The mitochondrial genome of Arabidopsis is composed of both native and immigrant information.

Plants contain large mitochondrial genomes, which are several times as complex as those in animals, fungi or algae. However, genome size is not correlated with information content. The mitochondrial genome (mtDNA) of Arabidopsis specifies only 58 genes in 367 kb, whereas the 184 kb mtDNA in the liverwort Marchantia polymorpha codes for 66 genes, and the 58 kb genome in the green alga Prototheca wickerhamii encodes 63 genes. In Arabidopsis' mtDNA, genes for subunits of complex II, for several ribosomal proteins and for 16 tRNAs are missing, some of which have been transferred recently to the nuclear genome. Numerous integrated fragments originate from alien genomes, including 16 sequence stretches of plastid origin, 41 fragments of nuclear (retro)transposons and two fragments of fungal viruses. These immigrant sequences suggest that the large size of plant mitochondrial genomes is caused by secondary expansion as a result of integration and propagation, and is thus a derived trait established during the evolution of land plants.

Mosaic open reading frames in the Arabidopsis thaliana mitochondrial genome.

In the mitochondrial genome of Arabidopsis thaliana eight mosaic open reading frames arose by recombination of fragments duplicated from one or more mitochondrial genes. These duplications represent unedited sequences, suggesting their derivation from genomic DNA rather than RNA. Five of the chimeric reading frames contain the information for the N-terminus of the original polypeptide and 5' upstream regions. These observations suggest that the generation of novel open reading frames in plant mitochondria can occur rather easily by chance extensions of duplicated gene fragments. The presence of so many mosaic open reading frames in the normal Arabidopsis thaliana mitochondrial genome suggests that such recombined sequences interfere only occasionally and fortuitously with the peak mitochondrial performance presumably required during pollen maturation, and usually do not cause a cytoplasmic male sterile phenotype.

The mitochondrial genome of Arabidopsis thaliana contains 57 genes in 366,924 nucleotides.

We have determined the complete sequence of the mitochondrial DNA in the model plant species Arabidopsis thaliana, affording access to the first of its three genomes. The 366,924 nucleotides code for 57 identified genes, which cover only 10% of the genome. Introns in these genes add about 8%, open reading frames larger than 100 amino acids represent 10% of the genome, duplications account for 7%, remnants of retrotransposons of nuclear origin contribute 4% and integrated plastid sequences amount to 1%-leaving 60% of the genome unaccounted for. With the significant contribution of duplications, imported foreign DNA and the extensive background of apparently functionless sequences, the mosaic structure of the Arabidopsis thaliana mitochondrial genome features many aspects of size-relaxed nuclear genomes.

Genomic recombination of the mitochondrial atp6 gene in Arabidopsis thaliana at the protein processing site creates two different presequences.

In the mitochondrial genome of the flowering plant Arabidopsis thaliana the atp6 open reading frame is located on the border of one of the repeats resulting in two copies with different presequence extensions. The two presequences of 135 and 97 amino acids respectively show no similarity to each other, while the mature protein sequences are identical. Both preproteins are most likely synthesized in Arabidopsis mitochondria from promoter elements upstream of each copy. The presence of two arrangements in the mitochondrial genome of fertile Arabidopsis plants suggests this recombination to be unrelated to a cytoplasmic male sterile phenotype. This recombination precisely at the mature protein terminus is reminiscent of the domain shuffling model in protein evolution.

copia-, gypsy- and LINE-like retrotransposon fragments in the mitochondrial genome of Arabidopsis thaliana.

Several retrotransposon fragments are integrated in the mitochondrial genome of Arabidopsis thaliana. These insertions are derived from all three classes of nuclear retrotransposons, the Ty1/copia-, Ty3/gypsy- and non-LTR/LINE-families. Members of the Ty3/gypsy group of elements have not yet been identified in the nuclear genome of Arabidopsis. The varying degrees of similarity with nuclear elements and the dispersed locations of the sequences in the mitochondrial genome suggest numerous independent transfer-insertion events in the evolutionary history of this plant mitochondrial genome. Overall, we estimate remnants of retrotransposons to cover > or = 5% of the mitochondrial genome in Arabidopsis.

The maize NCS2 abnormal growth mutant has a chimeric nad4-nad7 mitochondrial gene and is associated with reduced complex I function.

The molecular basis of the maternally inherited, heteroplasmic NCS2 mutant of maize was investigated. Analysis of the NCS2 mtDNA showed that it closely resembles the progenitor cmsT mitochondrial genome, except that the mutant genome contains a fused nad4-nad7 gene and is deleted for the small fourth exon of nad4. The rearrangement has occurred at a 16-bp repeat present in the third intron of the nad4 gene and in the second intron of the nad7 gene. Transcripts containing exon 4 of the nad4 gene are greatly reduced in mtRNA preparations from heteroplasmic NCS2 plants; larger transcripts are associated with the first three nad4 exons. Identical 5' ends of the nad4 transcripts have been mapped 396 and 247 bp upstream of the start codon in mtRNAs from both NCS2 and related non-NCS plants. The putative transcription termination signal of nad4 is deleted in mutant DNA, resulting in the production of the unique longer transcripts. The complex transcript pattern associated with nad7 is also altered in the mutant. Both nad4 and nad7 encode subunits of complex I (NADH dehydrogenase) of the mitochondrial electron transfer chain. Oxygen uptake experiments show that the functioning of complex I is specifically reduced in mitochondria isolated from NCS2 mutant plants.

The first analysed archegoniate mitochondrial gene (COX3) exhibits extraordinary features.

The first mitochondrial-encoded gene of an archegoniate has been identified, cloned and sequenced. The cytochrome oxidase III gene (cox3) of the moss Physcomitrella patens consists of a 618 bp open reading frame with high homology (around 72%) to known cox3 sequences of higher plants. Nevertheless, it is a quarter shorter than these. The cox3 gene of P. patens contains no introns and reveals a G + C-content of 41.3%. The region containing the cox3 gene exists as a single copy in the mitochondrial genome as shown by restriction mapping. In the 5' flanking sequence a putative ribosome binding site and a putative secondary structure were found. Two main transcripts of 2.4 kb and 2.6 kb were detected indicating a complex mitochondrial transcription pattern possibly due to co-transcription. Additional open reading frames were found downstream from, as well as upstream of, the cox3 gene. In Western blots a polyclonal cox3 antibody from yeast detected one single band with an apparent molecular weight of 22 kDa.