In vitro reconstruction of the mitochondrial translation system of yeast

Mitochondrial metabolomics using high-resolution Fourier-transform mass spectrometry

High-resolution Fourier-transform mass spectrometry (FTMS) provides important advantages in studies of metabolism because more than half of common intermediary metabolites can be measured in 10 min with minimal pre-detector separation and without ion dissociation. This capability allows unprecedented opportunity to study complex metabolic systems, such as mitochondria. Analysis of mouse liver mitochondria using FTMS with liquid chromatography shows that sex and genotypic differences in mitochondrial metabolism can be readily distinguished. Additionally, differences in mitochondrial function are readily measured, and many of the mitochondria-related metabolites are also measurable in plasma. Thus, application of high-resolution mass spectrometry provides an approach for integrated studies of complex metabolic processes of mitochondrial function and dysfunction in disease.

NCA3, a nuclear gene involved in the mitochondrial expression of subunits 6 and 8 of the Fo-F1 ATP synthase of S. cerevisiae

Respiratory-competent nuclear mutants have been isolated which presented a cryosensitive phenotype on a non-fermentative carbon source, due to a dysfunctioning of the mitochondrial F1-Fo ATP synthase which results from a relative defect in subunits 6 and 8 of the Fo sector. Both proteins are mtDNA-encoded, but the defect is due to the simultaneous presence of a mutation in two unlinked nuclear genes (NCA2 and NCA3, for Nuclear Control of ATPase) promoting a modification of the expression of the ATP8-ATP6 co-transcript (formerly denoted AAP1-OLI2). This co-transcript matures at a unique site to give two cotranscripts of 5.2 and 4.6 kb in length: in the mutant, the 5.2-kb co-transcript was greatly lowered. NCA3 was isolated from a wild-type yeast genomic library by genetic complementation. The level of the 5.2-kb transcript, like the synthesis of subunits 6 and 8, was partly restored in the transformed strain. A 1011-nucleotide ORF was identified that encodes an hydrophilic protein of 35417 Da. Disruption of chromosomal DNA within the reading frame promoted a dramatic decrease of the 5.2-kb mRNA but did not abolish the respiratory competence of a wild-type strain. NCA3 is located on chromosome IV and produces a single 1780-b transcript.

In-vitro translation of mitochondrial mRNAs by yeast mitochondrial ribosomes is hampered by the lack of start-codon recognition

In an attempt to reconstitute an homologous in-vitro translation system for yeast mitochondrial mRNAs, we have isolated ribosomes, supernatant factors, and tRNAs from mitochondria of Saccharomyces carlsbergensis. While poly(U) is translated faithfully in this system, no translation of in-vitro synthesised cytochrome c oxidase subunit II (COX2) mRNA could be detected. Formation of formylmethionyl-puromycin on mitochondrial ribosomes is stimulated by ApUpG, but not by COX2 mRNA, although mitochondrial small ribosomal subunits bind to this mRNA in vitro, even without added tRNA and initiation factors. We conclude, therefore, that the inability to faithfully translate mitochondrial mRNAs in vitro may be the result of an inability of mitochondrial ribosomes to recognize the initiation codon.

Ser-tRNAs from bovine mitochondrion form ternary complexes with bacterial elongation factor Tu and GTP

Transfer ribonucleic acids were isolated from mitochondria of bovine heart and aminoacylated in vitro by a crude mitochondrial enzyme. Ser-tRNASerUCN and Ser-tRNASerAGY were isolated and characterized by partial sequencing. Although these tRNAs possess unique structural features not found in any bacterial tRNA, they form a ternary complex with elongation factor from the extreme thermophilic bacterium Thermus thermophilus and GTP.

Kinetoplast DNA segments function as promoters in Escherichia coli cells

By means of coupled transcription-translation in Escherichia coli cell-free system, an open reading frame was found in the Crithidia oncopelti maxi-circle kDNA segment cloned in the hybrid plasmid pCo1. Subfragments from this region were tested for their ability to function as promoters in E. coli cells. For this purpose the vector pVE8 was constructed using the aminoglycoside phosphotransferase II (APTII) gene from the Tn5 transposon, and the pHC79 cosmid. After cloning of Sau3A fragments of pCo1 by insertion into pVE8 three types of plasmids containing promoter sequences were obtained. Two of these plasmids displayed promoter strength in E. coli cells greater than that of the normal promoter of the APTII gene. However the promoters found are not necessary for the coupled transcription-translation of kDNA in the E. coli cell-free system.