The transcription of DNA in chicken mitochondria initiates from one major bidirectional promoter

The minicircular and extremely heteroplasmic mitogenome of the holoparasitic plant Rhopalocnemis phalloides

The plastid and nuclear genomes of parasitic plants exhibit deeply altered architectures,^1-13 whereas the few examined mitogenomes range from deeply altered to conventional.^14-20 To provide further insight on mitogenome evolution in parasitic plants, we report the highly modified mitogenome of Rhopalocnemis phalloides, a holoparasite in Balanophoraceae. Its mitogenome is uniquely arranged in 21 minicircular chromosomes that vary in size from 4,949 to 7,861 bp, with a total length of only 130,713 bp. All chromosomes share an identical 896 bp conserved region, with a large stem-loop that acts as the origin of replication, flanked on each side by hypervariable and semi-conserved regions. Similar minicircular structures with shared and unique regions have been observed in parasitic animals and free-living protists,^21-24 suggesting convergent structural evolution. Southern blots confirm both the minicircular structure and the replication origin of the mitochondrial chromosomes. PacBio reads provide evidence for chromosome recombination and rolling-circle replication for the R. phalloides mitogenome. Despite its small size, the mitogenome harbors a typical set of genes and introns within the unique regions of each chromosome, yet introns are the smallest among seed plants and ferns. The mitogenome also exhibits extreme heteroplasmy, predominantly involving short indels and more complex variants, many of which cause potential loss-of-function mutations for some gene copies. All heteroplasmic variants are transcribed, and functional and nonfunctional protein-coding variants are spliced and RNA edited. Our findings offer a unique perspective into how mitogenomes of parasitic plants can be deeply altered and shed light on plant mitogenome replication.

Insights into the Mitochondrial and Nuclear Genome Diversity of Two High Yielding Strains of Laying Hens

Simple Summary

Mitochondria are commonly known as “the powerhouse of the cell”, influencing the fitness, lifespan and metabolism of eukaryotic organisms. In our study we examined mitochondrial and nuclear genomic diversity in two high yielding strains of laying hens. We tested if the mitochondrial genome affects functional traits such as body weight and phosphorus utilization. We discovered a surprisingly low mitochondrial genetic diversity and an unequal distribution of the haplotypes among both strains, leading to limitations of robust links to phenotypic traits. In contrast, we found similar levels of nuclear genome diversity in both strains. Our study explores the potential influence of the mitochondrial genome on phenotypic traits and thus contributes to a better understanding of the function of this organelle in laying hens. Further, we focus on its usefulness as a genetic marker, which is often underestimated in breeding approaches, given the different inheritance mechanism compared to the nuclear genome.

Abstract

Mitochondria are essential components of eukaryotes as they are involved in several organismic key processes such as energy production, apoptosis and cell growth. Despite their importance for the metabolism and physiology of all eukaryotic organisms, the impact of mitochondrial haplotype variation has only been studied for very few species. In this study we sequenced the mitochondrial genome of 180 individuals from two different strains of laying hens. The resulting haplotypes were combined with performance data such as body weight, feed intake and phosphorus utilization to assess their influence on the hens in five different life stages. After detecting a surprisingly low level of genetic diversity, we investigated the nuclear genetic background to estimate whether the low mitochondrial diversity is representative for the whole genetic background of the strains. Our results highlight the need for more in-depth investigation of the genetic compositions and mito-nuclear interaction in individuals to elucidate the basis of phenotypic performance differences. In addition, we raise the question of how the lack of mitochondrial variation developed, since the mitochondrial genome represents genetic information usually not considered in breeding approaches.

Characterization and Comparative Analyses of Mitochondrial Genomes in Single-Celled Eukaryotes to Shed Light on the Diversity and Evolution of Linear Molecular Architecture

Determination and comparisons of complete mitochondrial genomes (mitogenomes) are important to understand the origin and evolution of mitochondria. Mitogenomes of unicellular protists are particularly informative in this regard because they are gene-rich and display high structural diversity. Ciliates are a highly diverse assemblage of protists and their mitogenomes (linear structure with high A+T content in general) were amongst the first from protists to be characterized and have provided important insights into mitogenome evolution. Here, we report novel mitogenome sequences from three representatives (Strombidium sp., Strombidium cf. sulcatum, and Halteria grandinella) in two dominant ciliate lineages. Comparative and phylogenetic analyses of newly sequenced and previously published ciliate mitogenomes were performed and revealed a number of important insights. We found that the mitogenomes of these three species are linear molecules capped with telomeric repeats that differ greatly among known species. The genomes studied here are highly syntenic, but larger in size and more gene-rich than those of other groups. They also all share an AT-rich tandem repeat region which may serve as the replication origin and modulate initiation of bidirectional transcription. More generally we identified a split version of ccmf, a cytochrome c maturation-related gene that might be a derived character uniting taxa in the subclasses Hypotrichia and Euplotia. Finally, our mitogenome comparisons and phylogenetic analyses support to reclassify Halteria grandinella from the subclass Oligotrichia to the subclass Hypotrichia. These results add to the growing literature on the unique features of ciliate mitogenomes, shedding light on the diversity and evolution of their linear molecular architecture.

Characterization and Comparative Analysis of Complete Mitogenomes of Three Cacatua Parrots (Psittaciformes: Cacatuidae)

Cacatua alba, Cacatua galerita, and Cacatua goffiniana are parrots of the family Cacatuidae. Wild populations of these species are declining with C. alba listed by the International Union for the Conservation of Nature and Natural Resources (IUCN) as Endangered. In this study, complete mitogenomes were sequenced for a comparative analysis among the Cacatua species, and a detailed analysis of the control region. Mitogenome lengths of C. alba,C. galerita, and C. goffiniana were 18,894, 18,900, and 19,084 bp, respectively. They included 13 protein coding genes, two ribosomal RNA genes, 24 transfer RNA genes, three degenerated genes, and two control regions. Ten conserved motifs were found in three domains within each of the two control regions. For an evolution of duplicated control regions of Cacatua, domain I and the 3′ end of domain III experienced an independent evolution, while domain II and most of the regions of domain III was subjected to a concerted evolution. Based on a phylogenetic analysis of 37 mitochondrial genes, the genus Cacatua formed a well-supported, monophyletic, crown group within the Cacatuidae. Molecular dating results showed that Cacatua diverged from other genera of Cacatuinae in the middle of Miocene.

Higher Order Organization of the mtDNA: Beyond Mitochondrial Transcription Factor A

The higher order organization of eukaryotic and prokaryotic genomes is pivotal in the regulation of gene expression. Specifically, chromatin accessibility in eukaryotes and nucleoid accessibility in bacteria are regulated by a cohort of proteins to alter gene expression in response to diverse physiological conditions. By contrast, prior studies have suggested that the mitochondrial genome (mtDNA) is coated solely by mitochondrial transcription factor A (TFAM), whose increased cellular concentration was proposed to be the major determinant of mtDNA packaging in the mitochondrial nucleoid. Nevertheless, recent analysis of DNase-seq and ATAC-seq experiments from multiple human and mouse samples suggest gradual increase in mtDNA occupancy during the course of embryonic development to generate a conserved footprinting pattern which correlate with sites that have low TFAM occupancy in vivo (ChIP-seq) and tend to adopt G-quadruplex structures. These findings, along with recent identification of mtDNA binding by known modulators of chromatin accessibility such as MOF, suggest that mtDNA higher order organization is generated by cross talk with the nuclear regulatory system, may have a role in mtDNA regulation, and is more complex than once thought.

