Angomonas deanei

The body of Angomonas deanei is elliptical in shape, with a prominent tail-like flagellum at its posterior end for locomotion. The bacterial endosymbiont is inside its body and is surrounded by two membranes typical of Gram-negative bacteria, but its cell membrane presents unusual features, such as the presence of phosphatidylcholine, a major membrane lipid (atypical of bacterial membranes), and the highly reduced peptidoglycan layer, which shows reduced or absence of rigid cell wall. The cell membrane of the protozoan host contains an 18-domain β-barrel porin, which is a characteristic protein of Gram-negative bacteria.[9] In addition it contains cardiolipin and phosphatidylcholine as the major phospholipids, while sterols are absent.[10] Cardiolipin is a typical lipid of bacterial membranes; phosphatidylcholine, on the other hand, is mostly present in symbiotic prokaryotes of eukaryotic cells. For symbiotic adaptation, the host trypanosome has undergone alterations such as reduced paraflagellar rod, which is required full motility of the bacterial flagella. Yet the paraflagellar rod gene PFR1 is fully functional.[11] The bacteria are known to provide essential nutrients to the host, and provide electron transport system for the production of cellular energy, the ATP molecules through its glycosomes.[1] The bacteria synthesise amino acids,[12] vitamins,[13] and haem[14] for the protozoan. In return the protozoan offers its enzymes for the complete metabolic pathways for the biosysnthesis of amino acids, lipids and nucleotides, that are absent in the bacterium.[15] Phosphatidylinositol, a membrane lipid required for cell-cell interaction, in the bacteria is also synthesised by the protozoan.[16] Thus the two organisms intimately share and exchange their metabolic systems. When the bacterium is killed using antibiotics, the protozoan can no longer infect insects,[6] due to the altered glycosylphosphatidylinositol (gp63) in the protozoan flagellum.[17]

The cellular reproduction shows a strong synergistic adaptation between the bacterium and the protozoan. As each symbiont is each of a single bacterium and a protozoan, and each daughter cell contains the same number, the two cells divide in a coordinated process. The bacterium divides first, followed by the protozoan organelles, and lastly the nucleus.[18]

Symbiotic bacteria in the trypanosomatid protozoa are descended from a β-proteobacterium of the genus Bordetella.[19] With A. deanei, the bacteria have co-evolved in a mutualistic relationship characterised by intense metabolic exchanges. The endosymbiont contains enzymes and metabolic precursors that complete essential biosynthetic pathways of the host protozoan, such as those in the urea cycle and the production of haemin and polyamine.[20]

The symbiotic bacterium belongs to β-proteobacterium family Alcaligenaceae. Based on the 16S rRNA gene sequences, it is known that it originated from a common ancestor kinetoplastid with Blastocrithidia species. The two groups are assumed to enter two different host protozoans to evolve into different species. Hence the scientific name (Candidatus) Kinetoplastibacterium crithidii was given to the bacterium.[21] Although it was initially proposed that the bacterium evolved from a common ancestor with members of Bordetella,[2] however, detailed phylogenomic analysis revealed that it is more closely related to members of the genus Taylorella.[5]

• Taxonomy at Glycobase
• Protein database and taxonomy at UniProt
• Taxonomic information
• Taxonomic hierarchy at The Taxonomicon
• Genome information at Wellcome Trust Sanger Institute