The cytochrome oxidase subunit I gene of Tetrahymena: a 57 amino acid NH2-terminal extension and a 108 amino acid insert

Spatial genetic variation, phylogeography and barcoding of the peritrichous ciliate Carchesium polypinum

Most recent studies of geographic distribution of microbial eukaryotes have focused on marine rather than freshwater protists. Here, we used the freshwater peritrich ciliate Carchesium polypinum to quantify the degree of genetic diversity of four closely related and previously described lineages and to determine whether patterns of genetic differentiation showed geographic partitioning. Using an expanded dataset of 100 isolates and employing the mitochondrial marker cytochrome oxidase c subunit I (cox-1), we enriched the 6 previously identified clades of Carchesium polypinum. We found a large degree of geographic overlap among the different clades (e.g. to the level of range of sampling), but also a spatially restricted clade (e.g. to the level of one river basin). Furthermore, we present evidence of a clear geographic separation in one of the lineages with Canadian and North Carolinian isolates grouping in two distinct clusters.

Dinoflagellates: a mitochondrial genome all at sea

Dinoflagellate algae are notorious for their highly unusual organization of nuclear and chloroplast genomes. Early studies on the dinoflagellate mitochondrial genome indicated that it encodes the same three protein-coding genes found in Plasmodium spp., but with a complex organization and transcript editing. Recent work has extended this view, showing that the dinoflagellate mitochondrial genome contains a wide array of gene fragments and genes interspersed with noncoding inverted repeats. The genome seems to require noncanonical start and stop codons, as well as high levels of editing, trans-splicing and the addition of oligonucleotide caps at the 5' and 3' ends of transcripts. Despite its small coding content, the dinoflagellate mitochondrial genome is one of the most complex known.

Barcoding ciliates: a comprehensive study of 75 isolates of the genus Tetrahymena

The mitochondrial cytochrome-coxidase subunit 1 (cox1) gene has been proposed as a DNA barcode to identify animal species. To test the applicability of thecox1gene in identifying ciliates, 75 isolates of the genusTetrahymenaand three non-Tetrahymenaciliates that are close relatives ofTetrahymena,Colpidium campylum,Colpidium colpodaandGlaucoma chattoni, were selected. All tetrahymenines of unproblematic species could be identified to the species level using 689 bp of thecox1sequence, with about 11 % interspecific sequence divergence. Intraspecific isolates ofTetrahymena borealis,Tetrahymena lwoffi,Tetrahymena patulaandTetrahymena thermophilacould be identified by theircox1sequences, showing <0.65 % intraspecific sequence divergence. In addition, isolates of these species were clustered together on acox1neighbour-joining (NJ) tree. However, strains identified asTetrahymena pyriformisandTetrahymena tropicalisshowed high intraspecific sequence divergence values of 5.01 and 9.07 %, respectively, and did not cluster together on acox1NJ tree. This may indicate the presence of cryptic species. The mean interspecific sequence divergence ofTetrahymenawas about 11 times greater than the mean intraspecific sequence divergence, and this increased to 58 times when all isolates of species with high intraspecific sequence divergence were excluded. This result is similar to DNA barcoding studies on animals, indicating that congeneric sequence divergences are an order of magnitude greater than conspecific sequence divergences. Our analysis also demonstrated low sequence divergences of <1.0 % between some isolates ofT. pyriformisandTetrahymena setosaon the one hand and some isolates ofTetrahymena furgasoniandT. lwoffion the other, suggesting that the latter species in each pair is a junior synonym of the former. Overall, our study demonstrates the feasibility of using the mitochondrialcox1gene as a taxonomic marker for ‘barcoding’ and identifyingTetrahymenaspecies and some other ciliated protists.