Human primitive brain displays negative mitochondrial-nuclear expression correlation of respiratory genes

Oxidative phosphorylation (OXPHOS), a fundamental energy source in all human tissues, requires interactions between mitochondrial (mtDNA)- and nuclear (nDNA)-encoded protein subunits. Although such interactions are fundamental to OXPHOS, bi-genomic coregulation is poorly understood. To address this question, we analyzed ∼8500 RNA-seq experiments from 48 human body sites. Despite well-known variation in mitochondrial activity, quantity, and morphology, we found overall positive mtDNA-nDNA OXPHOS genes' co-expression across human tissues. Nevertheless, negative mtDNA-nDNA gene expression correlation was identified in the hypothalamus, basal ganglia, and amygdala (subcortical brain regions, collectively termed the "primitive" brain). Single-cell RNA-seq analysis of mouse and human brains revealed that this phenomenon is evolutionarily conserved, and both are influenced by brain cell types (involving excitatory/inhibitory neurons and nonneuronal cells) and by their spatial brain location. As the "primitive" brain is highly oxidative, we hypothesized that such negative mtDNA-nDNA co-expression likely controls for the high mtDNA transcript levels, which enforce tight OXPHOS regulation, rather than rewiring toward glycolysis. Accordingly, we found "primitive" brain-specific up-regulation of lactate dehydrogenase B (LDHB), which associates with high OXPHOS activity, at the expense of LDHA, which promotes glycolysis. Analyses of co-expression, DNase-seq, and ChIP-seq experiments revealed candidate RNA-binding proteins and CEBPB as the best regulatory candidates to explain these phenomena. Finally, cross-tissue expression analysis unearthed tissue-dependent splice variants and OXPHOS subunit paralogs and allowed revising the list of canonical OXPHOS transcripts. Taken together, our analysis provides a comprehensive view of mito-nuclear gene co-expression across human tissues and provides overall insights into the bi-genomic regulation of mitochondrial activities.

Comparative mitogenome analysis on species of four apple snails (Ampullariidae: Pomacea)

The genus Pomacea contains most of the economically important Ampullariid apple snails. Five Pomacea species were reported introduced out of their native ranges, including three highly invasive species, i.e. P. maculata, P. canaliculata and an unidentified species, Pomacea sp. In this study, the mitogenome of P. maculata was determined, which carried typical gene set of metazoan mitogenomes and shared the same gene rearrangement of Ampullariidae mitogenomes. The control region of the P. maculata has a 13-bp inverted repeat unit. We compared the mitogenome of P. maculata with P. canaliculata, Pomacea sp. and the aquatic pet species, P. diffusa. The three highly invasive species showed high sequence similarity of PCGs and RNAs. atp8 and nad2 showed the lowest similarity and the highest Ka/Ks ratios, indicating that both genes have potential for studying species identification and populations genetics in apple snails. All PCGs have the Ka/Ks ratios <1, indicating the existence of purifying selection in Pomacea species. We reconstructed phylogenetic trees using 14 Caenogastropda species based on sequences of PCGs and rRNAs using Bayesian inference. Pomacea maculata grouped with other Ampullariids and was most closely related to Pomacea sp. The mitogenome of P. maculata provides useful genetic resource for exploring the genetics and evolution of P. maculata and other apple snails.

Initiation of mtDNA transcription is followed by pausing, and diverges across human cell types and during evolution

Mitochondrial DNA (mtDNA) genes are long known to be cotranscribed in polycistrones, yet it remains impossible to study nascent mtDNA transcripts quantitatively in vivo using existing tools. To this end, we used deep sequencing (GRO-seq and PRO-seq) and analyzed nascent mtDNA-encoded RNA transcripts in diverse human cell lines and metazoan organisms. Surprisingly, accurate detection of human mtDNA transcription initiation sites (TISs) in the heavy and light strands revealed a novel conserved transcription pausing site near the light-strand TIS. This pausing site correlated with the presence of a bacterial pausing sequence motif, with reduced SNP density, and with a DNase footprinting signal in all tested cells. Its location within conserved sequence block 3 (CSBIII), just upstream of the known transcription–replication transition point, suggests involvement in such transition. Analysis of nonhuman organisms enabled de novo mtDNA sequence assembly, as well as detection of previously unknown mtDNA TIS, pausing, and transcription termination sites with unprecedented accuracy. Whereas mammals (Pan troglodytes, Macaca mulatta, Rattus norvegicus, and Mus musculus) showed a human-like mtDNA transcription pattern, the invertebrate pattern (Drosophila melanogaster and Caenorhabditis elegans) profoundly diverged. Our approach paves the path toward in vivo, quantitative, reference sequence-free analysis of mtDNA transcription in all eukaryotes.

Animal Mitochondrial DNA Replication

Recent advances in the field of mitochondrial DNA (mtDNA) replication highlight the diversity of both the mechanisms utilized and the structural and functional organization of the proteins at mtDNA replication fork, despite the relative simplicity of the animal mtDNA genome. DNA polymerase γ, mtDNA helicase and mitochondrial single-stranded DNA-binding protein-the key replisome proteins, have evolved distinct structural features and biochemical properties. These appear to be correlated with mtDNA genomic features in different metazoan taxa and with their modes of DNA replication, although substantial integrative research is warranted to establish firmly these links. To date, several modes of mtDNA replication have been described for animals: rolling circle, theta, strand-displacement, and RITOLS/bootlace. Resolution of a continuing controversy relevant to mtDNA replication in mammals/vertebrates will have a direct impact on the mechanistic interpretation of mtDNA-related human diseases. Here we review these subjects, integrating earlier and recent data to provide a perspective on the major challenges for future research.

Mitochondrial divergence between slow- and fast-aging garter snakes

Mitochondrial function has long been hypothesized to be intimately involved in aging processes--either directly through declining efficiency of mitochondrial respiration and ATP production with advancing age, or indirectly, e.g., through increased mitochondrial production of damaging free radicals with age. Yet we lack a comprehensive understanding of the evolution of mitochondrial genotypes and phenotypes across diverse animal models, particularly in species that have extremely labile physiology. Here, we measure mitochondrial genome-types and transcription in ecotypes of garter snakes (Thamnophis elegans) that are adapted to disparate habitats and have diverged in aging rates and lifespans despite residing in close proximity. Using two RNA-seq datasets, we (1) reconstruct the garter snake mitochondrial genome sequence and bioinformatically identify regulatory elements, (2) test for divergence of mitochondrial gene expression between the ecotypes and in response to heat stress, and (3) test for sequence divergence in mitochondrial protein-coding regions in these slow-aging (SA) and fast-aging (FA) naturally occurring ecotypes. At the nucleotide sequence level, we confirmed two (duplicated) mitochondrial control regions one of which contains a glucocorticoid response element (GRE). Gene expression of protein-coding genes was higher in FA snakes relative to SA snakes for most genes, but was neither affected by heat stress nor an interaction between heat stress and ecotype. SA and FA ecotypes had unique mitochondrial haplotypes with amino acid substitutions in both CYTB and ND5. The CYTB amino acid change (Isoleucine → Threonine) was highly segregated between ecotypes. This divergence of mitochondrial haplotypes between SA and FA snakes contrasts with nuclear gene-flow estimates, but correlates with previously reported divergence in mitochondrial function (mitochondrial oxygen consumption, ATP production, and reactive oxygen species consequences).

