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le jeudi 19 mai à 11:00

Maria Cristina Machado Motta (Universidade Federal do Rio de Janeiro, Brazil)

Salle de Formation du PRABI

par Vincent Daubin - 19 mai 2011

Endosymbiosis in trypanosomatid protozoa : an eternal love affair

Some trypanosomatids harbor a symbiotic bacterium, which maintains a
close association with the host, constituting an excellent model to study
organelle origin and cellular evolution. Molecular data show that all
endosymbiont containing trypanosomatids are grouped together in a single
phylogenetic branch. Endosymbionts of different species are similar, being
classified in the beta division of Proteobacteria, thus suggesting that a single
evolutionary event gave rise to the symbiosis in the Trypanosomatidae
family. The bacterium is enclosed by two unit membranes and presents a
reduced peptidoglycan layer, which is essential for cell division and
morphological maintenance. Regarding the protein composition, the number
of intramembrane particles in the endosymbiont envelope is similar to that
described for Gram-negative bacteria. Lipid analyses of purified endosymbionts
show absence of sterols and indicate phosphatidylcholine as a major component
of the envelope, as described in other intracellular bacteria. The endosymbiont
promotes ultrastructural and physico-chemical alterations in the trypanosomatid
and its presence influences the protozoan interaction with the insect host and
mammalian cells. Symbiont-containing trypanosomatids are able to infect and
to replicate inside fibroblasts and macrophages, whose microbicidal activity was
deactivated by HIV-1 infection. The symbiosis in trypanosomatids is characterized
by intensive metabolic exchanges ; the bacterium contains enzymes and
metabolic precursors that complete essential biosynthetic pathways of the
protozoan. Conversely, the symbiont is capable of obtaining part of the required
energetic molecules from the host glycosomes. Taken together, data suggest that
the endosymbiont in trypanosomatids represents an intermediate evolutionary
step between bacteria and eukaryotic cell organelles..