RNA delivery into mitochondria

Mitochondria, though containing their own genome, import the vast majority of their macromolecular components from the cytoplasm. If the mechanisms of pre-protein import are well understood, the import of nuclear-coded RNAs into mitochondria was investigated to a much lesser extent. This targeting, if not universal, is widely spread among species. The origin and the mechanisms of RNA import seem to differ from one system to another and striking differences are observed even in closely related species. We describe data concerning the various experimental systems of studying RNA import with emphasis on the model of the yeast Saccharomyces cerevisiae, which was studied in our laboratory. We compare various requirements of RNA import into mitochondria in different species and demonstrate that this pathway can be transferred from yeast to human cells, in which tRNAs normally are not imported. We speculate on the possibility to use RNA import for biomedical purposes.

Complete sequence of the mitochondrial genome of Tetrahymena pyriformis and comparison with Paramecium aurelia mitochondrial DNA

We report the complete nucleotide sequence of the Tetrahymena pyriformis mitochondrial genome and a comparison of its gene content and organization with that of Paramecium aurelia mtDNA. T. pyriformis mtDNA is a linear molecule of 47,172 bp (78.7 % A+T) excluding telomeric sequences (identical tandem repeats of 31 bp at each end of the genome). In addition to genes encoding the previously described bipartite small and large subunit rRNAs, the T. pyriformis mitochondrial genome contains 21 protein-coding genes that are clearly homologous to genes of defined function in other mtDNAs, including one (yejR) that specifies a component of a cytochrome c biogenesis pathway. As well, T. pyriformis mtDNA contains 22 open reading frames of unknown function larger than 60 codons, potentially specifying proteins ranging in size from 74 to 1386 amino acid residues. A total of 13 of these open reading frames ("ciliate-specific") are found in P. aurelia mtDNA, whereas the remaining nine appear to be unique to T. pyriformis; however, of the latter, five are positionally equivalent and of similar size in the two ciliate mitochondrial genomes, suggesting they may also be homologous, even though this is not evident from sequence comparisons. Only eight tRNA genes encoding seven distinct tRNAs are found in T. pyriformis mtDNA, formally confirming a long-standing proposal that most T. pyriformis mitochondrial tRNAs are nucleus-encoded species imported from the cytosol. Atypical features of mitochondrial gene organization and expression in T. pyriformis mtDNA include split and rearranged large subunit rRNA genes, as well as a split nad1 gene (encoding subunit 1 of NADH dehydrogenase of respiratory complex I) whose two segments are located on and transcribed from opposite strands, as is also the case in P. aurelia. Gene content and arrangement are very similar in T. pyriformis and P. aurelia mtDNAs, the two differing by a limited number of duplication, inversion and rearrangement events. Phylogenetic analyses using concatenated sequences of several mtDNA-encoded proteins provide high bootstrap support for the monophyly of alveolates (ciliates, dinoflagellates and apicomplexans) and slime molds.

The cytochrome oxidase subunit 1 gene (cox1) from the dinoflagellate, Crypthecodinium cohnii

To date, no genes have been characterized from dinoflagellate mitochondrial DNA. Here we present the complete sequence of the gene (cox1) encoding subunit 1 of cytochrome c oxidase in the dinoflagellate, Crypthecodinium cohnii. Analysis of nucleotide and deduced amino acid sequences predicts a protein of 523 amino acids that is translated using universal initiation, stop and tryptophan codons. COX1 amino acid identity and phylogenetic tree analyses strongly support a close evolutionary relationship between dinoflagellates and apicomplexans; however, inclusion of the ciliates in this clade is less well supported, a result likely due to the highly derived nature of ciliate COX1 sequences.

