A direct study of the relative synthesis of petite and grande mitochondrial DNA in zygotes from crosses involving suppressive petite mutants of Saccharomyces cerevisiae

Mitochondrial Genome Instability in W303-SK1 Yeast Cytoplasmic Hybrids

Unlike most animals, some fungi, including baker's yeast, inherit mitochondrial DNA (mtDNA) from both parents. When haploid yeast cells fuse, they form a heteroplasmic zygote, whose offspring retain one or the other variant of mtDNA. Meanwhile, some mutant mtDNA (rho-), with large deletions in the nucleotide sequence, can displace wild-type (rho+) mtDNA. Consequently, offspring of zygotes with such rho- mtDNA predominantly carry the mutant variant. This phenomenon is called suppressivity. In this study, we investigated how the suppressivity of rho- mtDNA depends on the mitochondrial and nuclear genomes of the rho+ strain during crossing. Comparing two diverged laboratory strains, SK1 and W303, we measured suppressivity in crosses with four rho- strains. One rho- strain showed significantly higher suppressivity when crossed with SK1 than with W303. We then created cytoplasmic hybrids by swapping mtDNAs between these strains. Surprisingly, we found that the mtDNA of the rho+ strain, rather than its nuclear DNA, determines high suppressivity in crosses of SK1 rho+ with the rho- strain. Additionally, mtDNA replacement reduced respiration rate and growth rate on non-fermentable substrates while increasing the likelihood of functional mtDNA loss. Our data demonstrate that a mutant mtDNA variant's ability to displace another mitochondrial DNA variant in a heteroplasmic cell depends more on mtDNA sequences than on the biochemical and structural context created by the nuclear genome background.

Decreasing mitochondrial RNA polymerase activity reverses biased inheritance of hypersuppressive mtDNA

Faithful inheritance of mitochondrial DNA (mtDNA) is crucial for cellular respiration/oxidative phosphorylation and mitochondrial membrane potential. However, how mtDNA is transmitted to progeny is not fully understood. We utilized hypersuppressive mtDNA, a class of respiratory deficient Saccharomyces cerevisiae mtDNA that is preferentially inherited over wild-type mtDNA (rho+), to uncover the factors governing mtDNA inheritance. We found that some regions of rho+ mtDNA persisted while others were lost after a specific hypersuppressive takeover indicating that hypersuppressive preferential inheritance may partially be due to active destruction of rho+ mtDNA. From a multicopy suppression screen, we found that overexpression of putative mitochondrial RNA exonuclease PET127 reduced biased inheritance of a subset of hypersuppressive genomes. This suppression required PET127 binding to the mitochondrial RNA polymerase RPO41 but not PET127 exonuclease activity. A temperature-sensitive allele of RPO41 improved rho+ mtDNA inheritance over a specific hypersuppressive mtDNA at semi-permissive temperatures revealing a previously unknown role for rho+ transcription in promoting hypersuppressive mtDNA inheritance.

Selfish little circles: transmission bias and evolution of large deletion-bearing mitochondrial DNA in Caenorhabditis briggsae nematodes

Selfish DNA poses a significant challenge to genome stability and organismal fitness in diverse eukaryotic lineages. Although selfish mitochondrial DNA (mtDNA) has known associations with cytoplasmic male sterility in numerous gynodioecious plant species and is manifested as petite mutants in experimental yeast lab populations, examples of selfish mtDNA in animals are less common. We analyzed the inheritance and evolution of mitochondrial DNA bearing large heteroplasmic deletions including nad5 gene sequences (nad5Δ mtDNA), in the nematode Caenorhabditis briggsae. The deletion is widespread in C. briggsae natural populations and is associated with deleterious organismal effects. We studied the inheritance patterns of nad5Δ mtDNA using eight sets of C. briggsae mutation-accumulation (MA) lines, each initiated from a different natural strain progenitor and bottlenecked as single hermaphrodites across generations. We observed a consistent and strong drive toward higher levels of deletion-bearing molecules in the heteroplasmic pool of mtDNA after ten generations of bottlenecking. Our results demonstrate a uniform transmission bias whereby nad5Δ mtDNA accumulates to higher levels relative to intact mtDNA in multiple genetically diverse natural strains of C. briggsae. We calculated an average 1% per-generation transmission bias for deletion-bearing mtDNA relative to intact genomes. Our study, coupled with known deleterious phenotypes associated with high deletion levels, shows that nad5Δ mtDNA are selfish genetic elements that have evolved in natural populations of C. briggsae, offering a powerful new system to study selfish mtDNA dynamics in metazoans.

