Mitochondrial termination factor

Although transcription of the heavy strand of mtDNA initiates from the heavy strand promoter (PH) and proceeds completely around the circular mtDNA molecule, this full-length transcript is only made about 10% of the time. The other 90% of the time a truncated transcript from the heavy strand promoter terminates after the 16S rRNA gene. How does this happen The nucleotides marked with an asterisk in Figure 8-8 form a transcription termination signal that, when bound by a mitochondrial termination factor, causes the mitochondrial RNA polymerase to dissociate from the mtDNA and cease transcription. It is proposed that alterations in the DNA sequence of this tRNA gene therefore not only may affect the role of the tRNA in protein synthesis but... 

Three cytosolic factors were recently characterized that contribute specifically to the maturation of cytosolic and nuclear FeS proteins. Cfdl (Roy et al. 2003) and Nbp35 (Hausmann et al. 2005) are essential soluble P-loop ATPases. Except for a short N-terminal extension in Nbp35 that itself carries an FeS cluster, these two proteins are structurally very similar. Together with the third component, yeast Narl (or human Narf, nuclear prelamin A recognition factor), these proteins have dual nuclear and cytosolic localization. It is not clear what the molecular role these cytosolic factors play in the maturation of FeS proteins is. An attractive hypothesis is that the FeS clusters or their precursors are transferred into the target apoproteins with the assistance of cytosolic factors after their export from mitochondria. Alternatively, the cytosolic proteins may facilitate de novo synthesis of FeS clusters in the cytosol using some compounds which are produced by mitochondrial FeS proteins (Lill and Miihlenhoff 2005). 

Fig. 12.5. Biogenesis and assembly of cytochrome 6-c, complex in the inner mitochondrial membrane. Cytochrome fc-Cj complex contains at least five different subunits COREI (corl), COREII (corll), nonheme iron protein (Fe-S), cytochrome c, (cyt Cj), and cytochrome b (cyt b). Cytochrome f> is a mitochondrial gene product and is probably assembled into the inner membrane (IM) via vectorial translation by mitochondrial ribosomes. The other subunits are synthesized on cytoplasmic ribosomes as larger precursors. The precursors, perhaps in association with a cytoplasmic factor , are attached to receptors on the mitochondrial outer membrane (OM). The complex laterally diffuses to the junctions of the outer and inner membranes, and with the help of a hypothetical translocator the precursors are imported across the membrane. Pre-Corl, pre-Corll, and the pre-nonheme iron protein cross the two membranes, whereas cytochrome c, becomes anchored to the outer face of the inner membrane, facing the intermembrane space (IMS). Cytochrome b is assembled inside the inner membrane, and the nonheme iron protein and Corl and Corll are assembled into the matrix side of the inner membrane. The N-terminal extensions are removed by a soluble matrix protease. The N-terminal extension of cytochrome c, is removed in two steps the first is catalyzed by the matrix protease and the second probably by a protease located on the outer face of the inner membrane.

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