Identifying and distinguishing sibling species in the Tetrahymena pyriformis complex (Ciliophora, Oligohymenophorea) using PCR/RFLP analysis of nuclear ribosomal DNA

We describe a riboprinting strategy for identifying and distinguishing among sibling species in the Tetrahymena pyriformis complex. It involves use of the polymerase chain reaction to amplify a large segment of the nuclear ribosomal DNA and internal transcribed spacers, and digestion of this DNA with restriction enzymes. Unique restriction fragment length patterns or haplotypes were then used to distinguish species into: (1) six taxa that were identifiable to the species level, (2) eight taxa that were separated into four pairs, and (3) a group of eight taxa that were identical to each other. The latter result indicates that a more variable molecule is needed to distinguish the most closely related species in the complex. There was no intraspecific variation between two strains from one species (Tetrahymena thermophila) nor among multiple isolates from another species (Tetrahymena empidokyrea). This approach provides an alternative to traditional techniques for identifying T. pyriformis species that require living reference specimens and/or that reveal high levels of intraspecific variation.

Molecular genetic and protein chemical characterization of the cytochrome ba3 from Thermus thermophilus HB8

Thermus thermophilus HB8 cells grown under reduced dioxygen tensions contain a substantially increased amount of heme A, much of which appears to be due to the presence of the terminal oxidase, cytochrome ba3. We describe a purification procedure for this enzyme that yields approximately 100 mg of pure protein from 2 kg of wet mass of cells grown in < or = 50 microM O2. Examination of the protein by SDS-polyacrylamide gel electrophoresis followed by staining with Coomassie Blue reveals one strongly staining band at approximately 35 kDa and one very weakly staining band at approximately 18 kDa as reported earlier (Zimmermann, B.H., Nitsche, C.I., Fee, J. A., Rusnak, F., and Münck, E. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 5779-5783). By contrast, treatment of the gels with AgNO3 reveals that the larger polypeptide stains quite weakly while the smaller polypeptide stains very strongly. These results suggested the presence of two polypeptides in this protein. Using partial amino acid sequences from both proteins to obtain DNA sequence information, we isolated and sequenced a portion of the Thermus chromosome containing the genes encoding the larger protein, subunit I (cbaA), and the smaller protein, subunit II (cbaB). The two polypeptides were isolated using reversed phase liquid chromatography, and their mole percent amino acid compositions are consistent with the proposed translation of their respective genes. The two genes appear to be part of a larger operon, but we have not extended the sequencing to identify initiation and termination sequences. The deduced amino acid sequence of subunit I includes the six canonical histidine residues involved in binding the low spin heme B and the binuclear center Cu(B)/heme A. These and other conserved amino acids are placed along the polypeptide among alternating hydrophobic and hydrophilic segments in a pattern that shows clear homology to other members of the heme- and copper-requiring terminal oxidases. The deduced amino acid sequence of the subunit II contains the CuA binding motif, including two cysteines, two histidines, and a methionine, but, in contrast to most other subunits II, it has only one region of hydrophobic sequence near its N terminus. Alignment of these two polypeptides with other cytochrome c and quinol oxidases, combined with secondary structure analysis and previous spectral studies, clearly establish cytochrome ba3 as a bona fide member of the superfamily of heme- and copper-requiring oxidases. The alignments further indicate that cytochrome ba3 is phylogenetically distant from other cytochrome c and quinol oxidases, and they substantially decrease the number of conserved amino acid residues.

Primary sequence and post-transcriptional modification pattern of an unusual mitochondrial tRNA(Met) from Tetrahymena pyriformis

In a previous investigation of the rDNA region in Tetrahymena pyriformis mitochondrial DNA, we identified a putative tRNA(Met) gene [Heinonen et al. (1987) J. Biol. Chem. 262, 2879-2887]. On the basis of Northern hybridization analyses, we suggested that this gene is expressed, even though the resulting tRNA would be unusually small and have an atypical dihydrouridine stem-loop domain. We report here the complete nucleotide sequence and post-transcriptional modification pattern of this tRNA(Met), confirming its predicted primary structure and supporting the view that this structurally aberrant species functions in translation in T. pyriformis mitochondria.