The complete mitochondrial genome sequence of the little egret (Egretta garzetta)

Many phylogenetic questions in the Ciconiiformes remain unresolved and complete mitogenome data are urgently needed for further molecular investigation. In this work, we determined the complete mitogenome sequence of the little egret (Egretta garzetta). The genome was 17,361 bp in length and the gene organization was typical of other avian mtDNA. In protein-coding genes (PCGs), a C insertion was found in ND3, and COIII and ND4 terminated with incomplete stop codons (T). tRNA-Val and tRNA-Ser (AGY) were unable to fold into canonical cloverleaf secondary structures because they had lost the DHU arms. Long repetitive sequences consisting of five types of tandem repeats were found at the 3' end of Domain III in the control region. A phylogenetic analysis of 11 species of Ciconiiformes was done using complete mitogenome data and 12 PCGs. The tree topologies obtained with these two strategies were identical, which strongly confirmed the monophyly of Ardeidae, Threskiorothidae and Ciconiidae. The phylogenetic analysis also revealed that Egretta was more closely related to Ardea than to Nycticorax in the Ardeidae, and Platalea was more closely related to Threskiornis than to Nipponia in the Threskiornithidae. These findings contribute to our understanding of the phylogenetic relationships of Ciconiiformes based on complete mitogenome data.

Comparative mitochondrial genomics and phylogenetic relationships of the Crossoptilon species (Phasianidae, Galliformes)

BACKGROUND

Phasianidae is a family of Galliformes containing 38 genera and approximately 138 species, which is grouped into two tribes based on their morphological features, the Pheasants and Partridges. Several studies have attempted to reconstruct the phylogenetic relationships of the Phasianidae, but many questions still remain unaddressed, such as the taxonomic status and phylogenetic relationships among Crossoptilon species. The mitochondrial genome (mitogenome) has been extensively used to infer avian genetic diversification with reasonable resolution. Here, we sequenced the entire mitogenomes of three Crossoptilon species (C. harmani, C. mantchuricum and C. crossoptilon) to investigate their evolutionary relationship among Crossoptilon species.

RESULTS

The complete mitogenomes of C. harmani, C. mantchuricum and C. crossoptilon are 16682 bp, 16690 bp and 16680 bp in length, respectively, encoding a standard set of 13 protein-coding genes, 2 ribosomal RNA genes, 22 transfer RNA genes, and a putative control region. C. auritum and C. mantchuricum are more closely related genetically, whereas C. harmani is more closely related to C. crossoptilon. Crossoptilon has a closer relationship with Lophura, and the following phylogenetic relationship was reconstructed: ((Crossoptilon + Lophura) + (Phasianus + Chrysolophus)). The divergence time between the clades C. harmani-C. crossoptilon and C. mantchuricum-C. auritum is consistent with the uplift of the Tibetan Plateau during the Tertiary Pliocene. The Ka/Ks analysis showed that atp8 gene in the Crossoptilon likely experienced a strong selective pressure in adaptation to the plateau environment.

CONCLUSIONS

C. auritum with C. mantchuricum and C. harmani with C. crossoptilon form two pairs of sister groups. The genetic distance between C. harmani and C. crossoptilon is far less than the interspecific distance and is close to the intraspecific distance of Crossoptilon, indicating that C. harmani is much more closely related to C. crossoptilon. Our mito-phylogenomic analysis supports the monophyly of Crossoptilon and its closer relationship with Lophura. The uplift of Tibetan Plateau is suggested to impact the divergence between C. harmani-C. crossoptilon clade and C. mantchuricum-C. auritum clade during the Tertiary Pliocene. Atp8 gene in the Crossoptilon species might have experienced a strong selective pressure for adaptation to the plateau environment.

Mito-nuclear co-evolution: the positive and negative sides of functional ancient mutations

Most cell functions are carried out by interacting factors, thus underlying the functional importance of genetic interactions between genes, termed epistasis. Epistasis could be under strong selective pressures especially in conditions where the mutation rate of one of the interacting partners notably differs from the other. Accordingly, the order of magnitude higher mitochondrial DNA (mtDNA) mutation rate as compared to the nuclear DNA (nDNA) of all tested animals, should influence systems involving mitochondrial-nuclear (mito-nuclear) interactions. Such is the case of the energy producing oxidative phosphorylation (OXPHOS) and mitochondrial translational machineries which are comprised of factors encoded by both the mtDNA and the nDNA. Additionally, the mitochondrial RNA transcription and mtDNA replication systems are operated by nDNA-encoded proteins that bind mtDNA regulatory elements. As these systems are central to cell life there is strong selection toward mito-nuclear co-evolution to maintain their function. However, it is unclear whether (A) mito-nuclear co-evolution befalls only to retain mitochondrial functions during evolution or, also, (B) serves as an adaptive tool to adjust for the evolving energetic demands as species' complexity increases. As the first step to answer these questions we discuss evidence of both negative and adaptive (positive) selection acting on the mtDNA and nDNA-encoded genes and the effect of both types of selection on mito-nuclear interacting factors. Emphasis is given to the crucial role of recurrent ancient (nodal) mutations in such selective events. We apply this point-of-view to the three available types of mito-nuclear co-evolution: protein-protein (within the OXPHOS system), protein-RNA (mainly within the mitochondrial ribosome), and protein-DNA (at the mitochondrial replication and transcription machineries).

Complete mitochondrial genome of the Japanese wood pigeon, Columba janthina janthina (Columbiformes, Columbidae)

The complete mitochondrial genome of Columba janthina janthina was sequenced and its total length was 17,469 bp, containing 13 protein-coding genes, 2 ribosomal RNAs, 22 transfer RNAs and 1 non-coding control region (D-loop). The A+T content of the overall base composition of H-strand were 54.46% (30.38% A, 24.08% T, 32.00% C and 13.54% G). The arrangement of all genes was identical to the typical mitochondrial genomes of pigeon. Within the control region, conserved sequences were identified in three domains. These results provide basic information for phylogenetic analyses of pigeon, especially Columbiformes species.

Transcription could be the key to the selection advantage of mitochondrial deletion mutants in aging

The mitochondrial theory of aging is widely popular but confronted by several apparent inconsistencies. On the one hand, mitochondrial energy production is of central importance to the health and proper functioning of cells, and single-cell studies have shown that mtDNA deletion mutants accumulate in a clonal fashion in various mammalian species, displacing the wild-type mtDNAs. On the other hand, no explanation exists yet for the clonal expansion of mtDNA mutants that is compatible with experimental observations. We present here a new idea based on the distinctive connection between transcription and replication of metazoan mtDNA. Bioinformatic analysis of mtDNA deletion spectra strongly supports the predictions of this hypothesis and identifies specific candidates for proteins involved in transcriptional control of mtDNA replication. Computer simulations show the mechanism to be compatible with the available data from short- and long-lived mammalian species.

