Download Mitochondrial tRNA import in the parasitic protozoon

Département de Biologie, Unité de Zoologie
Université de Fribourg (Suisse)
Mitochondrial tRNA import in the parasitic protozoon Trypanosoma brucei and
its consequences on mitochondrial translation
Fabien Charrière
The protozoon parasite Trypanosoma brucei is the causing agent of human sleeping sickness.
Besides its clinical importance T. brucei is also an excellent model for basic research since it has
many unique features. The mitochondrion of T. brucei, for example, lacks tRNA genes. The
tRNAs required for mitochondrial translation are therefore encoded in the nucleus and imported
from the cytosol. Thus, except for the initiator tRNAMet and tRNASec, all trypanosomal tRNAs
function in both the cytosol and the mitochondrion. An important consequence of mitochondrial
tRNA import is that the imported tRNAs are of eukaryotic evolutionary origin. The
mitochondrion however derives from a bacterial ancestor. Thus, we wanted to investigate how the
bacterial-type translation system of the mitochondrion has adapted to eukaryotic-type tRNAs
during evolution.
Due to the exclusive cytosolic localization of the trypanosomal initiator tRNAMet, the only
tRNAMet present in the mitochondrion of T. brucei is the imported eukaryotic elongator tRNAMet.
In bacteria and organelles, the translation initiation process requires the specific formylation of
the initiator methionyl-tRNAMet by the methionyl-tRNA formyltransferase (MTF). The
subsequent binding of the resulting initiator formylmethionyl-tRNAMet to the bacterial-type
initiation factor 2 (IF2) promotes the interaction of the tRNA with the ribosome. In the
mitochondrion of T. brucei a fraction of the imported elongator methionyl-tRNAMet is
unexpectedly formylated by an extraordinary large MTF orthologue. In the present work we
identified the trypanosomal IF2 and we demonstrated that it is required for normal growth of the
parasite. Furthermore, we showed that it recognizes the formylmethionylated imported elongator
tRNAMet, but not its unformylated counterpart. Hence, together with previous studies, this work
emphasizes the dual use of a cytosolic elongator tRNAMet as initiator and elongator in a
mitochondrial translation system.
In order to be used in translation each tRNA needs to be attached to its cognate amino acid. The
process of attachment is called aminoacylation and is catalyzed by aminoacyl-tRNA synthetases.
Since cytosolic and imported tRNAs of T. brucei derive from the same set of nuclear genes they
are expected to be aminoacylated by the same enzymes. In agreement with this hypothesis, most
trypanosomal aminoacyl-tRNA synthetases are represented by single genes. Interestingly
however, the T. brucei genome contains two different genes for eukaryotic tryptophanyl-tRNA
synthetases. We show in this work that both of these enzymes are essential for normal growth.
Furthermore we demonstrate that the unexpected use of a second tryptophanyl-tRNA synthetase
in T. brucei is caused by a mitochondria-specific editing event of the tRNATrp which is required
for the mitochondrial reassignment of the UGA codon to tryptophan.
Jury:
- Prof. André Schneider, Université de Fribourg, Directeur de thèse
- Prof. Patrick Linder, Université de Genève, Expert externe
- Prof. Fritz Müller, Université de Fribourg, Président du Jury