Purification and characterization of membrane-bound CO-reactive hemoprotein from Tetrahymena pyriformis mitochondria

A CO-reactive hemoprotein was purified from the mitochondrial membrane fraction of Tetrahymena pyriformis. It showed absorption peaks at 615 and 455 nm in the reduced form and an alpha peak at 565 nm in the pyridine ferrohemochrome spectrum. Although the spectral properties were apparently similar to those of 'cytochrome a620' which was previously proposed as a mitochondrial terminal oxidase in T. pyriformis, it did not contain any molecules of heme a or copper atoms. Further, it showed neither cytochrome c oxidase nor cytochrome c peroxidase activity. These results suggest that 'cytochrome a620' may not be the terminal oxidase in the mitochondrial respiratory chain of T. pyriformis.

Genetic screening in Saccharomyces cerevisiae for large numbers of mitochondrial point mutations which affect structure and function of catalytic subunits of cytochrome-c oxidase

A new search for mitochondrial respiratory deficient mutants (Mit-) has been undertaken in order to accumulate a large number of point mutations in the coding portions of cytochrome-c-oxidase catalytic subunits and cytochrome b. Therefore, a mitochondrial DNA which retains the exons and lacks all the introns of the cytochrome oxidase subunit I and of the cytochrome-b split genes has been introduced into a strain carrying a nuclear recessive mutation affecting the adenine-nucleotide translocator, the op1 mutation, which is known to prevent the accumulation of large deletion petite mutants and this was used as the parental strain. After a moderate MnCl2 mutagenesis in order to limit multiple mutations, 105 Mit- mutants were isolated from 15,000 mutagenised clones in Saccharomyces cerevisiae. Mutations were mapped to the three catalytic subunits encoding genes (COX1, COX2 and COX3) of the cytochrome-c oxidase (70 mutations) and to the cytochrome-b gene (15 mutations). More than 50% of the mutants tested still exhibited mitochondrial translation products (subunits I, II and III), suggesting that they carry a missense mutation, rather than a nonsense mutation which would normally have led to a truncated protein. Mutations in the COX1 gene were allocated to four different subregions corresponding to exons 4 and 8 or to groups of exons, exons 1, 2, 3 or exons 5, 6, 7. Seven missense monosubstitution mutations and two frameshift mutations were also identified. The amino acid changes of the missense mutations were located in the vicinity of the CuB-heme alpha 3 binuclear centre ligands.

Cytochrome b of protozoan mitochondria: relationships between function and structure

1. The sensitivity of ubiquinol:cytochrome c reductase to its most powerful inhibitors has been characterized in mitochondria from three ciliate and two trypanosome protozoans and compared with that in mitochondria of animals and plants. 2. Mitochondria of ciliates, particularly those of Tetrahymena pyriformis, are resistant to antimycin. 3. Mitochondria of trypanosomes are quite resistant to stigmatellin, as they exhibit a 40-fold higher titer than that in ciliate or animals mitochondria. 4. Both ciliates and trypanosomes are highly resistant to myxothiazol. 5. Correlations have been drawn between the natural resistance of the protozoan mitochondria to antimycin, stigmatellin and myxothiazol and peculiar features in the structure of their apocytochrome b, on the basis of an accurate alignment of the sequences of this protein.

Rapid evolution of the plastid translational apparatus in a nonphotosynthetic plant: loss or accelerated sequence evolution of tRNA and ribosomal protein genes

The vestigial plastid genome of Epifagus virginiana (beechdrops), a nonphotosynthetic parasitic flowering plant, is functional but lacks six ribosomal protein and 13 tRNA genes found in the chloroplast DNAs of photosynthetic flowering plants. Import of nuclear gene products is hypothesized to compensate for many of these losses. Codon usage and amino acid usage patterns in Epifagus plastic genes have not been affected by the tRNA gene losses, though a small shift in the base composition of the whole genome (toward A+T-richness) is apparent. The ribosomal protein and tRNA genes that remain have had a high rate of molecular evolution, perhaps due to relaxation of constraints on the translational apparatus. Despite the compactness and extensive gene loss, one translational gene (infA, encoding initiation factor 1) that is a pseudogene in tobacco has been maintained intact in Epifagus.