The mitochondrial genome of the Cinnamon Bittern, Ixobrychus cinnamomeus (Pelecaniformes: Ardeidae): sequence, structure and phylogenetic analysis

Ixobrychus cinnamomeus is a member of the large wading bird family, known as Ardeidae. In the present study, we determined the complete mitochondrial genome of I. cinnamomeus for use in future phylogenetic analysis. This circular mitochondrial genome is 17,180 bp in length and composed of 13 protein-coding genes, 22 tRNA genes, two rRNA genes and one putative control region. Three conserved domains and a minisatellite of 17 nucleotides with 22 tandem repeats were detected at the end of the control region. Phylogenetic relationships were reconstructed using the nucleotide and corresponding amino acid datasets of 12 concatenated protein-coding genes from the mitochondrial genome. Using maximum likelihood, maximum parsimony and Bayesian inference methods, the monophyly of Ciconiidae, Ardeidae and Threskiornithidae were confirmed; however, the monophyly of traditional Ciconiiformes and Pelecaniformes failed to be recovered. Although further studies are recommended to clarify relationships among and within the orders of Ciconiiformes, Pelecaniformes, Suliformes and Phaethontiformes, our results provide preliminary exploratory results that can be useful in the current understanding of avian phylogenetics.

Evolution of linear mitochondrial genomes in medusozoan cnidarians

In nearly all animals, mitochondrial DNA (mtDNA) consists of a single circular molecule that encodes several subunits of the protein complexes involved in oxidative phosphorylation as well as part of the machinery for their expression. By contrast, mtDNA in species belonging to Medusozoa (one of the two major lineages in the phylum Cnidaria) comprises one to several linear molecules. Many questions remain on the ubiquity of linear mtDNA in medusozoans and the mechanisms responsible for its evolution, replication, and transcription. To address some of these questions, we determined the sequences of nearly complete linear mtDNA from 24 species representing all four medusozoan classes: Cubozoa, Hydrozoa, Scyphozoa, and Staurozoa. All newly determined medusozoan mitochondrial genomes harbor the 17 genes typical for cnidarians and map as linear molecules with a high degree of gene order conservation relative to the anthozoans. In addition, two open reading frames (ORFs), polB and ORF314, are identified in cubozoan, schyphozoan, staurozoan, and trachyline hydrozoan mtDNA. polB belongs to the B-type DNA polymerase gene family, while the product of ORF314 may act as a terminal protein that binds telomeres. We posit that these two ORFs are remnants of a linear plasmid that invaded the mitochondrial genomes of the last common ancestor of Medusozoa and are responsible for its linearity. Hydroidolinan hydrozoans have lost the two ORFs and instead have duplicated cox1 at each end of their mitochondrial chromosome(s). Fragmentation of mtDNA occurred independently in Cubozoa and Hydridae (Hydrozoa, Hydroidolina). Our broad sampling allows us to reconstruct the evolutionary history of linear mtDNA in medusozoans.

Mitochondrial-nuclear co-evolution and its effects on OXPHOS activity and regulation

Factors required for mitochondrial function are encoded both by the nuclear and mitochondrial genomes. The order of magnitude higher mutation rate of animal mitochondrial DNA (mtDNA) enforces tight co-evolution of mtDNA and nuclear DNA encoded factors. In this essay we argue that such co evolution exists at the population and inter-specific levels and affect disease susceptibility. We also argue for the existence of three modes of co-evolution in the mitochondrial genetic system, which include the interaction of mtDNA and nuclear DNA encoded proteins, nuclear protein - mtDNA-encoded RNA interaction within the mitochondrial translation machinery and nuclear DNA encoded proteins-mtDNA binging sites interaction in the frame of the mtDNA replication and transcription machineries. These modes of co evolution require co-regulation of the interacting factors encoded by the two genomes. Thus co evolution plays an important role in modulating mitochondrial activity. This article is part of a Special Issue entitled: Mitochondrial Gene Expression.

The mitochondrial genomes of two nemerteans, Cephalothrix sp. (Nemertea: Palaeonemertea) and Paranemertes cf. peregrina (Nemertea: Hoplonemertea)

The mitochondrial genome sequences were determined for two species of nemerteans, Cephalothrix sp. (15,800 bp sequenced, near-complete) and Paranemertes cf. peregrina (14,558 bp, complete). As seen in most metazoans, the genomes encode 13 protein, 2 ribosomal RNA and 22 transfer RNA genes. The nucleotide composition is strongly biased toward A and T, as is typical for metazoan mtDNAs. There is also a significant strand skew in the distribution of these nucleotides, with the coding strand being richer in T than A and in G than C. All genes are transcribed in the same direction except for trnP and trnT, which is consistent with that reported for Cephalothrix hongkongiensis and Lineus viridis. Gene arrangement of Cephalothrix sp. is identical to that of C. hongkongiensis, while in P. cf. peregrina it is similar to L. viridis, but differs significantly from the three Cephalothrix species in the position of four protein-coding genes and seven tRNAs. Some protein-coding genes have 3' end stem-loop structures, which may allow mRNA processing without flanking tRNAs. The major non-coding regions observed in the two genomes with potential to form stem-loop structures may be involved in the initiation of replication or transcription. The average Ka/Ks values, varying from 0.12 to 0.89, are markedly different among the 13 mitochondrial protein-coding genes, suggesting that there may exist different selective pressure among mitochondrial genes of nemerteans.

Index-free de novo assembly and deconvolution of mixed mitochondrial genomes

Second-generation sequencing technology has allowed a very large increase in sequencing throughput. In order to make use of this high throughput, we have developed a pipeline for sequencing and de novo assembly of multiple mitochondrial genomes without the costs of indexing. Simulation studies on a mixture of diverse animal mitochondrial genomes showed that mitochondrial genomes could be reassembled from a high coverage of short (35 nt) reads, such as those generated by a second-generation Illumina Genome Analyzer. We then assessed this experimentally with long-range polymerase chain reaction products from mitochondria of a human, a rat, a bird, a frog, an insect, and a mollusc. Comparison with reference genomes was used for deconvolution of the assembled contigs rather than for mapping of sequence reads. As proof of concept, we report the complete mollusc mitochondrial genome of an olive shell (Amalda northlandica). It has a very unusual putative control region, which contains a structure that would probably only be detectable by next-generation sequencing. The general approach has considerable potential, especially when combined with indexed sequencing of different groups of genomes.

