A cluster of six tRNA genes in Drosophila mitochondrial DNA that includes a gene for an unusual tRNAserAGY.

Genes for URF3, tRNAala, tRNAarg, tRNAasn, tRNAserAGY, tRNAglu, tRNAphe, and the carboxyl terminal segment of the URF5 gene have been identified within a sequenced segment of the mtDNA molecule of Drosophila yakuba. The genes occur in the order given. The URF5 and tRNAphe genes are transcribed in the same direction as replication while the URF3 and remaining five tRNA genes are transcribed in the opposite direction. Considerable differences exist in the relative arrangement of these genes in D. yakuba and mammalian mtDNA molecules. In the tRNAserAGY gene an eleven nucleotide loop, within which secondary structure formation seems unlikely, replaces the dihydrouridine arm, and both the variable loop (six nucleotides) and the T phi C loop (nine nucleotides) are larger than in any other D. yakuba tRNA gene. As available evidence is consistent with AGA codons specifying serine rather than arginine in the Drosophila mitochondrial genetic code, the possibility is considered that the 5'GCU anticodon of the D. yakuba tRNAserAGY gene can recognize AGA as well as AGY codons.     

Transfer RNA genes in Drosophila mitochondrial DNA: related 5' flanking sequences and comparisons to mammalian mitochondrial tRNA genes.

Genes for tRNAgly and tRNAserUCN have been identified within sequences of mtDNA of Drosophila yakuba. The tRNAgly gene lies between the genes for cytochrome c oxidase subunit III and URF3, and all three of these genes are contained in the same strand of the mtDNA molecule. The tRNAserUCN gene is adjacent to the URF1 gene. These genes are contained in opposite strands of the mtDNA molecule and their 3' ends overlap. The structures of the tRNAgly and tRNAserUCN genes, and of the four tRNA genes of D. yakuba mtDNA reported earlier (tRNAile, tRNAgln, tRNAf-met and tRNAval) are compared to each other, to non-organelle tRNAs, and to corresponding mammalian mitochondrial tRNA genes. Within 19 nucleotides upstream from the 5' terminal nucleotide of each of the Drosophila mitochondrial tRNAgly, tRNAserUCN, tRNAile, tRNAgln and tRNAf-met genes occurs the sequence 5'TTTATTAT, or a sequence differing from it by one nucleotide substitution. Upstream from this octanucleotide sequence, and separated from it by 3, 4 and 11 nucleotides, respectively, in the 5' flanking regions of the tRNAile, tRNAserUCN and tRNAgly genes occurs the sequence 5'GATGAG.     

Mitochondrial DNA of the sea anemone,Metridium senile (Cnidaria): Prokaryote-like genes for tRNAf-Met and small-subunit ribosomal RNA,and standard genetic code specificities for AGR and ATA codons

The nucleotide sequence of a segment of the mitochondrial DNA (mtDNA) molecule of the sea anemone Metridium senile (phylum Cnidaria, class Anthozoa, order Actiniaria) has been determined, within which have been identified the genes for respiratory chain NADH dehydrogenase subunit 2 (ND2), the small-subunit rRNA (s-rRNA), cytochrome c oxidase subunit II(COII), ND4, ND6, cytochrome b (Cyt b), tRNA^f-Met, and the large-subunit rRNA (1-rRNA). The eight genes are arranged in the order given and are all transcribed from the same strand of the molecule. The overall order of the M. senile mt-genes differs from that of other metazoan mtDNAs. In M. senile mt-protein genes, AGA and AGG codons appear to have the standard genetic code specification of arginine, rather than serine as found for other invertebrate mt-genetic codes. Also, ATA has the standard genetic code specification of isoleucine. TGA occurs in three M. senile mt-protein genes and may specify tryptophan as in other metazoan, protozoan, and some fungal mt-genetic codes. The M. senile mt-rRNA^f-Met gene has primary and secondary structure features closely resembling those of the Escherichia coli initiator tRNA, including standard dihydrouridine and TC loop sequences and a mismatch pair at the top of the aminoacyl stem. Determinations of the 5 and 3 end nucleotides of the M. senile mt-srRNAs indicated that these molecules have a homogenous size of 1,081 ntp, larger than any other known metazoan mt-s-rRNAs. Consistent with its larger size, the M. senile mt-s-rRNA can be folded into a secondary structure that more closely resembles that of the E. coli 16S rRNA than can any other metazoan mt-s-rRNA. These findings concerning M. senile mtDNA indicate that most of the unusual features regarding metazoan mt-genetic codes, rRNAs, and probably tRNAs developed after divergence of the Cnidarian line from the ancestral line common to other metazoa.Correspondence to: D.R. Wolstenholme     

The ribosomal RNA genes of Drosophila mitochondrial DNA.

The nucleotide sequence of a segment of the mtDNA molecule of Drosophila yakuba which contains the A+T-rich region and the small and large rRNA genes separated by the tRNAval gene has been determined. The 5' end of the small rRNA gene was located by S1 protection analysis. In contrast to mammalian mtDNA, a tRNA gene was not found at the 5' end of the D. yakuba small rRNA gene. The small and large rRNA genes are 20.7% and 16.7% G+C and contain only 789 and 1326 nucleotides. The 5' regions of the small rRNA gene (371 nucleotides) and of the large rRNA gene (643 nucleotides) are extremely low in G+C (14.6% and 9.5%, respectively) and convincing sequence homologies between these regions and the corresponding regions of mouse mt-rRNA genes were found only for a few short segments. Nevertheless, the entire lengths of both of the D. yakuba mt-rRNA genes can be folded into secondary structures which are remarkably similar to secondary structures proposed for the rRNAs of mouse mtDNA. The replication origin-containing, A+T-rich region (1077 nucleotides; 92.8% A+T), which lies between the tRNAile gene and the small rRNA gene, lacks open reading frames greater than 123 nucleotides.     

