Tetrahymena are free-living ciliate protozoa that can also switch from commensalistic to pathogenic modes of survival. They are common in freshwater ponds. Tetrahymena species used as model organisms in biomedical research are T. thermophila and T. pyriformis.
T. thermophila: a model organism in experimental biology
As a ciliated protozoan, Tetrahymena thermophila exhibits nuclear dimorphism: two types of cell nuclei. They have a bigger, non-germline macronucleus and a small, germline micronucleus in each cell at the same time and they both carry out different functions with distinct cytological and biological properties. This unique versatility allows scientists to use Tetrahymena to identify several key factors regarding gene expression and genome integrity. In addition, Tetrahymena possess hundreds of cilia and has complicated microtubule structures, making it an optimal model to illustrate the diversity and functions of microtubule arrays.
Because Tetrahymena can be grown in a large quantity in the laboratory with ease, it has been a great source for biochemical analysis for years, specifically for enzymatic activities and purification of sub-cellular components. In addition, with the advancement of genetic techniques it has become an excellent model to study the gene function in vivo. The recent sequencing of the macronucleus genome should ensure that Tetrahymena will be continuously used as a model system.
Tetrahymena thermophila exists in 7 different sexes (mating types) that can reproduce in 21 different combinations, and a single tetrahymena cannot reproduce sexually with itself. Each organism "decides" which sex it will become during mating, through a stochastic process.
Studies on Tetrahymena have contributed to several scientific milestones including:
- First cell which showed synchronized division, which led to the first insights into the existence of mechanisms which control the cell cycle.
- Identification and purification of the first cytoskeleton based motor protein such as dynein.
- Aid in the discovery of lysosomes and peroxisomes.
- Early molecular identification of somatic genome rearrangement.
- Discovery of the molecular structure of telomeres, telomerase enzyme, the templating role of telomerase RNA and their roles in cellular senescence and chromosome healing (for which a Nobel Prize was won).
- Nobel Prize–winning co-discovery (1989, in Chemistry) of catalytic ribonucleic acid (ribozyme).
- Discovery of the function of histone acetylation.
- Demonstration of the roles of posttranslational modification such as acetylation and glycylation on tubulins and discovery of the enzymes responsible for some of these modifications (glutamylation)
- Crystal structure of 40S ribosome in complex with its initiation factor eIF1
- First demonstration that two of the "universal" stop codons, UAA and UAG, will code for the amino acid glutamine in some eukaryotes, leaving UGA as the only termination codon in these organisms.
- Discovery of self-splicing RNA.
The life cycle of T. thermophila consists of an alternation of haploid and diploid stages. During vegetative growth diploid cells reproduce by binary fission. Cell division occurs by a sequence of morphogenetic events that result in the development of duplicate sets of cell structures, one for each daughter cell.
As is typical of ciliates, T. thermophila has two functionally differentiated types of nuclei, the micronucleus and macronucleus. The micronucleus is the germline nucleus; i.e., it contains the DNA information passed down from one sexual generation to the next. The micronucleus is diploid and contains five pairs of chromosomes. The genes of the micronucleus are transcriptionally inert during vegetative growth. The macronucleus is the somatic nucleus, i.e. its genes are actively expressed. However, the macronucleus is propagated only during the vegetative part of the life cycle and is not transmitted from one sexual generation to the next. The macronucleus is polyploid and contains 200–300 autonomously replicating DNA species derived from the five micronuclear chromosomes by site-specific fragmentation.
Tetrahymena can be induced to undergo conjugation by washing which causes rapid starvation. When starved, T. thermophila cells change into fast-swimming dispersal forms. When such cells of one mating type encounter cells of a complementary mating type conjugation can occur. During conjugation two cells pair, form a temporary junction and exchange gamete nuclei. They then generate and differentiate the nuclei of their sexual progeny. This process takes about 12 hours. The sequence of events during conjugation is outlined in the accompanying figure.
The Rad51 recombinase of T. thermophila is a homolog of the Escherichia coli RecA recombinase. In T. thermophila, Rad51 participates in homologous recombination during mitosis, meiosis and in the repair of double-strand breaks. During conjugation, Rad51 is necessary for completion of meiosis. Meiosis in T. thermophila appears to employ a Mus81-dependent pathway that does not use a synaptonemal complex and is considered secondary in most other model eukaryotes. This pathway includes the Mus81 resolvase and the Sgs1 helicase. The Sgs1 helicase appears to promote the non-crossover outcome of meiotic recombinational repair of DNA, a pathway that generates little genetic variation.
It is common among protists that the sexual cycle is inducible by stressful conditions such as starvation. Such conditions often cause DNA damage. A central feature of meiosis is homologous recombination between non-sister chromosomes. In T. thermophila this process of meiotic recombination may be beneficial for repairing DNA damages caused by starvation.
Exposure of T. thermophila to UV light resulted in a greater than 100-fold increase in Rad51 gene expression. Treatment with the DNA alkylating agent methyl methane sulfonate also resulted in substantially elevated Rad 51 protein levels. These findings suggest that ciliates such as T. thermophila utilize a Rad51-dependent recombinational pathway to repair damaged DNA.
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- Tetrahymena Stock Center at Cornell University
- ASSET: Advancing Secondary Science Education thru Tetrahymena
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