Characterization of the complete mitochondrial genome of the Rock pigeon, Columba livia (Columbiformes: Columbidae)

The rock pigeon (Columba livia), or Rock dove, is a member of the bird family Columbidae. We mapped the complete mitochondrial genome of the Rock pigeon. The mitochondrial genome of this species is a circular molecule of 17,229 bp in length, encoding a standard set of 13 protein-coding genes, two ribosomal RNA genes, and 22 transfer RNA genes, plus a putative control region, demonstrating a structure very similar to that of other birds. As found in other vertebrates, most of these genes are coded on the H-strand, except for NADH dehydrogenase subunit 6 (nad6) and eight tRNA genes (Gln, Ala, Asn, Cys, Tyr, Ser(UCN), Pro, Glu). The AT skew and GC skew of the whole genome, protein-coding genes, tRNA, rRNA, and the control region were calculated for the complete mitochondrial genomes of 30 avian species, representing 29 orders. All protein-coding genes initiated with ATG, except for cox1 and nad5, which began with GTG. One extra nucleotide 'C' was present in NADH dehydrogenase subunit 3 (nad3). All tRNA gene sequences have the potential to fold into typical cloverleaf secondary structures. Within the control region, conserved sequences were identified in three domains. Although the conserved blocks, such as ETAS1, ETAS2, CSB1, CSB1-like, and boxes C, D, E, and F, are readily identifiable in the C. livia control region, the typical origin of H-strand replication (O(H)), CSB2 and CSB3 could not be detected. These results provide basic information for phylogenetic analyses of birds, especially Columbiformes species.

Complete mitochondrial genome of Cabot's tragopan, Tragopan caboti (Galliformes: Phasianidae)

Cabot's tragopan, Tragopan caboti, is a globally threatened pheasant endemic to southeast China. The complete mitochondrial genome of Cabot's tragopan was sequenced. The circular genome contains 16,727 bp, encoding a standard set of 13 protein-coding genes, two ribosomal RNA genes, and 22 transfer RNA genes, plus the putative control region, a structure very similar to that of other Galliformes. As found in other vertebrates, most of these genes code on the H-strand, except for the NADH dehydrogenase subunit 6 (nad6) and eight tRNA genes (Gln, Ala, Asn, Cys, Tyr, Ser(UCN), Pro, Glu). All protein-coding genes initiated with ATG, except for cox1, which began with GTG, and had a strong skew of C vs G (GC skew = -0.29 to -0.73). One extra 'C' nucleotide was found in the NADH dehydrogenase subunit 3 (nad3). All the tRNA gene sequences have the potential to fold into typical cloverleaf secondary structures. Conserved sequences in three domains were identified within the control region (D-loop). These results provide basic information for phylogenetic analyses among Galliform birds, and especially Tragopan species.

Gene expression patterns of oxidative phosphorylation complex I subunits are organized in clusters

After the radiation of eukaryotes, the NUO operon, controlling the transcription of the NADH dehydrogenase complex of the oxidative phosphorylation system (OXPHOS complex I), was broken down and genes encoding this protein complex were dispersed across the nuclear genome. Seven genes, however, were retained in the genome of the mitochondrion, the ancient symbiote of eukaryotes. This division, in combination with the three-fold increase in subunit number from bacteria (N = ∼14) to man (N = 45), renders the transcription regulation of OXPHOS complex I a challenge. Recently bioinformatics analysis of the promoter regions of all OXPHOS genes in mammals supported patterns of co-regulation, suggesting that natural selection favored a mechanism facilitating the transcriptional regulatory control of genes encoding subunits of these large protein complexes. Here, using real time PCR of mitochondrial (mtDNA)- and nuclear DNA (nDNA)-encoded transcripts in a panel of 13 different human tissues, we show that the expression pattern of OXPHOS complex I genes is regulated in several clusters. Firstly, all mtDNA-encoded complex I subunits (N = 7) share a similar expression pattern, distinct from all tested nDNA-encoded subunits (N = 10). Secondly, two sub-clusters of nDNA-encoded transcripts with significantly different expression patterns were observed. Thirdly, the expression patterns of two nDNA-encoded genes, NDUFA4 and NDUFA5, notably diverged from the rest of the nDNA-encoded subunits, suggesting a certain degree of tissue specificity. Finally, the expression pattern of the mtDNA-encoded ND4L gene diverged from the rest of the tested mtDNA-encoded transcripts that are regulated by the same promoter, consistent with post-transcriptional regulation. These findings suggest, for the first time, that the regulation of complex I subunits expression in humans is complex rather than reflecting global co-regulation.

Transcription of the Dictyostelium discoideum mitochondrial genome occurs from a single initiation site

Transcription of the mitochondrial genome in Dictyostelium discoideum gives rise to eight major polycistronic RNA species that can be detected by Northern hybridization. In order to determine whether these transcripts could possibly derive from processing of even larger transcripts, reverse transcriptase polymerase chain reactions (RT-PCRs) were performed in an attempt to amplify the intervening regions between the eight major transcripts. All but one intervening region were successfully reverse transcribed and amplified, indicating that even larger transcripts existed and that the eight major transcripts detected previously may be the products of transcript processing. Southern hybridization analyses of DNA fragments representing the sequences between the eight major transcripts with in vitro capped mitochondrial RNA identified the 5' end of only one of the eight major transcripts as a genuine transcription start site. The ability to initiate transcription from DNA sequences upstream of the identified transcription initiation site was demonstrated in bacterial cells expressing the Dictyostelium mitochondrial RNA polymerase. We conclude that transcription of the Dictyostelium mitochondrial genome is initiated at a single site, generating a large polycistronic transcript that is very efficiently, probably cotranscriptionally processed into mature RNA species. This is the first report on a protist mitochondrial DNA that is, although much larger in size than its metazoan counterparts, transcribed from a single transcription initiation site.

Complete mitochondrial genome of Otis tarda (Gruiformes: Otididae) and phylogeny of Gruiformes inferred from mitochondrial DNA sequences

The complete nucleotide sequence of mitochondrial genome of the Great bustard (Otis tarda) was determined by using polymerase chain reaction (PCR) method. The genome is 16,849 bp in size, containing 13 protein-coding, 2 ribosomal and 22 transfer RNA genes. Sequences of the tRNA genes can be folded into canonical cloverleaf secondary structure except for tRNA-Cys and tRNA-Ser (AGY), which lose "DHU" arm. Sequence analysis showed that the O. tarda mitochondrial control region (mtCR) contained many elements in common with other avian mtCRs. A microsatellite repeat was found in the 3'-peripheral domain of the O. tarda mtCR. Based on the mitochondrial DNA sequences of 12S rRNA, 16S rRNA and tRNA-Val, a phylogenetic study of Gruiformes was performed. The result showed that Otididae was a sister group to "core Gruiformes" and Charadriiformes with strong support (97% posterior probability values) in Bayesian analysis. The taxonomic status of Rhynochetidae, Mesitornithidae, Pedionomidae and Turnicidae that traditionally belonged to Gruiformes was also discussed in this paper.

