Use of lycorine and DAPI staining in Saccharomyces cerevisiae to differentiate between rho0 and rho- cells in a cce1/delta cce1 nuclear background.

In the yeast Saccharomyces cerevisiae, mutants are viable with large deletions (rho-), or even complete loss of the mitochondrial genome (rho0). One class of rho- mutants, which is called hypersuppressive, is characterised by a high transmission of the mutated mitochondrial genome to the diploid progeny when mated to a wild-type (rho+) haploid. The nuclear gene CCE1 encodes a cruciform cutting endonuclease, which is located in the mitochondrion and is responsible for the highly biased transmission of the hypersuppressive rho- genome. CCE1 is a Holliday junction specific endonuclease that resolves recombination intermediates in mitochondrial DNA. The cleavage activity shows a strong preference for cutting after a 5'-CT dinucleotide. In the absence of the CCE1 gene product, the mitochondrial genomes remain interconnected and have difficulty segregating to the daughter cells. As a consequence, there is an increase in the fraction of daughter cells that are rho0. In this paper we demonstrate the usefulness of lycorine, together with staining by 4',6-diamidino-2-phenylindole (DAPI), to assay for the mitotic stability of a variety of mitochondrial genomes. We have found that rho+ and rho- strains that contain CT sequences produce a large fraction of rho0 progeny in the absence of CCE1 activity. Only those rho- mitochondrial genomes lacking the CT recognition sequence are unaffected by the cce1 allele.

A role for recombination junctions in the segregation of mitochondrial DNA in yeast.

In S. cerevisiae, mitochondrial DNA (mtDNA) molecules, in spite of their high copy number, segregate as if there were a small number of heritable units. The rapid segregation of mitochondrial genomes can be analyzed using mtDNA deletion variants. These small, amplified genomes segregate preferentially from mixed zygotes relative to wild-type mtDNA. This segregation advantage is abolished by mutations in a gene, MGT1, that encodes a recombination junction-resolving enzyme. We show here that resolvase deficiency causes a larger proportion of molecules to be linked together by recombination junctions, resulting in the aggregation of mtDNA into a small number of cytological structures. This change in mtDNA structure can account for the increased mitotic loss of mtDNA and the altered pattern of mtDNA segregation from zygotes. We propose that the level of unresolved recombination junctions influences the number of heritable units of mtDNA.

A nuclear mutation reversing a biased transmission of yeast mitochondrial DNA.

The highly biased transmission of p- mitochondrial DNA that occurs in hypersuppressive matings between p- and p+ cells of the yeast Saccharomyces cerevisiae is thought to be a consequence of the replication advantage of the p- mtDNA. A nuclear gene, MGT1, that is required for this displacement of p+ mtDNA from zygotic clones has been identified through mutation. When one haploid parent carries the mgt1 allele, transmission of p- mtDNA is substantially reduced. When both haploid parents carry the mgt1 allele, p- mtDNA is essentially eliminated from the zygotic progeny. Thus in the absence of the MGT1 gene there is a switch in the transmission bias; p+ mtDNA rather than the hypersuppressive p- mtDNA is inherited by most zygotic clones. In contrast to its semi-dominant behavior in haploid matings, mgt1 behaves as a recessive allele in diploid matings since the p+ genome in MGT1/mgt1 diploids is efficiently displaced when mated with a MGT1/mgt1 hypersuppressive p- diploid strain. We find that p+ genomes can be comaintained along with hypersuppressive p- mtDNA for extended periods in clonal lines derived from MGT1 x mgt1 matings. However, as expected from the recessive nature of the mgt1 mutation, these p+ genomes are eventually eliminated. Our work indicates that MGT1 plays a crucial role in the competition for inheritance between hypersuppressive p- mtDNAs and the p+ mitochondrial genome. The MGT1 gene product may be a component of a mtDNA replication system that acts preferentially at the rep sequences found in hypersuppressive mtDNAs.

Creation of ARS activity in yeast through iteration of non-functional sequences.

Replication origins in Saccharomyces cerevisiae have been identified through the cloning of autonomous replication sequence (ARS) elements that allow the extrachromosomal maintenance of plasmid molecules. ARS activity requires a close match to an 11 bp consensus sequence and A + T-rich flanking DNA. ARS elements with a wide range of capacities for promoting plasmid maintenance have been described. We determined the ARS activity of plasmids with inserts consisting of repetitions of a 64 bp 100% A + T sequence that has sequence similarities to known ARS elements. An insert with approximately four repeats did not yield transformants, but inserts with either eight or eleven repeats did. The cooperative production of ARS activity did not require a contiguous arrangement since a plasmid containing two inserts of four repeats each, separated by about 1 kb, was functional. Our results show that a change from non-function to function can be accomplished by the cumulative action of individually inactive sequences. We conclude that the probability of replication initiation is too low with only four repeats to allow plasmid maintenance, but the overall probability is increased by further sequence iteration to provide origin activity. We suggest that chromosomes may contain stretches with dispersed, weak origin elements, each undetected by the conventional ARS assay, that in sum provide origin function.