trans

280 PLASTIDS, MITOCHONDRIA, AND HYDROGENOSOMES
FIGURE 12.1 Various forms of the mitochondrion in parasites. (A) Mitochondrion (Mi) from Leishmania mexicana
showing a conspicuous kinetoplast of concatenated DNA minicircles. (B) Mitochondrion (Mi) of Toxoplasma
gondii showing double membrane and tubular cristae (arrows). (C) Hydrogenosome (Hy) of Trichomonas vaginalis
showing double bounding membrane and amorphous contents. (D) Mitochondrion (Mi) of Plasmodium falciparum
gametocyte showing tubular cristae (arrows) and unidentified dark bodies. Scale bars 200 nm.
around the nucleus. Thus, the nucleus-encoded
mRNAs for mitochondrial proteins cannot
enter the mitochondrion but must be translated
on ribosomes in the cytoplasm. The gene
products are then specifically targeted into the
organelle by an N-terminal extension that is
recognized by complex machinery on the surface
of the mitochondrion. Only proteins
bearing these N-terminal extensions, known as
transit peptides, are targeted into the organelle.
Once established, this targeting mechanism
facilitated the ongoing relocation of genes from
organelle to host genome. Each transferred
gene needs to acquire one of these transit peptides
before the product can be returned to
the organelle. Acquisition of a transit peptide
is probably expedited by the fact that transit
peptides have a fairly loose set of parameters
to function as targeting motifs.
MITOCHONDRIAL
METABOLISM
The canonical function of mitochondria is
the oxidation of pyruvate to CO 2 and H 2 O.
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA