Comparison of the maxicircle (mitochondrial) genomes of Leishmania tarentolae and Trypanosoma brucei at the level of nucleotide sequence

The entire 16.7-kilobase (kb) transcribed region of the Leishmania tarentolae maxicircle was compared to the entire 15-kb transcribed region of the Trypanosoma brucei maxicircle at the nucleotide sequence level by dot matrix analysis and by alignments of individual genes. The L. tarentolae NADH dehydrogenase subunit 1 (ND1) gene was identified in a newly obtained 2.9-kb sequence. All but two regions which flank the cytochrome b gene are highly conserved in both species. One 3.1-kb region in L. tarentolae that contains the cytochrome oxidase subunit III (COIII) gene and several open reading frames corresponds to a 2-kb sequence in T. brucei with limited sequence homology that lacks the COIII gene. Another 0.6-kb region that comprises an unidentified open reading frame (open reading frame 12) in L. tarentolae is substituted by a nonhomologous 0.4-kb open reading frame in T. brucei. A short intergenic region between the ND1 gene and the maxicircle unidentified reading frame 1 gene shows limited sequence homology, and the regions between the ND4 and ND5 genes and between the COI and ND4 genes are not conserved. All of the intergenic regions share G + C richness and a similar pattern of G versus C strand bias. 1.8 kb of the L. tarentolae divergent region (DV) and around 3 kb of the T. brucei DV were also obtained. The T. brucei DV sequences were not homologous to the L. tarentolae DV sequence but were organized in a similar fashion with tandem repeats of varying complexity.     

Maxicircle (mitochondrial) genome sequence (partial) of Leishmania major: gene content, arrangement and composition compared with Leishmania tarentolae

We report 8420 bp of DNA sequence data from the maxicircle (mitochondrial) genome of Leishmania major (MHOM/SU/73/5ASKH), a much larger portion of this genome than has been reported previously from any Leishmania species infecting humans. This region contains 10 partial and complete genes: 5 protein-encoding genes (COII, COIII, ND1, ND7 and Cyt b); two ribosomal RNA subunits (12S and 9S) and three unidentified open reading frames (MURF1, MURF4 (ATPase6) and MURF5), as in the lizard-infecting species L. tarentolae. The genes from L. major exhibit 85-87% identity with those of L. tarentolae at the nucleotide level and 71-94% identity at the amino acid level. Most differences between sequences from the two species are transversions. The gene order and arrangement within the maxicircle of L. major are similar to those in L. tarentolae, but base composition and codon usage differ between the species. Codons assigned for initiation for protein-coding genes available for comparison are similar in five genes in the two species. Pre-editing was identified in some of the protein-coding genes. Short intergenic non-coding regions are also present in L. major as they are in L. tarentolae. Intergenic regions between 9S rRNA and MURF5, MURF1 and ND1 genes are G+C rich and considered to be extensive RNA editing regions. The RNA editing process is likely to be conserved in similar pattern in L. major as in L. tarentolae.     

Cytochrome oxidase subunit II gene in mitochondria of Oenothera has no intron

The cytochrome oxidase subunit II gene has been localized in the mitochondrial genome of Oenothera berteriana and the nucleotide sequence has been determined. The coding sequence contains 777 bp and, unlike the corresponding gene in Zea mays, is not interrupted by an intron. No TGA codon is found within the open reading frame. The codon CGG, as in the maize gene, is used in place of tryptophan codons of corresponding genes in other organisms. At position 742 in the Oenothera sequence the TGG of maize is changed into a CGG codon, where Trp is conserved as the amino acid in other organisms. Homologous sequences occur more than once in the mitochondrial genome as several mitochondrial DNA species hybridize with DNA probes of the cytochrome oxidase subunit II gene.     

Isolation and analysis of the genes for cytochrome c oxidase in Paracoccus denitrificans

Synthetic oligonucleotide probes were used to clone two loci from the chromosomal DNA of Paracoccus denitrificans that contain the genes for cytochrome c oxidase (cytochrome aa₃). One locus seems to contain four or five genes probably forming an operon. Two of these code for the oxidase subunits II and III. Three open reading frames are found between the COII and COIII genes. The other locus codes for the subunit I. A short open reading frame is found upstream of this gene. All three subunits of the Paracoccus enzyme show remarkable homology to the corresponding subunits of the mitochondrial cytochrome oxidase. Possible protein products of the open reading frames have not yet been identified.     

Nucleotide sequence of the gene coding for four subunits of cytochrome c oxidase from the thermophilic bacterium PS3

The gene coding for four subunits of cytochrome aa3-type oxidase was isolated from a genomic DNA library of the thermophilic bacterium PS3 and sequenced. The N-terminus of each subunit was also sequenced to verify the initiation site of the reading frame. The deduced amino acid sequences contained 615 amino acid residues for subunit I (CO1/caaB product), 333 residues for subunit II (CO2/caaA product), 207 residues for subunit III (CO3/caaC product), and 109 residues for subunit IV (CO4/caaD product) after processing. Re-examination of the sequencing of caa revealed a longer open reading frame for CO1, which contains 14 transmembrane segments instead of 12 [Sone et al. (1988) J. Biochem. 103, 606-610], although the main portions of the sequences constituting cytochrome a (FeA), cytochrome a3 (FeB), and CuB are correct. PS3 CO2 has an additional sequence for cytochrome c after the CuA binding protein portion with 2 transmembrane segments, which is homologous to the mitochondrial counterpart. PS3 CO3 has DCCD-binding glutamyl residues but contains only 5 transmembrane segments, unlike the mitochondrial counterpart, which has 7 segments. The subunits of PS3 cytochrome oxidase (aa3-type) show clear similarity in amino acid sequences with those of cytochrome bo-type oxidase from Escherichia coli as well, in spite of the difference of hemes. PS3 CO3 and CO4 are much more similar to E. coli CO3 and CO4 than to mitochondrial CO3 and CO4, respectively.     

Saccharomyces cerevisiae positive regulatory gene PET111 encodes a mitochondrial protein that is translated from an mRNA with a long 5'' leader.

The yeast nuclear gene PET111 is required specifically for translation of the mitochondrion-coded mRNA for cytochrome c oxidase subunit II. We have determined the nucleotide sequence of a 3-kilobase segment of DNA that carries PET111. The sequence contains a single long open reading frame that predicts a basic protein of 718 amino acids. The PET111 gene product is a mitochondrial protein, since a hybrid protein which includes the amino-terminal 154 amino acids of PET111 fused to beta-galactosidase is specifically associated with mitochondria. PET111 is translated from a 2.9-kilobase mRNA which, interestingly, has an extended 5'-leader sequence containing four short open reading frames upstream of the long open reading frame. These open reading frames exhibit an interesting pattern of overlap with each other and with the PET111 reading frame.     

Molecular evolution of cytochrome c oxidase subunit I in primates: is there coevolution between mitochondrial and nuclear genomes?

Phylogenetic analyses carried out on cytochrome c oxidase (COX) subunit I mitochondrial genes from 14 primates representing the major branches of the order and four outgroup nonprimate eutherians revealed that transversions and amino acid replacements (i.e., the more slowly occurring sequence changes) contained lower levels of homoplasy and thus provided more accurate information on cladistic relationships than transitions (i.e., the more rapidly occurring sequence changes). Several amino acids, each with a high likelihood of functionality involving the binding of cytochrome c or interaction with COX VIII, have changed in Anthropoidea, the primate suborder grouping New World monkey, Old World monkey, ape, and human lineages. They are conserved in other mammalian lineages and in nonanthropoid primates. Maximum-likelihood ancestral COX I nucleotide sequences were determined utilizing a near most parsimonious branching arrangement for the primate sequences that was consistent with previously hypothesized primate cladistic relationships based on larger and more diverse data sets. Relative rate tests of COX I mitochondrial sequences showed an elevated nonsynonymous (N) substitution rate for anthropoid-nonanthropoid comparisons. This finding for the largest mitochondrial (mt) DNA-encoded subunit is consistent with previous observations of elevated nonsynonymous substitution/synonymous substitution (S) rates in primates for mt-encoded COX II and for the nuclear-encoded COX IV and COX VIIa-H. Other COX-related proteins, including cytochrome c and cytochrome b, also show elevated amino acid replacement rates or N/S during similar time frames, suggesting that this group of interacting genes is likely to have coevolved during primate evolution.     

Complete nucleotide sequence of the 6 kb element and conserved cytochrome b gene sequences among Indian isolates of Plasmodium falciparum.

The malaria parasite contains a nuclear genome with 14 chromosomes and two extrachromosomal DNA molecules of 6 kb and 35 kb in size. The smallest genome, known as the 6 kb element or mitochondrial DNA, has been sequenced from several Plasmodium falciparum isolates because this is a potential drug target. Here we describe the complete nucleotide sequence of this element from an Indian isolate of P. falciparum. It is 5967 bp in size and shows 99.6% homology with the 6 kb element of other isolates. The element contains three open reading frames for mitochondrial proteins-cytochrome oxidase subunit I (CoI), subunit III (CoIII) and cytochrome b (Cyb) which were found to be expressed during blood stages of the parasite. We have also sequenced the entire cyb gene from several Indian isolates of P. falciparum. The rate of mutation in this gene was very low since 12 of 14 isolates showed the identical sequence. Only one isolate showed a maximum change in five amino acids whereas the other isolate showed only one amino acid change. However, none of the Indian isolates showed any change in those amino acids of cyb which are associated with resistance to various drugs as these drugs are not yet commonly used in India.     

