Unique aspects of mitochondrial biogenesis in Trypanosoma brucei

• Mitochondrial morphology: T. brucei has a single mitochondrion only. Unlike the very dynamic mitochondria of mammalian cells it does not undergo any fission or fusion events throughout the cell cycle. The only mitochondrial fission event that occurs in T. brucei is highly regulated. It happens prior to completion of cytokinesis when the mitochondrion must divide in two in order to allow its transmission to the daughter cells.
• Mitochondrial genome organization: The organellar DNA of trypanosomatids is organized in two classes of circular molecules of different sizes, the maxi- and the minicircles. Maxi- and minicircles are topologically interlocked both among themselves as well as between each other forming a huge network (termed kDNA) that is unique in nature. The maxicircles are structurally and functionally analogous to the mitochondrial DNA of other organsims. Minicircles are heterogenous in sequence and encode gRNAs (see RNA editing).
• Localization of kDNA: The single kDNA network is physically connected to the basal body of the flagellum. A set of intramitochondrial filaments links the kDNA to the inner face of a differentiated zone of outer and inner mitochondrial membranes to which the basal body is connected to from the outside by another set of filaments.
• Replication of k-DNA: Unlike in other eukaryotes T. brucei has a mitochondrial S-Phase. Thus the k-DNA is replicated at a precise point of the cell cylce prior to the nuclear S-Phase
• RNA editing: Some of the maxicircle-encoded genes are cryptogenes meaning that their transcripts have to be remodelled by gRNA-mediated RNA editing in order to be converted into translatable substrates.
• Mitochondrial ribosomes: The composition of the mitochondrial ribosomes is unusual since they have among the shortest rRNAs of any known translation system.
• Mitochondrial tRNA import: The mitochondrial genome of T. brucei does not encode any tRNAs, thus all tRNAs required for mitochondrial translation need to be imported from the cytosol.
• Mitochondrial protein import: A survey of the genome suggests that T. brucei has a minimized mitochondrial protein import system. Moreover the mitochondrial targeting sequences in T. brucei are unusually short.
• Stage specific regulation of mitochondrial function: Many mitochondrial proteins are stage specifically regulated. Thus, all proteins required for oxidative phosphorylation and the citric acid cycle are only expressed in the procyclic stage of T. brucei.