The unusual reversion properties of a mitochondrial mutation in the structural gene of subunit I of cytochrome oxidase of Saccharomyces cerevisiae reveal a probable histidine ligand of the redox center

We have analyzed a mutation in the mitochondrial gene oxi3 coding for subunit I of cytochrome-oxidase in the yeast Saccharomyces cerevisiae. This mutation replaces one of the seven invariant histidines of the polypeptide (position 378) by a tyrosine, and leads to a respiratory deficient phenotype. A total of 157 revertants, which have recovered the ability to grow on a respiratory substrate, have been selected from this mutant (tyrosine 378). The nature of the reversion has been analysed by a rapid screening procedure and 32 of the revertants have been sequenced. They are all true back-mutations reintroducing the histidine in position 378. This very exceptional situation suggests that this histidine is a ligand of the redox center of cytochrome oxidase.

Cytochrome c oxidases: polypeptide composition, role of subunits, and location of active metal centers

The general structure of the enzyme, its polypeptide composition, and a proposal for a rational nomenclature are discussed. The mitochondrially coded and bacterial cytochrome c oxidase subunits have been analyzed with more attention focused on elucidating the number of metals present in the enzyme and the ligands available for their coordination. The picture of a 2 Cu/2 Fe enzyme has been compared with that of a 3 Cu/2 Fe enzyme and a new model is proposed for the location of the metal centers in the enzyme.

Nucleotide sequence of the mitochondrial genome of Paramecium

The nucleotide sequence for 40,469 bp of the linear Paramecium aurelia mitochondrial (mt) genome is presented with the locations of the known genes, presumed ORFs, and their transcripts. Many of the genes commonly encoded in mt DNA of other organisms have been identified in the Paramecium mt genome but several unusual genes have been found. Ribosomal protein genes rps14, rps12, and rpl2 are clustered in a region that also contains two other genes usually found in chloroplasts, but rpl14 is over 16 kbp away. The ATP synthase gene, atp9, is encoded in this mt genome, but the atp6, atp8, and COIII genes have not been identified. All of the identified genes are transcribed. Many mono- and poly- cistronic transcripts have been detected which cover most of the genome, including large regions where genes have yet to be identified. Based on sequence comparisons with known tRNAs, only those for phe, trp, and tyr are encoded in Paramecium mt DNA.

An unusual region of Paramecium mitochondrial DNA containing chloroplast-like genes

Based on DNA and amino acid comparisons with known genes and their products, a region of the Paramecium aurelia mitochondrial (mt) genome has been found to encode the following gene products: (1) photosystem II protein G (psbG); (2) a large open reading frame (ORF400) which is also found encoded in the chloroplast (cp) DNA of tobacco (as ORF393) and liverwort (as ORF392), and in the kinetoplast maxicircle DNA of Leishmania tarentolae (as ORFs 3 and 4); (3) ribosomal protein L2 (rpl2); (4) ribosomal protein S12 (rps12); (5) ribosomal protein S14 (rps14); and (6) NADH dehydrogenase subunit 2 (ndh2). All of these genes have been found in cp DNA, but the psbG gene has never been identified in a mt genome, and ribosomal protein genes have never been located in an animal or protozoan mitochondrion. The ndh2 gene has been found in both mitochondria and plastids. The Paramecium genes are among the most divergent of those sequenced to date. Two of the genes are encoded on the strand of DNA complementary to that encoding all other known Paramecium mt genes. No gene contains an identifiable intron. The rps12 and psbG genes are probably overlapping. It is not yet known whether these genes are transcribed or have functional gene products. The presence of these genes in the mt genome raises interesting questions concerning their evolutionary origin.