Nucleotide sequence of the gene coding for cytochrome oxidase subunit I from the thermophilic bacterium PS3

The gene coding for cytochrome oxidase subunit I (COI) was isolated from a genomic DNA library of the thermophilic bacterium PS3 and sequenced. The N-terminal of the COI protein was also sequenced to verify the initiation site of the reading frame. The deduced amino acid sequence of COI protein is composed of 536 amino acid residues and its molecular mass is 59,510. The protein is clearly homologous to the corresponding subunit in the mitochondrial cytochrome oxidase and similarly appears to have 12 trans-membrane segments. The proposed ligands to two hemes (cytochrome aa3) and a copper atom (CuB) in this protein (Holm et al. (1987) EMBO J. 6, 2819-2823) are conserved in the sequence.     

Two intron sequences in yeast mitochondrial COX1 gene: homology among URF-containing introns and strain-dependent variation in flanking exons

The DNA sequences of two optional introns in the gene for subunit I of cytochome c oxidase in yeast mitochondrial DNA have been determined. Both contain long unassigned reading frames (URFs). These display regions of amino acid homology with six other URFs, two of which encode proteins involved in mitochondrial RNA splicing. Such conserved regions may thus define functionally important domains of proteins involved in RNA processing. This homology also implies that these URFs had a common ancestral sequence, which has been duplicated and dispersed around the genome. Comparison of the flanking exons in the long strain KL14-4A with their unsplit counterpart in D273-10B reveals clustered sequence differences, which lead in D273-10B to codons rarely used in exons. These differences may be linked to the loss or absence of one of the optional introns.     

Structure and organization of the mitochondrial genome of the unicellular red alga Cyanidioschyzon merolae deduced from the complete nucleotide sequence.

The complete nucleotide sequence of the mitochondrial genome of a very primitive unicellular red alga, Cyanidioschyzon merolae , has been determined. The mitochondrial genome of C.merolae contains 34 genes for proteins including unidentified open reading frames (ORFs) (three subunits of cytochrome c oxidase, apocytochrome b protein, three subunits of F1F0-ATPase, seven subunits of NADH ubiquinone oxidoreductase, three subunits of succinate dehydrogenase, four proteins implicated in c-type cytochrome biogenesis, 11 ribosomal subunits and two unidentified open reading frames), three genes for rRNAs and 25 genes for tRNAs. The G+C content of this mitochondrial genome is 27.2%. The genes are encoded on both strands. The genome size is comparatively small for a plant mitochondrial genome (32 211 bp). The mitochondrial genome resembles those of plants in its gene content because it contains several ribosomal protein genes and ORFs shared by other plant mitochondrial genomes. In contrast, it resembles those of animals in the genome organization, because it has very short intergenic regions and no introns. The gene set in this mitochondrial genome is a subset of that of Reclinomonas americana , an amoeboid protozoan. The results suggest that plant mitochondria originate from the same ancestor as other mitochondria and that most genes were lost from the mitochondrial genome at a fairly early stage of the evolution of the plants.     

Paracoccus denitrificans mutants deleted in the gene for subunit II of cytochrome c oxidase also lack subunit I.

As a prerequisite to site-directed mutagenesis on cytochrome c oxidase, two different mutants are constructed by inactivating the cta gene locus encoding subunits II and III (ctaC and ctaE) of the Paracoccus denitrificans oxidase. Either a short fragment encoding part of the putative copper binding site near the C terminus of subunit II, or a substantial fragment, comprising parts of the coding region for both subunits and all of the intervening three open reading frames, are removed and replaced by the kanamycin resistance gene. Each construct, ligated into a suicide vector, is mated into Paracoccus, and mutants originating from double homologous recombination events are selected. We observe complete loss of alpha-type heme and of oxidase subunits, as well as a substantial decrease in the cytochrome c oxidase activity. Upon complementation with the ctaC gene (plus various lengths of downstream sequence extending into the operon), subunit II gets expressed in all cases. Wild-type phenotype, however, is only restored with the whole operon. Using smaller fragments for complementation gives interesting clues on roles of the open reading frames for the assembly process of the oxidase complex; two of the open reading frame genes most likely code for two independent assembly factors. Since homologous genes have been described not only for other bacterial oxidases, but their gene products shown to participate also in the assembly of the yeast enzyme, they seem to constitute a group of evolutionary conserved proteins.     

Molecular Characterization of Lengthy Mitochondrial DNA Duplications from the Parasitic Nematode Romanomermis Culicivorax

Complete nucleotide sequences, precise endpoints and coding potential of several 3.0-kilobase mitochondrial DNA (mtDNA) repeating units derived from two isofemale lineages of the mermithid nematode Romanomermis culicivorax have been determined. Endpoint analysis has allowed us to infer deletion and inversion events that most likely generated the present day repeat configuration. Each amplified unit contains the genes for NADH dehydrogenase subunits 3 and 6 (ND3 and ND6), an open reading frame (ORF 1) that represents a cytochrome P450-like gene, and three additional unidentified open reading frames. The primary nucleotide sequences of the R. culicivorax mt-repeat copies within individual haplotypes are highly conserved; three nearly complete copies of the repeat unit vary by 0.01% at the nucleotide level. These observations suggest that concerted evolution mechanisms may be active, resulting in sequence homogenation of these lengthy duplications.