which should promise Tetrahymena to be continuously utilized as a model system in the genomic and post-genomic era. Studies on Tetrahymena have contributed to several scientific milestones:

1. First cell whose division was synchronized , leading to the first insights into the existence of cell cycle control mechanisms.
2. Identification and purification of the first cytoskeleton motor protein, i.e., dynein and determination of its directional activity.
3. Participation in the discovery of lysosomes and peroxisomes.
4. One of earliest molecular descriptions of somatic genome rearrangement.
5. Discovery of the molecular structure of telomeres , telomerase enzyme , the templating role of telomerase RNA and their roles in cellular senescence and chromosome healing.
6. Nobel-prize winning co-discovery of catalytic RNA (ribozyme);
7. Discovery of the function of histone acetylation.
8. Discovery of the roles of RNA interference-like pathway in the heterochromatin formation
9. Demonstration of the physiological roles of the posttranslational modification (i.e. acetylation and glycylation) on tubulins and identification of the enzymes responsible for some of the modifications (glutamylation).

(1 to 7 were adapted from Tetrahymena Genome Sequencing White Paper)