Mitochondrial Genome Organization
Mitochondrial Genome OrganizationIn comparison to the chloroplast genome, the size of the mitochondrial genome is quite variable.
Further, in comparison to the mitochondrial genomes of other species the size is quite large and variable. For example, animal mitochondrial genomes range in size form 15-18 kb, and fungi mitochondrial genomes range form 18-78 kb.
Plants may code for more proteins than with species. For example, genes for ribosomes, subunits I and II of cytochrome oxidase and ATPase subunits are located on the mitochondrial genomes of plants.
When DNA from corn mitochondria was investigated with EM, several circular molecules of different sizes were detected. Once the genome was mapped it became apparent that a mechanism existed to generated these circles of different sizes. It is now understood how these molecules arise. First, lets look at the simple situation of turnip. Two direct repeats undergo intramolecular recombination to give the two smaller molecules:
218 kb -------> 135 kb + 83 kb
The mitochondrial genome of corn undergoes the same type of recombination, but the events are more complex. First, the master circles can be subdivided into two major subgroups:
570 kb -------> 488 kb + 82 kb
570 kb -------> 503 kb + 67 kb
The second group of molecules are still labile and can produce several other subpopulations. Further two subgenomic circles can unite to form a larger circle. This variability is possible because corn has 10 repeats with which intramolecular recombination can occur.
Introns have been located in the cytochrome oxidase subunit II gene (the cytochrome complex consists of 3 mitochondrial and 4 nuclear encoded genes). This gene contains one intron in rye, corn, wheat, rice, and carrot, but for other species such as Oenothera, broad bean, cucumber the gene has no intron.
Stern and Lonsdale (1982) hybridized mtRNA to a SstII digest of maize mt DNA and found that it hybridized to fragments known not to contain mt rRNA genes. The question of interest was - what was it hybridizing to? They next looked at a cosmid clone of corn mtDNA that hybridized to the mt RNA and found that it hybridized to a RNA molecule of the size of the cp 16S RNA gene. How could this have happened?
They next mapped the clone and compared it to the map of the corn cpDNA and found that the clone map was almost congruent with that of the the cpDNA 16S RNA region. This mapping showed that the two maps were nearly identical over a 12 kb region of DNA. These results suggest that cpDNA had been transferred to the mitochondrial genome.
The observation that organelle DNA was found in other DNA compartments of the cell was extended by other researcher. Stern and Palmer looked at corn, mung bean, spinach and pea and found extensive evidence of cpDNA/mtDNA homology. These observations were extended to other DNA locations in the plant cell. Kemble et al (1983) demonstrated that mitochondrial DNA sequences are located in the nucleus of corn. Scott and Timmis (1984) showed that cpDNA sequences are found in the nuclear DNA.
Copyright © 1998. Phillip McClean