The single mitochondrial chromosome typical of animals has evolved into 18 minichromosomes in the human body louse, Pediculus humanus

The mitochondrial (mt) genomes of animals typically consist of a single circular chromosome that is approximately 16-kb long and has 37 genes. Our analyses of the sequence reads from the Human Body Louse Genome Project and the patterns of gel electrophoresis and Southern hybridization revealed a novel type of mt genome in the sucking louse, Pediculus humanus. Instead of having all mt genes on a single chromosome, the 37 mt genes of this louse are on 18 minicircular chromosomes. Each minicircular chromosome is 3-4 kb long and has one to three genes. Minicircular mt chromosomes are also present in the four other species of sucking lice that we investigated, but not in chewing lice nor in the Psocoptera, to which sucking lice are most closely related. We also report unequivocal evidence for recombination between minicircular mt chromosomes in P. humanus and for sequence variation in mt genes generated by recombination. The advantages of a fragmented mt genome, if any, are currently unknown. Fragmentation of mt genome, however, has coevolved with blood feeding in the sucking lice. It will be of interest to explore whether or not life history features are associated with the evolution of fragmented chromosomes.

Species-specific mitochondrial gene rearrangements in biting midges and vector species identification

Partial mitochondrial gene sequences of 16 Culicoides species were determined to elucidate phylogenetic relations among species and to develop a molecular identification method for important virus vector species. In addition, the analysis found mitochondrial gene rearrangement in several species. Sequences of the mitochondrial genome region, cox1-trnL2-cox2 (1940-3785 bp) of 16 Culicoides and additional sequences were determined in some species, including whole mitochondrial genome sequences of Culicoides arakawae. Nine species showed common organization in this region, with three genes cox1-trnL2-cox2 and a small or no intergenic region (0-30 bp) between them. The other seven species showed translocation of tRNA and protein-coding genes and/or insertion of AT-rich non-coding sequences (65-1846 bp) between the genes. The varied gene rearrangements among species within a genus is very rare for mitochondrial genome organization. Phylogenetic analyses based on the sequences of cox1+cox2 suggest a few clades among Japanese Culicoides species. No relationships between phylogenetic closeness and mitochondrial gene rearrangements were observed. Sequence data were used to establish a polymerase chain reaction tool to distinguish three important vector species from other Culicoides species, for which classification during larval stages is not advanced and identification is difficult.

Conserved repeats in the kinetoplast maxicircle divergent region of Leishmania sp. and Leptomonas seymouri

The maxicircle control region [also termed divergent region (DR)] composed of various repeat elements remains the most poorly studied part of the kinetoplast genome. Only three extensive DR sequences demonstrating no significant similarity were available for trypanosomatids (Leishmania tarentolae, Crithidia oncopelti, Trypanosoma brucei). Recently, extensive DR sequences have been obtained for Leishmania major and Trypanosoma cruzi. In this work we have sequenced DR fragments of Leishmania turanica, Leishmania mexicana, Leishmania chagasi and two monogenetic trypanosomatids Leptomonas seymouri and Leptomonas collosoma. With the emergence of the additional extensive sequences some conserved features of DR structure become evident. A conserved palindromic sequence has been revealed in the DRs of the studied Leishmania species, L. seymouri, and T. cruzi. The overall DR structure appears to be similar in all the Leishmania species, their relative L. seymouri, and T. brucei: long relatively GC-rich repeats are interspersed with clusters of short AT-rich repeats. C. oncopelti, L. collosoma, and T. cruzi have a completely different DR structure. Identification of conserved sequences and invariable structural features of the DR may further our understanding of the functioning of this important genome fragment.

The Mitochondrial Genome of Xiphinema americanum sensu stricto (Nematoda: Enoplea): Considerable Economization in the Length and Structural Features of Encoded Genes

The complete sequence of the mitochondrial genome of the plant parasitic nematode Xiphinema americanum sensu stricto has been determined. At 12626bp it is the smallest metazoan mitochondrial genome reported to date. Genes are transcribed from both strands. Genes coding for 12 proteins, 2 rRNAs and 17 putative tRNAs (with the tRNA-C, I, N, S1, S2 missing) are predicted from the sequence. The arrangement of genes within the X. americanum mitochondrial genome is unique and includes gene overlaps. Comparisons with the mtDNA of other nematodes show that the small size of the X. americanum mtDNA is due to a combination of factors. The two mitochondrial rRNA genes are considerably smaller than those of other nematodes, with most of the protein encoding and tRNA genes also slightly smaller. In addition, five tRNAs genes are absent, lengthy noncoding regions are not present in the mtDNA, and several gene overlaps are present.

Novel Mitochondrial Gene Content and Gene Arrangement Indicate Illegitimate Inter-mtDNA Recombination in the Chigger Mite, Leptotrombidium pallidum

To better understand the evolution of mitochondrial (mt) genomes in the Acari (mites and ticks), we sequenced the mt genome of the chigger mite, Leptotrombidium pallidum (Arthropoda: Acari: Acariformes). This genome is highly rearranged relative to that of the hypothetical ancestor of the arthropods and the other species of Acari studied. The mt genome of L. pallidum has two genes for large subunit rRNA, a pseudogene for small subunit rRNA, and four nearly identical large noncoding regions. Nineteen of the 22 tRNAs encoded by this genome apparently lack either a T-arm or a D-arm. Further, the mt genome of L. pallidum has two distantly separated sections with identical sequences but opposite orientations of transcription. This arrangement cannot be accounted for by homologous recombination or by previously known mechanisms of mt gene rearrangement. The most plausible explanation for the origin of this arrangement is illegitimate inter-mtDNA recombination, which has not been reported previously in animals. In light of the evidence from previous experiments on recombination in nuclear and mt genomes of animals, we propose a model of illegitimate inter-mtDNA recombination to account for the novel gene content and gene arrangement in the mt genome of L. pallidum.

Molecular architecture and rates of DNA substitutions of the mitochondrial control region of cracid birds

The control region (CR) plays an important role in replication and transcription in the mitochondrial genome. Its supposedly high rate of DNA substitution makes it a suitable marker for studies of population and closely related species. Three domains can be identified in CR, each having its own characteristics regarding base composition, pattern of variability and rate of DNA substitution. We sequenced the complete CR for 27 cracids, including all 11 genera to characterize its molecular organization, describe patterns of DNA substitution along the gene, and estimate absolute rates of DNA substitution. Our results show that cracid CR organization and patterns of DNA substitution are typical of other birds. Conserved blocks C and B, fused CSB2/3, and a putative segment for bidirectional replication not usually present in birds were identified in cracids. We also suggest a new delimitation for domains to allow better comparisons among CRs at different taxonomic ranking. Our estimates of absolute rates of DNA substitution show that, in general, CR evolves at a rate slower than that of two protein-coding genes (CR, 0.14%-0.3%; ND2, 0.37%-0.47%; and cytochrome b, 0.29%-0.35% per site per million years within genera). Within CR domains, rates within genera ranged from 0.05% to 0.8% per site per million years.