The reductive hotspot hypothesis of mammalian aging: membrane metabolism magnifies mutant mitochondrial mischief

A severe challenge to the idea that mitochondrial DNA mutations play a major role in the aging process in mammals is that clear loss-of-function mutations accumulate only to very low levels (under 1% of total) in almost any tissue, even by very old age. Their accumulation is punctate: some cells become nearly devoid of wild-type mitochondrial DNA and exhibit no activity for the partly mitochondrially encoded enzyme cytochrome c oxidase. Such cells accumulate in number with aging, suggesting that they survive indefinitely, which is itself paradoxical. The reductive hotspot hypothesis suggests that these cells adjust their metabolism to use plasma membrane electron transport as a substitute for the mitochondrial electron transport chain in the reoxidation of reduced dinucleotides, and that, like mitochondrial electron transport, this process is imperfect and generates superoxide as a side-effect. This superoxide, generated on the outside of the cell, can potentially initiate classical free radical chemistry including lipid peroxidation chain reactions in circulating material such as lipoproteins. These, in turn, can be toxic to mitochondrially nonmutant cells that import them to satisfy their cholesterol requirements. Thus, the relatively few cells that have lost oxidative phosphorylation capacity may be toxic to the rest of the body. In this minireview, recent results relevant to this hypothesis are surveyed and approaches to intervening in the proposed process are discussed.

A proposed refinement of the mitochondrial free radical theory of aging

Over recent years, evidence has been accumulating in favour of the free radical theory of aging, first proposed by Harman. Despite this, an understanding of the mechanism by which cells might succumb to the effects of free radicals has proved elusive. This paper proposes such a mechanism, based on a previously unexplored hypothesis for the proliferation of mutant mitochondrial DNA: that mitochondria with reduced respiratory function, due to a mutation or deletion affecting the respiratory chain, suffer less frequent lysosomal degradation, because they inflict free radical damage more slowly on their own membranes. Once such a mutation occurs in a mitochondrion of a non-dividing cell, therefore, mitochondria carrying it will rapidly populate that cell, thereby destroying the cell's respiratory capability. The accumulation of cells that have undergone this transition results in aging at the organismal level. The consistency of the hypothesis with known facts is discussed, and technically feasible tests are suggested, of both the proposed mechanism and its overall contribution to mammalian aging.

Transmission of the yeast mitochondrial genome to progeny: the impact of intergenic sequences

In a previous publication it was shown that the output of yeast mitochondrial loci lacking nearby intergenic sequences (encompassing ori/rep elements) was reduced in crosses to strains with wild-type mtDNAs. In the present work, mitochondrial genomes carrying the intergenic deletions were marked at unlinked loci by introducing specific antibiotic resistance mutations against erythromycin, oligomycin and paromomycin. These marked genomes were used to follow the output of unlinked regions of the genome from crosses between the intergenic deletion mutants and wild-type strains. Transmission of genetically unlinked markers in coding regions was substantially reduced when an intergenic deletion was present on the same genome. In general the transmission of the antibiotic markers was the same as or slightly higher than the corresponding intergenic marker. These results indicate that the presence of an intergenic deletion in the regions studied impairs the transmission to progeny of a mitochondrial genome as a whole. More specifically, the results suggest that ori/rep sequences, present in the regions that have been deleted, confer a competitive advantage over genomes lacking a full complement of such sequences. These results support the hypothesis that intergenic sequences, and specifically ori/rep elements, have a biological role in the mitochondrial genome. However, because of the exclusive presence of ori/rep sequences in the genus Saccharomyces, it may be that these sequences evolved in (or invaded) the mitochondrial genome relatively late in the evolution of the yeasts.(ABSTRACT TRUNCATED AT 250 WORDS)

Transmission of yeast mitochondrial loci to progeny is reduced when nearby intergenic regions containing ori sequences are deleted

Mitochondrial DNAs (mtDNA) from four stable revertant strains generated from high frequency petite forming strains of Saccharomyces cerevisiae have been shown to contain deletions which have eliminated intergenic sequences encompassing ori1, ori2 and ori7. The deleted sequences are dispensable for expression of the respiratory phenotype and mutant strains exhibit the same relative amount of mtDNA per cell as the wild-type (wt) parental strain. These deletion mutants were also used to study the influence of particular intergenic sequences on the transmission of closely linked mitochondrial loci. When the mutant strains were crossed with the parental wt strains, there was a strong bias towards the transmission into the progeny of mitochondrial genomes lacking the intergenic deletions. The deficiency in the transmission of the mutant regions was not a simple function of deletion length and varied between different loci. In crosses between mutant strains which had non-overlapping deletions, wt mtDNA molecules were formed by recombination. The wt recombinants were present at high frequencies among the progeny of such crosses, but recombinants containing both deletions were not detected at all. The results indicate that mitochondrial genomes can be selectively transmitted to progeny and that two particular intergenic regions positively influence transmission. Within these regions other sequences in addition to ori/rep affect transmission.