Hyaloraphidium curvatum: a linear mitochondrial genome, tRNA editing, and an evolutionary link to lower fungi

We have sequenced the mitochondrial DNA (mtDNA) of Hyaloraphidium curvatum, an organism previously classified as a colorless green alga but now recognized as a lower fungus based on molecular data. The 29.97-kbp mitochondrial chromosome is maintained as a monomeric, linear molecule with identical, inverted repeats (1.43 kbp) at both ends, a rare genome architecture in mitochondria. The genome encodes only 14 known mitochondrial proteins, 7 tRNAs, the large subunit rRNA and small subunit rRNA (SSU rRNA), and 3 ORFs. The SSU rRNA is encoded in two gene pieces that are located 8 kbp apart on the mtDNA. Scrambled and fragmented mitochondrial rRNAs are well known from green algae and alveolate protists but are unprecedented in fungi. Protein genes code for apocytochrome b; cytochrome oxidase 1, 2, and 3, NADH dehydrogenase 1, 2, 3, 4, 4L, 5, and 6, and ATP synthase 6, 8, and 9 subunits, and several of these genes are organized in operon-like clusters. The set of seven mitochondrially encoded tRNAs is insufficient to recognize all codons that occur in the mitochondrial protein genes. When taking into account the pronounced codon bias, at least 16 nuclear-encoded tRNAs are assumed to be imported into the mitochondria. Three of the seven predicted mitochondria-encoded tRNA sequences carry mispairings in the first three positions of the acceptor stem. This strongly suggests that these tRNAs are edited by a mechanism similar to the one seen in the fungus Spizellomyces punctatus and the rhizopod amoeba Acanthamoeba castellanii. Our phylogenetic analysis confirms with overwhelming support that H. curvatum is a member of the chytridiomycete fungi, specifically related to the Monoblepharidales.     

Complete DNA Sequence of the Mitochondrial Genome of the Black Chiton, Katharina Tunicata

The DNA sequence of the 15,532-base pair (bp) mitochondrial DNA (mtDNA) of the chiton Katharina tunicata has been determined. The 37 genes typical of metazoan mtDNA are present: 13 for protein subunits involved in oxidative phosphorylation, 2 for rRNAs and 22 for tRNAs. The gene arrangement resembles those of arthropods much more than that of another mollusc, the bivalve Mytilus edulis. Most genes abut directly or overlap, and abbreviated stop codons are inferred for four genes. Four junctions between adjacent pairs of protein genes lack intervening tRNA genes; however, at each of these junctions there is a sequence immediately adjacent to the start codon of the downstream gene that is capable of forming a stem-and-loop structure. Analysis of the tRNA gene sequences suggests that the D arm is unpaired in tRNA(ser(AGN)), which is typical of metazoan mtDNAs, and also in tRNA(ser(UCN)), a condition found previously only in nematode mtDNAs. There are two additional sequences in Katharina mtDNA that can be folded into structures resembling tRNAs; whether these are functional genes is unknown. All possible codons except the stop codons TAA and TAG are used in the protein-encoding genes, and Katharina mtDNA appears to use the same variation of the mitochondrial genetic code that is used in Drosophila and Mytilus. Translation initiates at the codons ATG, ATA and GTG. A + T richness appears to have affected codon usage patterns and, perhaps, the amino acid composition of the encoded proteins. A 142-bp non-coding region between tRNA(glu) and CO3 contains a 72-bp tract of alternating A and T.     

Drosophila mitochondrial DNA: a novel gene order

Part of the replication origin-containing A+T-rich region of the Drosophila yakuba mtDNA molecule and segments on either side of this region have been sequenced, and the genes within them identified. The data confirm that the small and large rRNA genes lie in tandem adjacent to that side of the A+T-rich region which is replicated first, and establish that a tRNAval gene lies between the two rRNA genes and that URF1 follows the large rRNA gene. The data further establish that the genes for tRNAile, tRNAgln, tRNAf-met and URF2 lie in the order given, on the opposite side of the A+T-rich region to the rRNA genes and, except for tRNAgln, are contained in the opposite strand to the rRNA, tRNAval and URF1 genes. This is in contrast to mammalian mtDNAs where all of these genes are located on the side of the replication origin which is replicated last, within the order tRNAphe, small (12S) rRNA, tRNAval, large (16S) rRNA, tRNAleu, URF1, tRNAile, tRNAgln, tRNAf-met and URF2, and, except tRNAgln, are all contained in the same (H) strand. In D. yakuba URF1 and URF2, the triplet AGA appears to specify an amino acid, which is again different from the situation found in mammalian mtDNAs, where AGA is used only as a rare termination codon.     

Drosophila Melanogaster Mitochondrial DNA: Gene Organization and Evolutionary Considerations

The sequence of a 8351-nucleotide mitochondrial DNA (mtDNA) fragment has been obtained extending the knowledge of the Drosophila melanogaster mitochondrial genome to 90% of its coding region. The sequence encodes seven polypeptides, 12 tRNAs and the 3' end of the 16S rRNA and CO III genes. The gene organization is strictly conserved with respect to the Drosophila yakuba mitochondrial genome, and different from that found in mammals and Xenopus. The high A + T content of D. melanogaster mitochondrial DNA is reflected in a reiterative codon usage, with more than 90% of the codons ending in T or A, G + C rich codons being practically absent. The average level of homology between the D. melanogaster and D. yakuba sequences is very high (roughly 94%), although insertion and deletions have been detected in protein, tRNA and large ribosomal genes. The analysis of nucleotide changes reveals a similar frequency for transitions and transversions, and reflects a strong bias against G + C on both strands. The predominant type of transition is strand specific.