Functional domains of chicken mitochondrial transcription factor A for the maintenance of mitochondrial DNA copy number in lymphoma cell line DT40

Nuclear and mitochondrial (mt) forms of chicken mt transcription factor A (c-TFAM) generated by alternative splicing of a gene (c-tfam) were cloned. c-tfam mapped at 6q1.1-q1.2 has similar exon/intron organization as mouse tfam except that the first exons encoding the nuclear and the mt form-specific sequences were positioned oppositely. When cDNA encoding the nuclear form was transiently expressed in chicken lymphoma DT40 cells after tagging at the C terminus with c-Myc, the product was localized into nucleus, whereas the only endogenous mt form of DT40 cells was immunostained exclusively within mitochondria. c-TFAM is most similar to Xenopus (xl-) TFAM in having extended C-terminal regions in addition to two high mobility group (HMG) boxes, a linker region between them, and a C-terminal tail, also found in human and mouse TFAM. Similarities between c- and xl-TFAM are higher in linker and C-terminal regions than in HMG boxes. Disruption of both tfam alleles in DT40 cells prevented proliferation. The tfam+/tfam- cells showed a 50 and 40-60% reduction of mtDNA and its transcripts, respectively. Expression of exogenous wild type c-tfam cDNA in the tfam+/tfam- cells increased mtDNA up to 4-fold in a dose-dependent manner, whereas its transcripts increased only marginally. A deletion mutant lacking the first HMG box lost this activity, whereas only marginal reduction of the activity was observed in a deletion mutant at the second HMG box. Despite the essential role of the C-terminal tail in mtDNA transcription demonstrated in vitro, deletion of c-TFAM at this region reduced the activity of maintenance of the mtDNA level only by 50%. A series of deletion mutant at the tail region suggested stimulatory and suppressive sequences in this region for the maintenance of mtDNA level.

Characterization of the red knot (Calidris canutus) mitochondrial control region

We sequenced the complete mitochondrial control regions of 11 red knots (Calidris canutus). The control region is 1168 bp in length and is flanked by tRNA glutamate (glu) and the gene ND6 at its 5' end and tRNA phenylalanine (phe) and the gene 12S on its 3' end. The sequence possesses conserved sequence blocks F, E, D, C, CSB-1, and the bird similarity box (BSB), as expected for a mitochondrial copy. Flanking tRNA regions show correct secondary structure, and a relative rate test indicated no significant difference between substitution rates in the sequence we obtained versus the known mitochondrial sequence of turnstones (Charadriiformes: Scolopacidae). These characteristics indicate that the sequence is mitochondrial in origin. To confirm this, we sequenced the control region of a single individual using both purified mitochondrial DNA and genomic DNA. The sequences were identical using both methods. The sequence and methods presented in this paper may now serve as a reference for future studies using knot and other avian control regions. Furthermore, the discovery of five variable sites in 11 knots towards the 3' end of the control region, and the variability of this region in contrast to the more conserved central domain in the alignment between knots and other Charadriiformes, highlights the importance of this area as a source of variation for future studies in knots and other birds.

The crocodilian mitochondrial control region: general structure, conserved sequences, and evolutionary implications

We present the first comprehensive analysis of the crocodilian control region. We have analyzed sequences from all three families of Crocodylia (Crocodylidae, Gavialidae, Alligatoridae), incorporating all genera except Paleosuchus and Melanosuchus. Within the control region of other vertebrates, several sequence motifs and their order appear to be conserved. Herein, we compare aligned crocodilian D-loop sequences to homologous sequences from other vertebrates ranging from fish to birds. Among other findings, we have discovered that while domain I tends to be shorter than the same region in mammals and birds, it contains sequences similar in structure to both the goose-hairpin and termination associated sequences (TAS). Domain II is highly conservative with regard to size among the taxa examined and contains several of the conserved sequence boxes characterized in other vertebrates. Domain III contains several interesting sequence motifs including tandemly repeated sequences, a long poly-A region in the Crocodylidae, and possible bidirection promoter sequences.

Structure and evolution of the mitochondrial DNA complete control region in the Drosophila subobscura subgroup

The complete A + T-rich region of mitochondrial DNA (mtDNA) has been cloned and sequenced in the species of the Drosophila subobscura subgroup D. subobscura, D. madeirensis and D. guanche. Comparative analysis of these sequences with others already published has identified new sequence motifs that are conserved in Drosophila and other insects. A putative bi-directional promoter and a stop signal are proposed to be involved in the primary mtDNA strand replication of Drosophila. This region strongly resolves relationships of the species included in a phylogenetic analysis, both for closely related species and also at deeper phylogenetic levels when only the left and central domains are taken into account.

The complete sequence of the mitochondrial genome of Buteo buteo (Aves, Accipitridae) indicates an early split in the phylogeny of raptors

The complete sequence of the mitochondrial (mt) genome of Buteo buteo was determined. Its gene content and nucleotide composition are typical for avian genomes. Due to expanded noncoding sequences, Buteo possesses the longest mt genome sequenced so far (18,674 bp). The gene order comprising the control region and neighboring genes is identical to that of Falco peregrinus, suggesting that the corresponding rearrangement occurred before the falconid/accipitrid split. Phylogenetic analyses performed with the mt sequence of Buteo and nine other mt genomes suggest that for investigations at higher taxonomic levels (e.g., avian orders), concatenated rRNA and tRNA gene sequences are more informative than protein gene sequences with respect to resolution and bootstrap support. Phylogenetic analyses indicate an early split between Accipitridae and Falconidae, which, according to molecular dating of other avian divergence times, can be assumed to have taken place in the late Cretaceous 65-83 MYA.

Duplication and concerted evolution of the mitochondrial control region in the parrot genus Amazona

We report a duplication and rearrangement of the mitochondrial genome involving the control region of parrots in the genus Amazona. This rearrangement results in a gene order of cytochrome b/tRNA(Thr)/pND6/pGlu/CR1/tRNA(Pro)/NADH dehydrogenase 6/tRNA(Glu)/CR2/tRNA(Phe)/12s rRNA, where CR1 and CR2 refer to duplicate control regions, and pND6 and pGlu indicate presumed pseudogenes. In contrast to previous reports of duplications involving the control regions of birds, neither copy of the parrot control region shows any indications of degeneration. Rather, both copies contain many of the conserved sequence features typically found in avian control regions, including the goose hairpin, TASs, the F, C, and D boxes, conserved sequence box 1 (CSB1), and an apparent homolog to the mammalian CSB3. We conducted a phylogenetic analysis of homologous portions of the duplicate control regions from 21 individuals representing four species of Amazona (A. ochrocephala, A. autumnalis, A. farinosa, and A. amazonica) and Pionus chalcopterus. This analysis revealed that an individual's two control region copies (i.e., the paralogous copies) were typically more closely related to one another than to corresponding segments of other individuals (i.e., the orthologous copies). The average sequence divergence of the paralogous control region copies within an individual was 1.4%, versus a mean value of 4.1% between control region orthologs representing nearest phylogenetic neighbors. No differences were found between the paralogous copies in either the rate or the pattern in which the two copies accumulated base pair changes. This pattern suggests concerted evolution of the two control regions, perhaps through occasional gene conversion events. We estimated that gene conversion events occurred on average every 34,670 +/- 18,400 years based on pairwise distances between the paralogous control region sequences of each individual. Our results add to the growing body of work indicating that under some circumstances duplicated mitochondrial control regions are retained through evolutionary time rather than degenerating and being lost, presumably due to selection for a small mitochondrial genome.

