Kinetoplastid Protozoa and Trypanosomes

 

The kinetoplastid protozoa comprise a large group of flagellated organisms that are thought to have had a common ancestor with the Euglenoid protozoa early in evolution of eukaryotic cells. There is some evidence that the common ancestor contained a plastid derived from endosymbiosis of an algae, which was secondarily lost in the kinetoplastid lineage but some of the genes of which were transferred to the nucleus.

 

The common taxonomic feature of these organisms is a single mitochondrion with one or more masses of mitochondrial DNA. The region of the mitochondrion containing the mitochondrial DNA is termed the "kinetoplast" and the DNA is termed "kinetoplast DNA". There are two major groups of kinetoplastid protists - the bodonids and the trypanomatids. The trypanosomatids are probably monophyletic and consist of all the well studied monogenetic genera such as Crithidia, Leptomonas, and Blastocrithidia, and the digenetic genera such as Leishmania and New World and Old World Trypanosoma. The bodonids are poorly studied due to the inability to grow in axenic culture (with the exception of Trypanoplasma borrelli), and are probably paraphyletic. A phylogenetic tree based on rRNA sequences is shown below, together with an indication of the type of mitochondrial DNA molecules in the kinetoplast.

 

The term, trypanosome, is used both as a general designation of a trypanosomatid protist and also for species in the genus, Trypanosoma. All of the medically important parasites belong to the Leishmania and Trypanosoma genera.

Trypanosomes as Parasites

Several digenetic trypanosomatids are the causal agents of several animal and human and even plant diseases. Leishmania species cause both old and new world visceral leishmaniasis and cutaneous leishmaniasis. Trypanosoma species cause Chagas disease in South America and Sleeping Sickness in Africa. Phytomonas species cause diseases in plants. Most of the initial interest in these cells was due to their biomedical importance as parasites.

Trypanosomes as Model Systems

Due to the ease of axenic culture and the abundance of reverse genetic techniques currently available, the trypanosomes have provided model systems to study several basic problems in cell and molecular biology. In addition, a number of novel phenomena have been discovered in these cells. These include addition of the spliced leader or miniexon sequence to the 5' end of all mRNAs by trans-splicing, GPI-anchored surface proteins, localization of all glycolytic enzymes in a peroxysomal like organelle (the glycosome), polycistronic chromosomal transcription units, chromosomal regulation of expression of surface glycoproteins in the African trypanosomes, uridine insertion/deletion RNA editing in the mitochondrion, and organization of mitochondrial DNA into maxicircle and minicircle genomes. There are also several ongoing genome projects on trypanosomes.

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