Genetic analysis of petite mutants of Saccharomyces cerevisiae: transmissional types

DarT-mediated mtDNA damage induces dynamic reorganization and selective segregation of mitochondria

Mitochondria are dynamic organelles that play essential roles in cell growth and survival. Processes of fission and fusion are critical for the distribution, segregation, and maintenance of mitochondria and their genomes (mtDNA). While recent work has revealed the significance of mitochondrial organization for mtDNA maintenance, the impact of mtDNA perturbations on mitochondrial dynamics remains less understood. Here, we develop a tool to induce mitochondria-specific DNA damage using a mitochondrial-targeted base modifying bacterial toxin, DarT. Following damage, we observe dynamic reorganization of mitochondrial networks, likely driven by mitochondrial dysfunction. Changes in the organization are associated with the loss of mtDNA, independent of mitophagy. Unexpectedly, perturbation to exonuclease function of mtDNA replicative polymerase, Mip1, results in rapid loss of mtDNA. Our data suggest that, under damage, partitioning of defective mtDNA and organelle are de-coupled, with emphasis on mitochondrial segregation independent of its DNA. Together, our work underscores the importance of genome maintenance on mitochondrial function, which can act as a modulator of organelle organization and segregation.

pif mutation blocks recombination between mitochondrial rho+ and rho- genomes having tandemly arrayed repeat units in Saccharomyces cerevisiae

Three allelic nuclear mutants affected in the recombination of mtDNA have been characterized in Saccharomyces cerevisiae and assigned to the PIF locus. In the mutants, the general recombination measured by the recombination frequency between linked or unlinked alleles is normal. However, the pif mutations prevent the integration into the rho+ genome of the markers (oli1, oli2, diu1, ery, oxi1, oxi2) of those rho- genomes that have tandemly arrayed repeat units. Therefore, these rho- genomes characterize a PIF-dependent recombination system. The pif mutations have also revealed the existence of a PIF-independent recombination system used by those rho- genomes that have an inverted organization of their repeat units. The markers of such palindromic rho- genomes exhibit high integration frequency into the rho+ genome even in the presence of the pif mutation. In addition, the pif mutations greatly increase suppressiveness in crosses between pif rho+ strains and PIF-dependent as well as PIF-independent rho- clones. We conclude that the recombination between rho+ and rho- genomes involves at least two distinct systems that depend on the organization of the rho- genome.

Gene conversion at the var1 locus on yeast mitochondrial DNA

Alleles of the var1 locus on yeast mtDNA determine the apparent size of the mitochondrial translation product, var1 polypeptide. We have analyzed most of the different var1 alleles in our collection, which number at least 15, and have developed procedures and a genetic rationale for determining their origin and predicting their behavior in crosses. The var1 alleles are characterized by two genetically defined segments, designated a and b, which can move from one var1 allele to another by asymmetric gene conversion. We show that the a segment behaves as an entity in recombination; it is either present in or absent from different var1 alleles. The b segment usually, but not always, recombines as an entity; in some cases, only portions of the b segment recombine by gene conversion. Thus, the total number of electrophoretically resolvable var1 species we observe is explained by the assortment of a, b, and partial b segments. Each segment recombines at a characteristic frequency; however, one example is presented which shows that the recipient can modulate the frequency of gene conversion. Finally, we show that, like the 21S rDNA region (omega), there is polarity of gene conversion within var1.

Genetic and physical characterization of a segment of yeast mitochondrial DNA involved in the control of genetic recombination

Genetic recombination between the 3 RIB (ribosomal) loci of yeast mitochondrial DNA is under the control of a mitochondrial locus named omega (with alleles omega+ and omega-) which is tightly linked to the RIBI locus. We have attempted to elucidate the molecular mechanisms(s) involved by using rho- mutants with similar (RIBI+ RIB2+ RIB3(0) genotype but different recombination properties in rho- x rho+ crosses. These were obtained through pedigree analysis and their mitochondrial DNAs were mapped on a high resolution physical map of the RIB section that had been built by analysis of thermal denaturation profiles and electron microscopy of partially denatured molecules. By comparison of physical and genetic data it can be shown that possession of the omega+ allele by the rho- cell is not sufficient for its expression in crosses, some additional DNA segments(s) in the ribosomal region being needed. This result and several features of the rho+ x rho- crosses are discussed in the light of current concepts in mitochondrial genetics of yeast and the recently discovered fact that omega+ and omega- strains differ by the presence of a 1000 base pairs insertion in the former.