A nice web page from the Kovac lab from Bratislava concerning telomerase and widespread existence of linear mitochondrial chromosomes.
At variance with the earlier belief that mitochondrial genomes are represented by circular DNA molecules, a large number of organisms have been found to carry linear mitochondrial DNA. The studies of linear mitochondrial genomes evoked several questions addressing the evolution of organellar genophores and the role of their terminal structures, mitochondrial telomeres.
- The occurence of linear genophores in mitochondria is relatively frequent.
- Linear and circular mitochondrial genomes (i) do not belong to the independent evolutionary lineages and apparently (ii) do not represent radically different life styles.
- Evolutionary emergence of linear mitochondrial genomes was accompanied by (i) the generation of various types of terminal structures and (ii) the adaptation of the component(s) of mtDNA replication machinery.
- Mitochondrial telomeres seem to play similar role(s) as the terminal structures of other linear genophores (i.e. nuclear chromosomes, plasmids, viruses).
- Mitochondrial telomeres might shed some light on alternative (i.e. telomerase-independent) solutions to the ‘end-replication problem’.
- Mitochondrial telomeres are promising target for specific intervention in human pathogens harboring linear mtDNA.
Also the FAQ:
What is the frequency of occurence of linear mitochondrial DNAs?: Surprisingly high. The first linear mitochondrial genome was discovered in the ciliate Tetrahymena by Suyama and Miura in 1968. Since then other species harboring linear mitochondrial genome were discovered. These organisms belong to a large variety of taxonomical groups (ciliates, apicomplexan protozoa, algae, slime molds, oomycetous fungi, yeasts and several species of the metazoan phyllum Cnidaria). Even mtDNA of yeast Saccharomyces cerevisiae, portrayed by geneticists as a circular molecule of 25 µm circumference, is in vivo mainly linear.
What is the difference in genetic organization between circular and linear mitochondrial genomes? Essentially there are no striking differences. Linear mitochondrial genomes comprise a set of genes which is very similar to that on the circular mtDNAs. The genetic maps of yeast Candida parapsilosis and Williopsis saturnus var. mrakii demonstrate that their linear mitochondrial genomes encode sequences homologous to the ‘standard set’ of mitochondrial genes.
What is the evolutionary origin of linear mitochondrial genomes? Evidence for the existence of genophore in mitochondria served as an argument in favor of the endosymbiotic origin of these organelles. It has been generally held that the molecule of the mitochondrial DNA is circular and that the circularity of mitochondrial genome represents the vestigial structure of the ancestral prokaryotic endosymbiont genome. However, certain bacteria (i.e. Streptomyces, Borrelia) possess linear chromosomes. Does it mean that organisms containing linear mtDNA represent a different evolutionary lineage that separates them substantially from those with circular DNA? The study of yeast mitochondrial DNA demonstrated frequent occurrences of linear genomes in genera Pichia, Williopsis and Candida. An inspection of the evolutionary relationships of different yeast species does not favor the idea of independent origin of linear mitochondrial genomes. Rather, they seem to be almost randomly scattered on the phylogenetic tree (Figure9). Moreover, two strains of the same species of Williopsis saturnus var. suaveolens have either circular or linear mitochondrial genome (Fukuhara et al., 1993, Drissi et al., 1994).