Increased mitochondrial-encoded gene transcription in immortal DF-1 cells

We have established, in continuous cell culture, a spontaneously immortalized chicken embryo fibroblast (CEF) cell line (DF-1) as well as several other immortal CEF cell lines. The immortal DF-1 cells divided more rapidly than primary and other immortal CEF cells. To identify the genes involved in rapidly dividing DF-1 cells, we have used differential display RT-PCR. Of the numerous genes analyzed, three mitochondrial-encoded genes (ATPase 8/6, 16S rRNA, and cytochrome b) were shown to express at higher levels in DF-1 cells compared to primary and other immortal CEF cells. The inhibition of mitochondrial translation by treatment with chloramphenicol markedly decreased ATP production and cell proliferation in DF-1 cells, while not affecting growth in either primary or other immortal CEF cells. This result suggests a correlation between rapid cell proliferation and the increased mitochondrial respiratory functions. We also determined that the increased transcription of mitochondrial-encoded genes in DF-1 cells is due to increased de novo transcript synthesis as shown by mitochondrial run-on assays, and not the result of either increased mitochondrial biogenesis or mitochondrial transcript half-lives. Together, the present studies suggest that the transcriptional activation of mitochondrial-encoded genes and the elevated respiratory function should be one of the characteristics of rapidly dividing immortal cells.

Slow rate of evolution in the mitochondrial control region of gulls (Aves: Laridae)

We sequenced part of the mitochondrial control region and the cytochrome b gene in 72 specimens from 32 gull species (Laridae, Larini) and 2 outgroup representatives (terns: Laridae, Sternini). Our control region segment spanned the conserved central domain II and the usually hypervariable 3' domain III. Apart from some heteroplasmy at the 3' end of the control region, domain III was not more variable than domain II or the cytochrome b gene. Furthermore, variation in the tempo of evolution of domain III was apparent between phyletic species groups. The lack of variation of the gull control region could not be explained by an increase in the proportion of conserved sequences in these birds, and the gull control region showed an organization similar to those of other avian control regions studied to date. A novel invariant direct repeat was identified in domain II of gulls, and in domain III, two to three inverted, sometimes imperfect, repeats are able to form a significantly stable stem-and-loop structure. These putative secondary structures have not been reported before, and a comparison between species groups showed that they are more stable in the group with the more conserved control region. The unusually slow rate of evolution of control region part III of the gulls could thus be partly explained by the existence of secondary structures in domain III of these species.

A repeat complex in the mitochondrial control region of Adélie penguins from Antarctica

We have determined the nucleotide sequence of the entire mitochondrial control region (CR) of the Adélie penguin (Pygoscelis adeliae) from Antarctica. Like in most other birds, this CR region is flanked by the gene nad6 and transfer (t)RNA trnE(uuc) at the 5' end and the gene rns and trnF(gaa) at the 3' end. Sequence analysis shows that the Adelie penguin CR contains many elements in common with other CRs including the termination associated sequences (TAS), conserved F, E, D, and C boxes, the conserved sequence block (CSB)-1, as well as the putative light and heavy strand promoters sites (LSP-HSP). We report an extraordinarily long avian control region (1758 bp) which can be attributed to the presence, at the 3' peripheral domain, of five 81-bp repeat sequences, each containing a putative LSP-HSP, followed by 30 tetranucleotide microsatellite repeat sequences consisting of (dC-dA-dA-dA)30. The microsatellite and the 81-bp repeat reside in an area known to be transcribed in other species.

Tandemly repeated sequences in mtDNA control region of whitefish, Coregonus lavaretus

Length variation of the mitochondrial DNA control region was observed with PCR amplification of a sample of 138 whitefish (Coregonus lavaretus). Nucleotide sequences of representative PCR products showed that the variation was due to the presence of an approximately 100-bp motif tandemly repeated two, three, or five times in the region between the conserved sequence block-3 (CSB-3) and the gene for phenylalanine tRNA. This is the first report on the tandem array composed of long repeat units in mitochondrial DNA of salmonids.

Complete sequence of the mitochondrial DNA of the primitive opisthobranch gastropod Pupa strigosa: systematic implication of the genome organization

The complete sequence (14,189 bp) of the mitochondrial DNA of the opisthobranch gastropod Pupa strigosa was determined. The genome contains 13 protein, 2 rRNA, and 22 tRNA genes typical of metazoan mtDNA. The Pupa mitochondrial genome is highly compact and shows the following unusual features, like pulmonate land snails: (1) extremely small genome size, (2) absence of lengthy noncoding regions (with the largest intergenic spacer being only 46 nt), (3) size reduction of encoded genes, and (4) many overlapping genes. Several tRNA genes exhibit bizarre secondary structures with reduced T or D stems, and many tRNA genes have unstable acceptor stems that might be corrected by posttranscriptional RNA editing. The Pupa mitochondrial gene arrangement is almost identical to those of pulmonate land snails but is radically divergent from those of the prosobranch gastropod Littorina saxatilis and other molluscs. Our finding that the unique gene arrangement and highly compact genome organization are shared between opisthobranch and pulmonate gastropods strongly suggests their close phylogenetic affinity.

Transcription initiation and RNA processing in the mitochondria of the red alga Chondrus crispus: convergence in the evolution of transcription mechanisms in mitochondria

The mitochondrial DNA (mt DNA) of the red alga Chondrus crispus is shown to be transcribed into two large RNA molecules. These primary transcripts are cleaved once, at the level of a tRNA, then the resulting products are processed via multiple maturation events into either mono- or poly-cistronic RNAs. Transcripts were detected for all genes and open reading frames, except for rps11 and orf172. For both transcription units the initiation of transcription was mapped by in vitro RNA capping and primer extension experiments within inverse repeated sequences at the north pole of the molecule. Consistent with primer extension mapping, putative promoter motifs sharing significant similarities with both chicken and Xenopus mitochondrial promoters were found in the C. crispus mitochondrial genome. Altogether C. crispus mitochondrial DNA appears to be transcribed as animal mtDNA is, suggesting that transcription mechanisms in mitochondria are dependent on the overall organization of the mitochondrial genome irrespective of the eukaryotic phylogeny.

Heteroplasmy, length and sequence variation in the mtDNA control regions of three percid fish species (Perca fluviatilis, Acerina cernua, Stizostedion lucioperca)

The nucleotide sequence of the control region and flanking tRNA genes of perch (Perca fluviatilis) mtDNA was determined. The organization of this region is similar to that of other vertebrates. A tandem array of 10-bp repeats, associated with length variation and heteroplasmy was observed in the 5' end. While the location of the array corresponds to that reported in other species, the length of the repeated unit is shorter than previously observed for tandem repeats in this region. The repeated sequence was highly similar to the Mt5 element which has been shown to specifically bind a putative D-loop DNA termination protein. Of 149 perch analyzed, 74% showed length variation heteroplasmy. Single-cell PCR on oocytes suggested that the high level of heteroplasmy is passively maintained by maternal transmission. The array was also observed in the two other percid species, ruffe (Acerina cernua) and zander (Stizostedion lucioperca). The array and the associated length variation heteroplasmy are therefore likely to be general features of percid mtDNAs. Among the perch repeats, the mutation pattern is consistent with unidirectional slippage, and statistical analyses supported the notion that the various haplotypes are associated with different levels of heteroplasmy. The variation in array length among and within species is ascribed to differences in predicted stability of secondary structures made between repeat units.